THE EFFECT OF TREATMENT COUCHES ON DELIVERED DOSE

DURING RADIOTHERAPY TREATMENTS

_______________

A Thesis

Presented to the

Faculty of

San Diego State University

_______________

In Partial Fulfillment

of the Requirements for the Degree

Master of Science

in

Radiological Health Physics

_______________

by

Brittany Elizabeth Cochran

Spring 2012

iii

Copyright © 2012

by

Brittany Elizabeth Cochran

All Rights Reserved

iv

ABSTRACT OF THE THESIS

The Effect of Treatment Couches on Delivered Dose During Radiotherapy Treatments

by Brittany Elizabeth Cochran

Master of Science in Radiological Health Physics San Diego State University, 2012

This study measured the photon beam attenuation characteristics of the Varian

EXACT, Varian IGRT treatment couches and two CIVCO Universal extensions used during external beam radiotherapy; and assessed the accuracy of Varian’s Eclipse treatment planning system in modeling the beam attenuation of each couch. Dose Measurements were taken using a Farmer ion chamber inside a cylindrical acrylic phantom. They were taken for two photon energies (6MV and 15MV), at three different field sizes, for various gantry angles through the couch. The EXACT couch was tested both with the adjustable rails pushed to the outside of the couch, and pushed to the center of the couch. The IGRT couch was divided by thickness into three sections for more accurate representation of attenuation along the length of the couch. Effects on surface dose are measured using a Tissue Maximum Ratio for the Varian EXACT couch, and Percent Depth Dose curves for the Varian IGRT and CIVCO extensions. Both CIVCO extensions were introduced into the Eclipse planning system by tracing the couch structures as in viewed in Ct images. All measurements were modeled in the Eclipse treatment planning system for comparison. Maximum attenuation was measured at 6MV with the smallest field for all couch setups. The EXACT couch produced a maximum attenuation of 19.2% and a surface dose increase of 47% of Dmax through the couch. Maximum attenuation for the IGRT couch and both CIVCO extensions were between 6-8%. Through the couch, the IGRT couch showed a surface dose increase of 29% of Dmax,; the CIVCO extensions produced a surface dose increase of 33% and 7% of Dmax. Maximum difference between measured and planned dose was 7%, for the EXACT couch, 1.9% for the IGRT couch and 2.4% for both CIVCO extensions. These treatment couches were shown to affect delivered and surface dose. Including the couch structure during treatment planning can account for the couch attenuation in most cases, but some attenuation values are underestimated in the planning software. Largest effects are for oblique treatment angles using low energy and small field sizes.

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TABLE OF CONTENTS

PAGE

ABSTRACT ............................................................................................................................. iv

LIST OF TABLES .................................................................................................................. vii

LIST OF FIGURES ............................................................................................................... viii

CHAPTER

1 TREATMENT OVERVIEW .........................................................................................1

1.1 Basics of Photon Radiotherapy ..........................................................................2

1.2 Treatment Beam Characteristics ........................................................................6

1.3 Dose Distribution ...............................................................................................7

1.4 Current Treatment Modalities ............................................................................9

1.5 Composition and Design of Typical Treatment Couch ...................................10

1.6 Previous Studies Concerning Couch Attenuation ............................................11

2 OVERVIEW OF STUDY ............................................................................................13

2.1 Materials and Methods .....................................................................................14

2.1.1 Direct Attenuation Measurements: General Setup .................................14

2.1.2 Varian EXACT Couch ............................................................................16

2.1.3 Varian IGRT Couch ................................................................................16

2.1.4 CIVCO Couch Extensions ......................................................................16

2.1.5 Tennis Racket Support ............................................................................17

2.1.6 Tissue Maximum Ratio Measurement of EXACT Couch with Rails Out ..........................................................................................................17

2.1.7 Percent Depth Dose Measurements of IGRT and CIVCO couches .............................................................................................................17

2.2 Eclipse Modeling .............................................................................................18

2.3 Error Considerations ........................................................................................19

2.4 Results ..............................................................................................................21

2.4.1 Varian Tennis Racket Insert....................................................................22

2.4.2 Varian EXACT Couch Attenuation ........................................................22

2.4.3 IGRT Couch Attenuation ........................................................................27

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2.4.4 CIVCO Couch Attenuation .....................................................................29

3 DISCUSSION ..............................................................................................................33

3.1 Conclusion .......................................................................................................35

REFERENCES ........................................................................................................................36

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LIST OF TABLES

PAGE

Table 2.1. Attenuation Measurements for the Varian Tennis Racket, when Beam is Passing Straight Through the Grid Surface .................................................................22

Table 2.2. Attenuation Measurements for 6MV Photon Beam Passing Intersecting the Varian EXACT Couch then the Adjustable Rails are Pushed Under the Center of the Treatment Couch (Rails-In Position) .................................................................23

Table 2.3. Percent Attenuation Measurements for 15MV Photon Beam Passing Intersecting the Varian EXACT Couch when the Adjustable Rails are Pushed Under the Center of the Treatment Couch (Rails-In Position) ....................................24

Table 2.4. Surface Dose Measurements for the Varian EXACT Couch, Taken from the TMR Curve Produced Using the Solid Water Phantom ........................................24

Table 2.5. Attenuation Measurements for a 6MV Photon Beam Passing Intersecting the Varian EXACT Couch when the Adjustable Rails are Pushed Under the Center of the Treatment Couch (Rails-Out Position) ..................................................25

Table 2.6. Attenuation Measurements for a 15MV Photon Beam Passing Intersecting the Varian EXACT Couch when the Adjustable Rails are Pushed Under the Center of the Treatment Couch (Rails-Out Position) ..................................................26

Table 2.7. Attenuation Measurements for a 6MV Photon Beam Passing Intersecting the Varian IGRT Couch for Thickness Divisions ........................................................27

Table 2.8. Attenuation Measurements for a 15MV Photon Beam Passing Intersecting the Varian IGRT Couch for Thickness Divisions ........................................................28

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LIST OF FIGURES

PAGE

Figure 1.1. Schematic of a linear accelerator.. ...........................................................................3

Figure 2.1. Gantry Rotation around isocenter. .........................................................................14

Figure 2.2. Experimental set-up for direct attenuation measurements. ...................................15

Figure 2.3. Measurement set-up for TMR measurement on EXACT couch. ..........................18

Figure 2.4. (a) CT scan slice of the CIVCO Universal couch extension, (b) Contour traced couch structures from CT scan slice, (c) Resultant couch structure set produced in the Eclipse treatment planning system, after couch contouring and Hounsfield unit assignment of each couch structure. ..................................................20

Figure 2.5. Attenuation measurements for a 10x10cm2 field, 6MV beam passing through the Varian EXACT couch with the adjustable rails pushed to the center of the couch. ......................................................................................................23

Figure 2.6. Attenuation measurements for a 10x10cm2 field, 6MV beam passing through the Varian EXACT couch with the adjustable rails pushed to the sides of the couch. .................................................................................................................25

Figure 2.7. Tissue maximum ratio curve produced for the Varian EXACT couch using a 6MV, 10x10cm2 treatment beam. ....................................................................26

Figure 2.8. Measured Attenuation values for the thickest section of the Varian IGRT couch for a 6MV treatment beam, using a 10x10cm2 field size. .................................28

Figure 2.9. Percent depth dose measurements for a 6MV beam, 10x10cm2 field, using the thickest section of the Varian IGRT couch. ...........................................................29

Figure 2.10. Attenuation measurements for the CIVCO universal extension using a 6MV treatment beam, with a 10x10cm2 field size. ......................................................30

Figure 2.11. Attenuation measurements for the CIVCO sample extension using a 6MV treatment beam, with a 10x10cm2 field size. ......................................................31

Figure 2.12. Comparison for attenuation measurements between the CIVCO universal and CIVCO sample extensions for a 6MV beam, 10x10cm2 field. .............................31

Figure 2.13. Eclipse modeled percent depth dose curves for the CIVCO universal and CIVCO sample extensions. ..........................................................................................32

1

CHAPTER 1

TREATMENT OVERVIEW

A neoplasm, also known as a tumor, is the result of abnormal tissue cells whose

reproductive cycles are malfunctioning, causing overactive reproduction and division.1 A

tumor is both benign and self-contained, or malignant (cancerous) with rapidly dividing cells

that kill surrounding tissue cells and metastasis throughout the body using the blood system.1

Removal of tumors can be difficult depending on the location and size of the tumor, and is

performed using either surgery, chemotherapy or radiation techniques.1

Radiation is defined as the emission and propagation of energy through space or a

material medium.2 Similarly, dose is defined as the amount of energy deposited in a medium

per unit mass. This transfer of energy is the basis for External Photon Beam Radiotherapy,

which is used in the treatment of both benign and malignant tumors. External Photon Beam

radiotherapy is performed using an x-ray beam from a source located outside the patient to

irradiate a target volume inside the patient.2 Through this targeting, energy is transferred to

particles inside tissue cells with the purpose of stopping cell reproduction attributed to tumor

growth.

Radiation exposure is closely monitored and regulated by federal organizations for

occupational, public, and medical exposure. The United States Nuclear Regulatory

Commission has published radiation exposure limits for non-medical exposures in Title 10,

Chapter 20 of the Code of Federal Regulations.3 Medical exposure limits are more complex,

the International Commission on Radiation Units and Measurements (ICRU) presents

recommended tolerance levels for calculation accuracy of treatment planning systems with

regards to delivered dose in Report 42. This report suggests that the calculated dose

distribution across high gradient distributions should be within 2% of the relative dose or

within 0.2cm of the isodose curve position in the target volume.4 Over the total planning

target volume, the ICRU recommends the delivered dose be within 5% of the actual

prescribed dose. Although the tolerances put forth by the ICRU are suggested tolerances

2

rather than required tolerance limits, these limits contribute to the principle of ALARA (As

Low As Reasonably Achievable) treatment doses.

Over the past 50 years or so, the use of linear accelerators has improved to allow for

more control over definition of treatment areas and required treatment beam characteristics,

resulting in improved control over dose delivery within patients. Furthermore, the use of

external photon beam radiotherapy has undergone several changes in radiation delivery

techniques and modalities. Over time the method of radiation treatment has switched from

the use of a single treatment beam to the utilization of multiple beams and field sizes,

although the basic principles of external beam radiation therapy still remains the same; to use

radiation to destroy tumor cells without damaging any surrounding normal tissue beyond

their ability to repair themselves.5 However with the advent of new treatment techniques, the

requirement for vigilant quality control requires the re-evaluation of elements that previously

may have been considered of no importance during early treatment techniques.

1.1 BASICS OF PHOTON RADIOTHERAPY External Photon Beam Therapy utilizes a linear accelerator to produce a focused,

beam of x-rays that can be used for patient exposure. A linear accelerator produces this x-ray

beam by first producing a beam of high-energy electrons and aiming the electron beam at a

target made of material with a high atomic number. The interactions of these electrons with

the target result in Bremsstrahlung x-ray emissions that form a resultant treatment x-ray

beam. This x-ray beam is shaped first by a primary set of collimators, found directly behind

the x-ray target. Because the x-ray beam is heterogeneous a flattening filter is used to

influence the energy uniformity of the beam before it passes through a second set of

moveable collimators. The primary collimators are stationary, but the secondary collimators

are made of lead or tungsten pieces, which allow for treatment field size variation through

adjustment of each piece.2

Linear accelerators used for isocentric treatments have a Gantry that allows for a full

360° rotation around the system’s isocenter, the point where the axis of rotation of the

secondary collimator perpendicularly intersects the gantry’s rotational axis (Figure 1.1).5

This rotation allows the treatment beam to enter the target medium at a designated angle of

incidence. While the gantry can move rotationally, the radius of rotation cannot be altered.

3

Figure 1.1. Schematic of a linear accelerator. Source: E. B. Podgorsak and J. H. Hendry, Radiation Oncology Physics: A Handbook for Teachers and Students (International Atomic Energy Association, Vienna, 2005).

The distance between the beam source and the target surface can be controlled

though, rotational, vertical and horizontal movement of the treatment table.

In air or vacuum mediums, the intensity of a photon beam can be described through

the Inverse Square Law; where intensity is inversely proportional to the squared distance

between a point and the source location.5 As photons pass through a medium they are

subjected to interactions with matter through absorption and scattering interactions, primarily

consisting of photoelectric absorption, Compton scattering, pair production, as well as

attenuation and intensity loss governed by the Inverse Square Law.6 Scattering interactions

are dependent on the density of electrons in the incident medium, which can be described

through atomic number of the medium.

Photoelectric effects occur when low energy photons are completely absorbed by an

atom and results in the emission of a high-energy photoelectron from the tightest bound

electron shell. Photoelectric absorption is most probable for low energy photons passing

through a material of high atomic number. Compton scattering involves the interaction of a

high-energy incident photon with a loosely bound electron, through which energy is

transferred from the incident photon to the stationary electron before both the photon and

electron are scattered into the medium. The probability of Compton interactions increases

4

linearly with the atomic number of a material. Pair production is strongly dependent on the

energy of the incident photon. For photons with energy greater than 1.02MeV, it is possible

for the photon to interact with the electromagnetic field of an atom’s nucleus, producing an

electron and positron pair. The probability of this interaction increases with the square of a

material’s atomic number.2,5,6

All of the primary interactions photons with matter cause secondary charged particles

to be released in the medium. Photons are thus considered to be indirectly ionizing radiation,

such that as high-energy photons pass into a medium they cause the medium to release

ionizing particles, electrons, which subsequently interact with and transfer energy to

surrounding particles.2 In the case of megavoltage external photon beam radiotherapy, the

energy of incident photons on the patient’s surface is on the order of 4 to 25MV.5 When the

relatively low effective atomic number of tissue ~7.5, is also taken into consideration, the

most probable interaction of incident photons is Compton scattering.6 As such, the transfer of

energy through the medium follows the following steps after the photon is incident on the

patient surface. For external photon beams on tissue, the megavoltage photon enters the skin

and continues to travel through the tissue until it strikes a stationary, loosely bound electron

in a tissue cell. The photon transfers a portion of its energy to the electron and is then

scattered at an angle relative to the photon’s original direction. At megavoltage energies this

scattering angle is relatively small and the photon still moves in a forward direction.7 At the

same time the electron is released to move through surrounding tissue.

Tissue cells are made primarily of water molecules, H2O. As electrons are freed

through Compton scattering, free radicals (molecules with an unpaired electron) are produced

in the tissue cell, which can damage DNA structure. In tissue, the produced Hydroxyl radical

(OH) can be highly damaging to DNA structure.8 The unpaired valence electron of the free

radical makes it highly reactive resulting in a short lifetime and an inability to travel far in

the tissue before reacting with another molecule. Still free radicals in tissue cells break apart

chemical bonds that alter DNA and cause biological effects.5 Unfortunately, this procedure is

not unique to target cells.

According to the Law of Bergonié and Tribondeau, cells that are more rapidly

reproducing and dividing are more sensitive to ionizing radiation than cells that reproduce at

slower rates.5 External Photon Beam Radiotherapy is generally used for deep tumors;

5

consequently the treatment beam passes through several layers of normal tissue cells before

reaching the target volume of tissues below the skin’s surface. Of the normal cells exposed to

radiation, by the definition of the law of Bergonié and Tribondeau, some of the most

radiosensitive cells include germinal epithelium, sebaceous glands and hair follicles. These

cells are all held within the epidermis skin layer, which undergoes a complete renewal and

replacement every 2 weeks.9

Radiation effects are classified in two types, stochastic and deterministic effects. A

stochastic effect is an effect that is dependent on a probability of occurrence, which is

increases with dose, but the effect does not always appear at a certain exposure, and there is

no minimum exposure needed for manifestation. The probability of a stochastic reaction is

expressed by organ weighting factors, given in the ICRP publication 60. Skin, for example,

has a weighting factor of 0.01 relative to the probability of stochastic effects being produced

with during total body irradiation.9 This corresponds to a low probability of ionizing

radiation causing any stochastic effects in skin, rather than another organ of the body.

On the other hand, deterministic effects have a high threshold dose, which must be

reached before they appear. Instead of the probability of deterministic effects increasing with

dose, the severity of the deterministic effect increases with dose beyond the threshold value.

The high threshold dose of deterministic effects results in cell death in exposed tissue

through interruptions of cell function and in structure.2

The surface skin layer is one of the first places to exhibit deterministic signs of

radiation exposure or damage. Before the Roentgen was instituted as the standard unit of

measure for Exposure, therapists used to use a unit called the Skin Erythema Dose.2 This unit

was defined as the amount of radiation exposure required to induce a reddening of the skin’s

surface within a given time period. This unit, however, was dropped due to the fact that

reactions of cells to radiation exposure are dependent on several different factors. Cell

response to radiation exposure is dependent on the amount of radiation, rate of exposure,

extent of exposure, and the quality of the beam used for exposure.9

Skin reactions were discovered through the first reported adverse skin radiation effect

in 1901, when Henri Becquerel suffered a skin burn after holding vial of radium in his

pocket.5 With the advent of radiation safety and protection, the threshold dose levels for

different skin reactions have been studied for various severities of skin reaction. The

6

minimum exposure for a reddening of the skin surface, erythema, is around 1Gy within a

relatively short time.9 For short exposures between 2 and 6 Gy, erythema can appear on

exposed skin within 1 to 2 days of exposure. If the exposure is over 3 Gy the erythema can be

accompanied by a temporary loss of hair over the small exposed area. These symptoms will

generally heal within about 4 to 8 weeks, during which the Erythema reappears for several

weeks, with the worst manifestation occurring the 3rd week after exposure.

Higher exposures result in a faster and more severe manifestation of erythema on the

exposed area. If the exposure over an approximate 4 week period, reaches a high enough

total dose between 20 to 50Gy, the skin reactions can include, but are not limited to,

permanent hair loss, acute radiation dermatitis with moist desquamation (peeling), vascular

damage, permanent darkening of the skin, and extreme erythema over the treatment area.

This amount of exposure produces permanent changes in skin tone and hair growth, however

the other symptoms can heal within 2 to 3 months of the first exposure. Depending on the

extent of the tissue damage caused by these large exposures, the skin can also become

atrophic, highly susceptible to damage and develop lesions or infections at the exposure site.9

1.2 TREATMENT BEAM CHARACTERISTICS Tumors can develop in any location and at any size within the body, which means

some tumors cannot be accessed or treated without passing radiation through surrounding

normal tissues. Because normal tissue cells, including those located in the skin surface, are

radiosensitive, effort has to be made to spare normal tissue as much as possible to reduce the

occurrence of skin reactions. The field of photon radiotherapy has changed to reflect this

need since the first introduction of linear accelerators as clinical delivery systems for external

photon beams in 1956.10

The earliest clinical linear accelerators were only capable of generating single photon

beam energies, with limited range of rotation (approximately 120°) of the source point

around the system’s isocenter.10 Modern linear accelerators are capable of producing multiple

photon beams of different beam energy, in addition to several electron beams with full gantry

rotation across 360 degrees. Beam field shaping methods have developed since the first

installation of a primary collimator in the 1960s, to the use of hand-positioned blocks and

wedges for normal tissue sparing in the 1970s, to the development and implementation of

7

multi-leaf collimators in the 1990s.10 Currently, external photon beams produced with

modern linear accelerators can be shaped and controlled for case-by-case treatment planning.

Some of the controllable aspects of treatment beams include beam energy, beam field size

and the angle of incidence between the treatment beam and target surface.

One of the most important factors in external photon beam radiotherapy is the energy

of the treatment beam used. As the energy of the incident photon increases the depth at which

maximum dose is delivered also increases. Consequently megavoltage beams provide an

advantage over lower energy beams such as orthovoltage (150-500kV), and supervoltage

(500-1000kV) beams, because they produce a lower deposited dose in surface and superficial

tissues.2 This is called skin sparing and is important when treating deep-seated tumors, such

as those found in the abdomen because the target volume may be defined several centimeters

below the skin surface and a lower deposited dose reduces the risk of deterministic effects in

normal tissue.

1.3 DOSE DISTRIBUTION Recall that dose is defined by the amount of energy deposited per unit mass. For

photon beams, the distribution of dose changes between the target surface, within the buildup

region, and declines after the depth of maximum dose deliver, dmax. As previously described,

surface dose is inversely related to the energy of incident photons on the medium. Because

photon beams are indirectly ionizing, surface dose is actually delivered due to contamination

of the incident photon beam by secondary electrons.2 These secondary electrons are produced

by backscatter in the target medium and interactions between the photon beam with

collimators, air, and any other material in the beam path.5 The surface dose therefore

increases with field size and source to skin distance, because the treatment beam has more

opportunities to interact before reaching the target surface. The higher the contamination of

the beam, the more electrons are present to transfer energy to surface cells, increasing the

delivered surface dose.

As previously discussed, as photon beams travel through a medium it causes the

release of ionizing particles. For megavoltage beams in a clinical setting, these ionizing

particles are recoil electrons produced by Compton interactions. As the beam passes through

the medium, the intensity of the beam decreases due to the scattering of photons and lost

8

energy given to released recoil electrons.5 Fewer high-energy photons at deeper depths

means the number of electrons released in the medium decreases with increasing depth. Since

dose is delivered by the electrons depositing energy at the end of their range in a medium, the

dose delivered through the medium reaches a maximum value at a depth dmax.2 This depth is

determined by the range of the electrons released near the surface and superficial layers of

the medium, since the highest numbers of electrons released are from interactions close to the

target surface.

The distance between the target surface and the depth at which maximum dose is

delivered is known as the build up region. Within this region the delivered dose increases

until reaching the maximum delivered dose at dmax.5 Beyond the build up region the delivered

dose decreases as depth increases. The distribution of dose through a medium can be

qualified using a Percent Depth Dose (PDD) curve or a Tissue Maximum Ratio (TMR). PDD

is defined as the ratio of delivered dose at a depth to the delivered dose at a reference depth.

Similarly the TMR is defined as the ratio of dose delivered at a given point in a phantom to

the dose at the same point for the reference depth of maximum delivered dose.5 As depth

increases both PDD and TMR values increase before reaching a maximum value at the given

reference depth, then decrease beyond the build up region. As beam energy increases both

PDD and TMR increase comparatively for depths beyond dmax, between energies due to

increased penetration of higher energy photon beams. The PDD and TMR values also

increase with an increased field size due to increased electron contamination in the beam

caused by scatter and interactions before intersecting the target.5 The difference between

these quantities lies in the method by which measurements are made.

Both PDD and TMR measurements can be made using an ionization chamber inside a

water phantom, and are mainly taken using small field sizes to reduce the effects of electron

contamination of the photon beam. PDD measurements are made using the same source to

surface distance for each depth measurement, that is the distance between the source and the

surface of the phantom remains constant while the detector moves within the phantom. TMR

measurements are made by placing the detector at isocenter, using a constant source to axis

of gantry rotation distance, but the source to surface distance decreases as depth increases.5

Through inverse square law, PDD values are dependent on the source to surface distance,

9

while source to surface distance has very little influence on TMR measurements because the

distance between the radiation source and the detector is unchanged.

1.4 CURRENT TREATMENT MODALITIES The development of linear accelerators with more control over the use of multiple

beams, field sizes, beam energies and gantry rotation, has led to the development of

rotational radiation therapy techniques using optimized treatment plans. The optimal

treatment plan for a treatment would deliver a lethal amount of dose to all the abnormal,

tumor cells without delivering any dose to normal tissues. While such a plan is not possible,

treatment plans can be ‘optimized’ in part through the use of multiple fields, beam modifiers

and proper beam angles, determined using 3D images of the patient.2 Taking advantage of

plan optimization through the use of multiple treatment beams has allowed therapists to use

radiation as a non-invasive treatment method for tumors that are in difficult locations in the

body, as defined by the depth of the tumor, or the radio-sensitivity of surrounding organs and

tissues.11

This can be performed through modalities such as Intensity Modulated Radiation

Therapy (IMRT) and Intensity Modulated Arc Therapy (IMAT). Both of these treatment

modalities use dynamic multi-leaf collimators to shape the treatment field specifically to the

desired treatment area for multiple treatment fields, while allowing for gantry rotation during

patient treatment. X-ray beams produced by linear accelerators are not necessarily of uniform

fluence without the use of a flattening filter. The basic principle of intensity-modulated

therapy is that multiple non-uniform treatment fields can be combined together to produce a

uniform fluence and subsequent absorbed dose across the target volume.12 Simultaneously,

the combination of non-uniform fluence fields can allow for greater sparing of surrounding

normal tissue, reducing damage done to surrounding tissue. The stricter definition of the

irradiated volume, allows for the use of higher radiation doses for treatment, with less

opportunity for adverse side effects in normal tissues. This is a benefit in tumor treatment

because higher radiation doses are more effective in damaging cells and thus are more

effective as treatment beams.11

In some IMRT treatments multiple fields can be utilized through a stop and shoot

method; the gantry moves to a certain angle position, the multi-leaf collimator shapes the

10

field for the given angle and the beam is initiated for a given exposure. The gantry then

moves to the next prescribed angle and the process repeats until all the prescribed fields have

been treated. IMAT treatments differ in that it stays in constant motion rotating a single, full

360° rotation arc around the patient, with varying rotation speed and exposure rate. During

this arc, the multi-leaf collimator constantly changes the shape of the source aperture in order

to shape the treatment volume to the tumor volume.13,11 Both IMRT and IMAT techniques

share the benefit of reduced treatment time, which limits the opportunity for patient or tumor

movement during treatment. Reduced treatment time also allows for more patients to under

go treatment on the same day.14

Although advances in radiation treatment techniques and technology have been

effective in the treatment of tumors, the advent of new treatment techniques brings new

challenges and at times a need for reevaluation of pre-existing treatment components. One

such evaluation has arisen concerning the treatment couches used for external photon beam

radiation treatments on linear accelerators. As the logistics of radiation delivery have shifted

towards the use of more fields, treatment beams start to pass through the treatment couches.

Up until the use of multiple treatment fields and beams, the treatment couches may have been

considered negligible in treatment planning, as they previously had little interference with the

delivered radiation beam.

1.5 COMPOSITION AND DESIGN OF TYPICAL TREATMENT COUCH

The interaction of a photon beam with a treatment couch is dependent on the same

variables prevalent during the photon beam’s interaction with skin tissue; field size, beam

energy, source to surface distance, and beam angle. The penetration of the beam depends on

the energy of the incident photons and the atomic number of the material being traversed

influences the attenuation of coefficient if the absorbing material.6 This is due to the idea that

the attenuation coefficient represents the likelihood of the photon beam running into a

particle with which it will interact. Typical treatment couches installed on clinically used

linear accelerators are composed of multiple layers. One of the most common combinations

of couch materials is the use of carbon fiber layers around an inner foam section. Carbon has

an atomic number of 6, which reduces the probability of pair production and photoelectric

11

absorption interactions occurring with megavoltage treatment photon beams. This reduces

the amount of attenuation of the treatment beams before target medium interactions.7

Under current federal regulations, treatment couches and linear accelerators used for

therapeutic radiation exposure fall under the classification of Class II devices and are subject

to regulation by the Center for Devices and Radiological Health (CDRH), held within the

United States Food &Drug Administration.7 Under this designation, treatment couches are

evaluated for motion and weight support characteristics. For example, during couch rotation,

the location of the system isocenter on the couch surface must be within 2mm for any degree

of rotation.2 As of yet regulations have not been published for attenuation limits of treatment

couches used in multi-beam treatments. This presents a possible problem if the attenuation

characteristics of a treatment couch are not accurately modeled in the treatment planning

system, the effect of the couch on delivered dose could exceed the ICRU suggested 5%

margin for prescribed to delivered dose.

1.6 PREVIOUS STUDIES CONCERNING COUCH ATTENUATION

As the number of fields used for patient treatment increases, the effect of treatment

couches becomes more significant.15 The investigation of the impact of treatment couches on

radiation delivery has been conducted from a vast array of perspectives and with focus on

several different treatment couches. The effect of treatment couches on skin sparing effects is

one of the most studied characteristics. An investigation of 8 different couch and insert

models, Seppala et al. found that megavoltage skin sparing benefits were reduced.16 A

photon treatment beam intersecting a iBEAM evo carbon fiber couch (Medical Intelligence,

Schwabmunchen, Germany) experienced an increased skin dose percentage of Dmax, from

17.9% to 92% of Dmax, using a 10x10cm, 6MV beam. A skin dose percentage of 92% of

maximum dose is almost a complete removal of any skin sparing characteristics of the

beam.17 This change in measured surface dose is tied to decreases in the depth at which

maximum dose is delivered. Gerig, et al. proposed that the shift in depth of maximum dose

delivery can be treated as a ‘radiological thickness’ for the treatment couch.18

Mihaylov, et al. evaluated a method for adding treatment couch structures into

treatment planning systems by importing CT scans of treatment couch and tracing contours

of each couch component. These traced contours can then be assigned densities or

12

Hounsfield units based on composition materials and added to patient images used for

planning.19 Using this method, Mihaylov, et al. added an ExacTrac couch into the Pinnacle

treatment planning system (Phillips Medical Systems, Fitchburg, WI) with resultant dose

calculations within 2% of delivered values.19

Similar couch tracing techniques have been used by Munjal, et al. for a Med-tec

carbon fiber couch during IMRT planning and Zhihui, et al. for accurate modeling of a

Siemens ZXT couch using the Pinnacle3 8.0 treatment planning system.20,21 Also, Wagner

and Vorwerk performed a study on the accuracy of the assigned Hounsfield unit values

present in the Varian Eclipse treatment planning system for the Varian EXACT couch. This

study showed that while the Eclipse software uses default Hounsfield numbers as -300HU for

the carbon layers, -1000HU for the internal foam, and 200 HU for the rails, better dose

predictions can be made by adjusting these values.22

The Varian EXACT couch is a frequently used treatment couch that has been

investigated for attenuation by several researchers. In 2003, Veira et al. found attenuation of

up to 15% for oblique beams passing through the EXACT couch.23 Furthermore according to

research done by Li, Lee, Johnson, Zhu and Kudchadker, this attenuation value can be as

high as 26.8% during multi-field treatments using 5x5cm2, 6MV beams passing through the

support rails and couch top.24 Similarly, Proojen, et al. published an article stating the

EXACT couch produced a 3% lower delivered dose through the planning target volume.

Their study also used Mihaylov-like techniques to predict dose within 2% accuracy. The

study generated a maximum attenuation of ~17.5%, for a 6MV beam passing through with

rails pushed to the side for an n approximate 237°±1° gantry angle.25

Measurements made on the Varian IGRT couch by Munjal, et al. showed a maximum

attenuation of 4.8% through the thickest section of the IGRT couch.21 The surface dose using

a 6MV beam for an IGRT couch was estimated to change from ~16% to ~90% of the

maximum delivered dose.26 The interference of treatment couches has been to shown to alter

delivered dose in clinical settings. As such, the four couch inserts investigated in this study

are expected to show attenuation of the treatment beam prior to entering the phantom.

13

CHAPTER 2

OVERVIEW OF STUDY

This study took place at University of California, San Diego Moores Cancer Center

facilities in La Jolla and Encinitas, California. Data was collected using three linear

accelerators, a Varian Truebeam 23iX Clinac accelerator and two Varian Trilogy medical

accelerators. In this investigation we evaluated the attenuation characteristics of 4 different

treatment couches used in multi-field dose treatments. These treatment couches are the

Varian EXACT couch, the Varian IGRT couch, the Varian tennis racket insert, the CIVCO

Universal treatment couch extension and a CIVCO Sample treatment couch extension for a

new construction of the original universal model. This evaluation was performed through

observation of direct attenuation measurements made using a tissue equivalent phantom, and

through determination of the effect of beam-couch interference on the surface dose of the

target medium. Direct attenuation measurements were made for two different beam energies,

using three field sizes in order to observe how energy and field size affect the severity of

couch attenuation effects. The accuracy of inclusion of treatment couches during treatment

planning will be evaluated through comparison of directly measured values with those

predicted by the treatment planning system.

Based on past research and studies, the attenuation of treatment couches is expected

to be most significant under relatively low treatment beam energies, using a small field sizes,

at oblique beam angles. One of the differences in this investigation is that although the

Varian tennis racket insert is considered, and modeled to be free of attenuation properties, the

tennis racket is expected to actually contribute to beam attenuation. Furthermore the study of

the CIVCO Sample extension has not been completed before, and it is expected that the

thinner overall design of the new construction, compared to the original Universal extension,

will allow for less beam attenuation.

14

2.1 MATERIALS AND METHODS This study consisted of two types of measurements, direct attenuation measurements

and percent depth dose measurements.

2.1.1 Direct Attenuation Measurements: General Setup

Direct measurements of couch attenuation were made using a 0.66c farmer-type ion

chamber isocentrically placed in the center of a homogeneous cylindrical acrylic CT dose

phantom (Fluke Biomedical, USA). The positioning of the ionization chamber at the

system’s isocenter results is a constant source to detector distance as the gantry is rotated

around the phantom (see Figure 2.1). Conveniently, the cylindrical phantom resulted in a

constant detector depth in the phantom across all gantry angles. The phantom was positioned

using triangular cushions along the central axis of the gantry as defined by system laser

guides (see Figure 2.2). This set up was used to make initial dose measurements at the

machine’s isocenter given an angle of incidence of 0 degrees. This measurement corresponds

to the dose collected at the isocenter by a beam not passing through a treatment couch.

Measurements were also taken from an angle of 180 degrees such that the beam passes

perpendicular through the treatment couch before reaching the system’s isocenter.

Figure 2.1. Gantry Rotation around isocenter.

15

Further measurements were taken at varying degree intervals between the 180

incident beam to beams reaching one side of the couch, for the purpose of monitoring the

Figure 2.2. Experimental set-up for direct attenuation measurements.

change in attenuation for given beam paths through the treatment couch. These

measurements represent the attenuation distributions across the couch from the center to one

side of the treatment couch during isocentric treatments. Comparing these measurements

with the dose collected without passing through the treatment couch produced the percentage

by which the beam was attenuated by the treatment couch using the equation:

%Attenuation =100 * (1 −Dtc

Dnc

)

Where Dtc represents the dose measured with the beam passing through the treatment

couch and Dnc represents the dose measured while the beam did not intersect the treatment

couch, and was always treated as the dose measured with the machine set for a 0 degree

angle of incidence. Each of these measurements were made at two different beam energies

of 6MV and 15MV, and for 3 different field sizes; 5x5cm2, 10x10cm2 and 20x20cm2.

16

2.1.2 Varian EXACT Couch Two different EXACT treatment couches were employed during the attenuation

measurements, both in use on Varian Trilogy treatment delivery systems. Initially attenuation

measurements were taken with regards to measurements made using a tennis racket support,

however it was later found that the tennis racket itself provided a small amount of attenuation

and in some cases increased scatter, and therefore dose. The EXACT couch measurements

were later corrected by subtracting direct attenuation measurements taken implementing the

tennis racket support from the attenuation measurements taken using the EXACT couch.

The EXACT couch is designed using a 2.5cm couch top, with two 8.5cm moveable

rails on the underside of the couch. The couch top is divided into three layers, a 1mm carbon

top carbon layer, a 2.5mm carbon bottom layer and an internal foam layer. The support

system is designed such that the rails can be positioned on the exterior sides or together in the

center of the couch.22 This rail movement reduces the necessity for patient repositioning

during treatment, but it makes it necessary to measure the attenuation of the couch in both

rails position setup cases for each energy and all three different field sizes within each

energy.27

2.1.3 Varian IGRT Couch The IGRT couch was installed on a Varian Truebeam treatment system and is

composed mostly of carbon fibers, and no metal components allowing for its use in

imaging.28 The IGRT couch increases in thickness moving longitudinally from the section of

the couch used for head treatments towards the section used for abdomen treatments. As such

the couch was subdivided such that three sections of the couch were subjected to attenuation

measurements at the region of average thickness, resulting in average attenuation

measurements for each of the three thickness sections. Direct attenuation measurements were

taken for each region of thickness for each energy and field size combination using the

general attenuation measurement set up discussed previously.

2.1.4 CIVCO Couch Extensions The CIVCO couch consists of a series of inserts, which can be changed depending on

the needs of each treatment. In this study two of the inserts were measured for attenuation

characteristics. The first of these inserts is a current model known as the CIVCO Universal

17

treatment extension that employs a thick couch top, about 5cm, with approximately 1cm

thick side wings, but has no supporting rails. The second insert is a new construction given to

the University of California, San Diego Moores Cancer Center as a beta sample. This insert,

called the CIVCO Sample extension from this point forward, employs a small, stationary

support rail, but the table thickness is much thinner that that of the currently used Universal

insert. Each construction was installed on a Varian Trilogy treatment system for

measurements, once again using the general attenuation measurement setup.

2.1.5 Tennis Racket Support A Tennis Racket insert consists of a thin carbon fiber grid that is considered air

equivalent during dose delivery.16 However it was noticed during measurements of the

EXACT couch that the Tennis Racket had an impact, although small, on the measured dose

of the system. As such, the attenuation of the tennis racket was directly measured using the

same general attenuation measurement setup as the other couch sets.

2.1.6 Tissue Maximum Ratio Measurement of EXACT Couch with Rails Out

For the EXACT couch, a TMR curve was created using measurements taken directly

through the use of a solid water phantom and the farmer-type ion chamber. This was done by

placing solid water directly onto the tennis racket insert, with the farmer chamber placed

inside the solid water phantom at isocenter and the EXACT couch insert, with rails out,

placed on top of the solid water phantom (Figure 2.3). Solid water was added to the top of the

phantom and the couch replaced on top of the phantom such that the farmer chamber

remained at isocenter but was submerged at increasing depths of solid water. These

measurements were compared to the measured dose at the depth of maximum dose for the

given beam energy. These measurements were repeated for both a 6MV and 15MV beam,

under the same three different field size scenarios used for the attenuation measurements and

without the EXACT couch insert between the phantom and the beam for comparison.

2.1.7 Percent Depth Dose Measurements of IGRT and CIVCO couches

EBT2 Gafchromic film (International Specialty Products, NJ) was used to produce a

Percent Depth Dose for the largest thickness of the IGRT couch using a 10x10 field size, at

18

Figure 2.3. Measurement set-up for TMR measurement on EXACT couch.

beam energy of 6MV. Of the energies and field sizes being investigated in this study, this

combination was determined to exhibit the highest percentage of attenuation. Placing the

Gafchromic film between slabs of solid water and placing the solid water such that the edge

of the film was against the couch surface produced the Percent Depth Dose curve after the

film was exposed using a beam from directly below the table. A second percent depth curve

was created for treatments not through the treatment couch by placing a piece of film

between slabs of solid water such that the top edge of the film was even with the central axis

of the treatment delivery system. This film was then exposed using the same 10x10cm2 field

size and 6MV photon beam, from above such that the beam did not pass through the

treatment couch.

Both constructions of the CIVCO couch were not directly measured to produce a

percent depth dose curve, but instead were modeled in the Eclipse planning software.

2.2 ECLIPSE MODELING The Moores Cancer Center utilizes Varian Eclipse treatment planning software

(Varian Medical Systems, Palo Alto, CA) for patient treatment and research. As such, the

accuracy of the Eclipse planning software was evaluated by modeling all of the direct

measurements made in this investigation for comparison, including the attenuation

19

measurements and TMR/PDD measurements. Both the Varian EXACT couch and Varian

IGRT couch have model structures that already exist in the Varian treatment planning

software. As such, the couches were included in the treatment planning by simply including

the structures in the initial plan and evaluating the Eclipse Software in its accuracy in

compensating for the added structures.

The CIVCO couch inserts were not as easily introduced in the treatment planning

software because they are not a Varian product and thus do not have pre-existing models in

the Eclipse software. In order to include the CIVCO couches in the planning system, each

insert was CT scanned and the images uploaded into the Eclipse program. From these CT

images each structure of the couch was traced and assigned a Hounsfield number based on

the value measured during the CT scan. The structure set was then copied into a treatment

plan including a model of the acrylic phantom, and dose calculations were made using the

new plan including the copied couch structure set (Figure 2.4).

Although the Eclipse software has a pre-existing structure set for the use of a tennis

racket, the pre-existing model is based on the idea that the tennis racket should have no

affect, be it attenuation or scatter contribution for the dose measurement. Therefore the tennis

racket insert was also CT scanned, uploaded to the Eclipse system, and was traced and

modeled for comparison with the pre-existing model and the direct attenuation measurements

made on the Trilogy treatment delivery system.

2.3 ERROR CONSIDERATIONS Uncertainty in the dose measurements within a phantom is dependent on the accuracy

of the ionization measurement and the positioning of the chamber inside the phantom. It was

difficult to judge the center of the ion chamber once placed inside the acrylic phantom. If our

setup was not accurately centered on the gantry’s isocenter our measurements may have been

lower than a clinical situation because as gantry angle increased the distance form the source

to the detector would increase, causing the intensity to decrease by the inverse square law. Or

if off center in the direction of gantry angle rotation, the dose measurements would be higher

based on the distance between the source and detector decreasing, causing the intensity to

increase. This would cause the intensity at the detector under an oblique angle setup to be

20

Figure 2.4. (a) CT scan slice of the CIVCO Universal couch extension, (b) Contour traced couch structures from CT scan slice, (c) Resultant couch structure set produced in the Eclipse treatment planning system, after couch contouring and Hounsfield unit assignment of each couch structure.

a

b

c

21

higher than at the comparison measurement, which would result in a lower calculated

attenuation. Subsequently if the chamber was off set away from the direction of rotation, the

intensity at the chamber would be lower at the oblique angle than at the degree comparison

measurement resulting in a higher calculation for attenuation.

Inherent in the measurement set up of the acrylic phantom and the Farmer type ion

chamber there is a small gap of air between the acrylic and the ion chamber. The radius of

the detector chamber was slightly smaller than the radius of the hole at the center of the

acrylic phantom. Medical tape had to be wrapped around the arm of the ionization chamber

to secure the chamber in the center of the phantom. Air increases scatter, so our

measurements may be higher than what may actually occur in a patient. However, since this

gap was present across all the measurements taken for each field size and energy pairing, we

can ignore this increase for evaluation of attenuation. That is, the scatter added by the air

surrounding the chamber for a measurement using the 6MV beam in a 5x5 field size should

be independent of the gantry angle and thus be negligible in comparison of two

measurements made using the same energy and field size.

Similarly if during the direct attenuation measurements, the acrylic was sitting up off

of the table through the use of the triangle cushions used to secure the cylindrical phantom,

then an air gap between the phantom and couch surface will result in increased scatter

contributions to dose. Still when measurements are taken under the same energy and field

size, scatter should be the same and negligible during attenuation calculations. According to

the measurements of Wagner and Vorwerk, calculated doses in the Eclipse planning system

have an uncertainty if 1% and measured values positioned using laser guides can be

estimated to have an uncertainty of 2.2% of the measured dose.22

2.4 RESULTS For all of the couches tested in this study, maximum attenuation was observed under

the smallest field size tested (5x5), using the lower energy of 6MV. Also for each couch and

across all three field sizes, the higher energy treatment beam of 15MV, was affected less by

the introduction of the treatment couch into the beam path, than the lower energy beam.

These measurements were all taken across half of the treatment couch; therefore any graphs

only represent the attenuation spectrum from the lengthwise central axis of the couch to one

22

side of the couch surface. However due to the symmetry present in all the couches studied in

this investigation the attenuation spectrums produced here should be representative of the

spectrum for each side of the couch.

2.4.1 Varian Tennis Racket Insert The straight measurement of the attenuation across the Varian Tennis Racket

produced attenuation values ranging from 0.06% to 0.39%, found in Table 2.1, depending on

beam energy and field size.

Table 2.1. Attenuation Measurements for the Varian Tennis Racket, when Beam is Passing Straight Through the Grid Surface

%Attenuation 6MV

beam

15MV

beam

5x5 field 0.39 0.33

10x10 field 0.19 0.06

20x20 field 0.18 0.22

The structure set corresponding to the use of a tennis racket in the treatment beam

produced no attenuation, and thus no effect on the calculated dose delivered through the

racket.

2.4.2 Varian EXACT Couch Attenuation The EXACT couch was measured to have a maximum attenuation of 19.2% when the

adjustable rails were pushed under the center of the couch. When the rails were in the outer

position, the EXACT couch measured a maximum attenuation of 5.98%. This value was

attained using an oblique treatment beam intersecting the side rail and couch surface. When

the rails were pushed to the center of the treatment couch, the distribution of attenuation

values across the couch surface produced a ‘M’ shaped curve (Figure 2.5), with two peaks of

maximum attenuation across gantry angles at which the beam intersected part of an

adjustable rail and the treatment couch. As energy of the treatment beam was increased, the

attenuation of the beam decreased relative to measurements made at the same gantry angle.

Both energies showed that an increased field size, decreased the effect of beam attenuation

despite gantry angle. Of further note is for situations where the rails are pushed under the

23

Figure 2.5. Attenuation measurements for a 10x10cm2 field, 6MV beam passing through the Varian EXACT couch with the adjustable rails pushed to the center of the couch.

center of the treatment couch, the angle of maximum attenuation differed from the angle of

maximum attenuation predicted by the Eclipse Treatment planning system. Values for the

attenuation measurements can be found in Table 2.2 and Table 2.3. The surface dose

measurements can be found in Table 2.4.

Table 2.2. Attenuation Measurements for 6MV Photon Beam Passing Intersecting the Varian EXACT Couch then the Adjustable Rails are Pushed Under the Center of the Treatment Couch (Rails-In Position) 6MV beam 5x5cm field 10x10cm field 20x20cm field

Gantry

Angle

Measured

%Attenuation

Calculated

%Attenuation

Measured

%Attenuation

Calculated

%Attenuation

Measured

%Attenuation

Calculated

%Attenuation

180 2.35 2.80 2.59 1.45 0.86 0.00

190 19.23 12.52 13.78 10.74 12.02 9.14

195 15.06 11.45 14.01 9.65 12.63 8.69

200 11.86 10.65 11.04 8.69 9.86 7.67

205 12.29 10.52 11.41 9.29 10.29 7.68

210 14.96 10.25 12.38 8.81 11.15 7.45

220 2.41 4.39 4.04 3.02 1.36 2.37

230 2.90 5.19 4.77 3.98 1.99 3.05

Average

Attenuation 10.13 8.47 9.25 6.95 7.52 5.76

24

Table 2.3. Percent Attenuation Measurements for 15MV Photon Beam Passing Intersecting the Varian EXACT Couch when the Adjustable Rails are Pushed Under the Center of the Treatment Couch (Rails-In Position)

15MV beam 5x5cm field 10x10cm field 20x20cm field

Gantry Angle

Measured

%Attenuation

Calculated

%Attenuation

Measured

%Attenuation

Calculated

%Attenuation

Measured

%Attenuation

Calculated

%Attenuation

180 1.24 2.22 0.28 2.27 -0.73 2.56

190 10.16 9.00 9.00 7.94 8.04 7.44

195 8.75 11.19 7.62 9.58 6.78 8.37

200 8.28 7.71 7.15 4.79 6.41 4.62

205 9.03 10.65 7.96 8.97 7.16 8.14

210 8.22 7.36 7.11 6.05 6.34 4.87

220 2.38 3.27 1.47 1.64 0.87 1.28

230 2.90 3.86 2.18 1.51 1.50 0.77

Average

Attenuation: 6.37 6.91 5.34 5.34 4.55 4.76

Table 2.4. Surface Dose Measurements for the Varian EXACT Couch, Taken from the TMR Curve Produced Using the Solid Water Phantom

6MV Beam No Couch in Beam Couch in Beam Difference

5x5 field 44.8 91.6 46.8

10x10 field 49.5 91.8 42.3

20x20 field 58.2 94.5 36.3

15MV

Beam No Couch in Beam Couch in Beam Difference

5x5 field 24.4 70.5 46.1

10x10 field 31 71.4 40.4

20x20 field 42.8 79.8 37

Positioning the rails to the outside of the treatment couch, the attenuation of the couch

insert itself was determined to be 1.64% in a 6MV beam and 1.27% for a 15MV beam. As

the gantry angle increased the attenuation also increased as seen in Figure 2.6. As with the

rails in set up, the maximum attenuation measurement was taken with the beam passing

directly

25

Figure 2.6. Attenuation measurements for a 10x10cm2 field, 6MV beam passing through the Varian EXACT couch with the adjustable rails pushed to the sides of the couch.

through one of the support rails at an oblique angle. For both energies and all three field sizes

the maximum attenuation occurred a gantry angle of 225°. All the measurement values for

the rails out set up can be found in Table 2.5 and Table 2.6.

Table 2.5. Attenuation Measurements for a 6MV Photon Beam Passing Intersecting the Varian EXACT Couch when the Adjustable Rails are Pushed Under the Center of the Treatment Couch (Rails-Out Position)

6MV beam 5x5cm field 10x10cm field 20x20cm field

Gantry Angle

Measured

Attenuation

Calculated

Attenuation

Measured

Attenuation

Calculated

Attenuation

Measured

Attenuation

Calculated

Attenuation

180 1.64 2.40 0.81 2.29 3.21 1.81

190 2.11 2.80 1.61 2.65 3.58 2.37

200 2.85 3.06 2.34 3.26 3.70 2.60

210 3.75 4.26 3.14 3.62 3.83 3.05

220 4.28 4.53 3.88 3.98 3.77 2.94

225 5.59 5.33 4.37 4.22 4.14 3.50

Average Attenuation: 3.37 3.73 2.69 3.34 3.70 2.71

26

Table 2.6. Attenuation Measurements for a 15MV Photon Beam Passing Intersecting the Varian EXACT Couch when the Adjustable Rails are Pushed Under the Center of the Treatment Couch (Rails-Out Position)

15MV beam 5x5cm field 10x10cm field 20x20cm field

Gantry Angle

Measured

Attenuation

Calculated

Attenuation

Measured

Attenuation

Calculated

Attenuation

Measured

Attenuation

Calculated

Attenuation

180 1.28 1.75 0.24 1.53 3.02 1.25

190 1.73 2.22 0.14 1.85 3.02 1.88

200 2.37 2.34 1.24 2.40 3.19 1.98

210 3.02 3.15 1.82 2.62 3.36 2.29

220 3.66 3.39 2.41 2.94 3.19 2.29

225 3.99 3.86 2.82 2.94 3.24 2.50

Average Attenuation: 2.68 2.78 1.44 2.38 3.17 2.03

The EXACT couch also showed a significant change in surface dose from 45% of

Dmax to 92% of Dmax with the introduction of the treatment couch into the 6MV beam for the

5x5cm2 field with the rails under the couch center. An example of the Tissue Maximum

Ratio curve for this couch can be found in Figure 2.7.

Figure 2.7. Tissue maximum ratio curve produced for the Varian EXACT couch using a 6MV, 10x10cm2 treatment beam.

The depth of maximum dose also decreased by 0.5cm across all experimental setups

when the EXACT couch was introduced into the treatment beam. Using the structure set

included in the Varian Eclipse Planning System for the Varian EXACT couch, the maximum

27

difference between the measured attenuation and planning system attenuation calculation is

7%, at the angle of maximum attenuation for the EXACT couch for the 6MV, 5x5cm2 field

with rails pushed in. When the rails were positioned under the sides of the couch, the

calculated values were within a range of 1.21% of the measured values.

2.4.3 IGRT Couch Attenuation As previously stated the Varian IGRT treatment couch was studied in three separate

sections determined by thickness divisions. As the IGRT couch is designed without the use

of support rails, the direct attenuation measurements increased as the gantry angle and

subsequent angle of incidence increased, reaching a maximum value at an oblique angle

between 240° and 250°, when the beam traversed through the side of the couch, depending

on field size. The IGRT couch produced a maximum attenuation of 7.22% with the treatment

beam passing through the thickest section of the couch. The maximum attenuation values for

each energy and field size can be found in Tables 2.7 and 2.8. Figure 2.8 shows distribution

of the attenuation measurements as the gantry angle increased.

Table 2.7. Attenuation Measurements for a 6MV Photon Beam Passing Intersecting the Varian IGRT Couch for Thickness Divisions

6MV Beam

Thinnest Section

Angle of Maximum

Attenuation

Measured

Attenuation

Calculated

Attenuation

5x5 field 240 6.7 5.47

10x10 field 240 5.93 4.48

20x20 field 240 5.29 3.73

Medium Thickness

Section

Angle of Maximum

Attenuation

Measured

Attenuation

Calculated

Attenuation

5x5 field 240 5.95 5.33

10x10 field 245 5.69 4.24

20x20 field 245 5.05 3.51

Thickest Section

Angle of Maximum

Attenuation

Measured

Attenuation

Calculated

Attenuation

5x5 field 250 7.22 5.87

10x10 field 250 6.88 4.97

20x20 field 250 6.18 4.41

28

Table 2.8. Attenuation Measurements for a 15MV Photon Beam Passing Intersecting the Varian IGRT Couch for Thickness Divisions

15MV Beam

Thinnest Section

Angle of Maximum

Attenuation

Measured

Attenuation

Calculated

Attenuation

5x5 field 240 4.45 3.86

10x10 field 240 3.88 3.17

20x20 field 240 3.48 2.71

Medium Thickness

Section

Angle of Maximum

Attenuation

Measured

Attenuation

Calculated

Attenuation

5x5 field 240 3.84 3.74

10x10 field 245 3.45 3.06

20x20 field 245 3.1 2.71

Thickest Section

Angle of Maximum

Attenuation

Measured

Attenuation

Calculated

Attenuation

5x5 field 250 4.96 4.21

10x10 field 250 4.78 3.5

20x20 field 250 4.34 2.92

Figure 2.8. Measured Attenuation values for the thickest section of the Varian IGRT couch for a 6MV treatment beam, using a 10x10cm2 field size.

29

Figure 2.9. Percent depth dose measurements for a 6MV beam, 10x10cm2 field, using the thickest section of the Varian IGRT couch.

As previously stated a Percent Depth Dose curve was produced for the thickest

section of the IGRT couch, using a 6MV beam with a 10x10 field size (see Figure 2.9). In

such a set up the IGRT couch showed an increase in surface from 68% of Dmax to 97% of

Dmax. The Percent Depth Dose curve also shows a decrease in the depth of maximum

delivered dose. Usually the depth of maximum dose delivery is considered to be 1.5cm for a

clinical 6MV beam, and the Gafchromic film measured dose delivered, without the couch in

the beam, a depth of maximum dose to be 1.45cm. However when the beam passes through

the IGRT treatment couch, the depth of maximum dose delivery was measured as 0.81cm, a

decrease of 0.64cm.

A structure set for each thickness delineated section is pre-existing in the Eclipse

treatment planning system. For each section of the couch, the treatment planning system’s

calculated values were within 2% of the direct measurement made in this study. The largest

separation between the measured and predicted value was 1.94% on the thickest section of

the couch, using a 6MV beam under a 10x10 field size.

2.4.4 CIVCO Couch Attenuation The original Universal CIVCO treatment couch extension was measured to have a

maximum attenuation of 6.47% when a 6MV treatment beam passed a 5x5 field through the

30

couch surface at an angle of 245°. Similar to the Varian IGRT couch, the Universal treatment

couch extension does not have an additional support rail beneath the treatment couch. As

such, the attenuation profile of the Universal couch insert resembles that of the IGRT couch,

with attenuation increasing as the gantry angle increased, increasing the angle of incidence

(Figure 2.10).

Figure 2.10. Attenuation measurements for the CIVCO universal extension using a 6MV treatment beam, with a 10x10cm2 field size.

On the other hand, the CIVCO Sample couch extension has a small immobile support

rail under a thinner couch surface. The resulting attenuation measurements produced a profile

with peak attenuation at a gantry angle of 245° for all field sizes at both energies. The

Sample extension had a measured maximum attenuation of 7.7% for the 6MV treatment

beam using a 10x10 field size, as seen in Figure 2.11.

It should be noted, however, that although the Sample insert produced a larger

maximum attenuation than the Universal insert, over the area through which the treatment

field did not intersect the Sample insert’s support rail, the Sample CIVCO measured much

smaller and more consistent attenuation, when compared to the Universal insert (Figure

2.12).

The imported CT images of the CIVCO extensions showed measured Hounsfield

units of -995 HU for the internal foam core, and -750HU for the surrounding carbon layers.

31

Figure 2.11. Attenuation measurements for the CIVCO sample extension using a 6MV treatment beam, with a 10x10cm2 field size.

Figure 2.12. Comparison for attenuation measurements between the CIVCO universal and CIVCO sample extensions for a 6MV beam, 10x10cm2 field.

By CT scanning the Universal CIVCO insert and the Sample CIVCO insert, the two CIVCO

products were reasonably modeled in the Varian Eclipse planning system. Once the planning

system had the CT scans uploaded and the Hounsfield units measured using eclipse tools, the

inclusion of the traced structures modeled attenuation values within approximately 2.4% of

32

the measured values for both inserts. The treatment planning system was then used to

produce Percent Depth Dose curves for both inserts under a 6MV beam with a 10x10 field

size, producing the results found in Figure 2.13.

The introduction of the Universal insert changed the surface dose from 52% of Dmax

to 85% of Dmax. The Sample insert changed the surface dose from 55% of Dmax to 68% of

Dmax, without any change in the depth at which maximum dose is delivered.

Figure 2.13. Eclipse modeled percent depth dose curves for the CIVCO universal and CIVCO sample extensions.

33

CHAPTER 3

DISCUSSION

Overall, the attenuation characteristics of the studied treatment couches followed the

expected trends of attenuation dependence. As the angle of incidence increases the beam path

through the treatment couch will also increase allowing the beam to interact for a longer

distance with the couch materials. This causes less of the beam to be transmitted through the

couch (inverse square law) and used for patient treatment. Our results also showed that the

treatment couches have a larger effect on the 6MV treatment beam rather than the 15MV

treatment beam, and greatest effects for smaller field sizes.

Of the studied couches, the EXACT couch was shown to have the highest attenuation,

19.2%, of the treatment beam, specifically during treatments intersecting the adjustable

support rail with a 5x5cm2 treatment field. Our measurement falls within the values

previously published values between 15 and 26.8%. The EXACT couch also produced the

most significant change in surface dose almost doubling the delivered dose to the surface of

the phantom.

As expected the IGRT couch affects the treatment the most at its widest section. This

section would generally be used during treatments conducted on the abdomen and pelvis of a

patient. As such, the interference of this section of the couch must be fully accounted for

because such cases are more likely to need to reach a larger depth of penetration before

delivering maximum dose. However if the interference of the couch into the beam causes the

beam to enter a patient with a higher initial dose, the beam may not have the desired dose

characteristics by the time it interacts with a tumor located deeper within a patient.

The differences between the two CIVCO extensions show a reduction in the overall

attenuation of the Sample extension compared to the Universal extension. The introduction

of the small immobile support rail under the couch top, however, makes it possible for this

newer construction to attenuate the beam more severely than original Universal extension

when treatment techniques utilize an oblique beam intersecting the support rail. The new

construction therefore is an improvement for treatments that would not need to utilize

34

multiple beams or gantry angles but for multi-field treatment techniques the support rail

would need to be avoided for even dose distribution or compensated for during treatment

planning.

During the Eclipse modeling of both CIVCO extensions, some data was missing near

the edge of the couch structures traced from the CT scans. This meant that for the 10x10 and

20x20 field size, the beam passed through sections of the couch, which were missing or

incomplete when taken from the contoured image for the 245° and higher gantry angles. Still,

the modeled delivered dose fell within 2.4% of the measured values. This variation is very

close to the expected difference of 2% predicted when using the techniques as described the

Mihaylov, et al. study.19

Based on the Eclipse modeling of the Percent Depth Dose curves of the CIVCO

Universal and Sample extensions, both couches have far less impact on the delivered surface

dose than the other studied couches. The Sample insert, specifically only increased the

surface dose by 13% of Dmax, which when compared to the 47% of Dmax increase produced

by the Varian EXACT couch, lends itself to the argument that the Sample CIVCO insert goes

a long way towards being less intrusive in treatment delivery.

The Eclipse treatment planning system calculated attenuations within a reasonable

2.4% range for the IGRT couch and CIVCO extensions. The 7% variation for the EXACT

couch planned dose predictions represents a significant difference between the planned

treatment dose and the actual measured dose at the point of reference. This difference has

already surpassed the ICRU suggested tolerance of 5% between delivered and prescribed

dose. Also, the differences in planned and delivered dose for all of the couches fell outside

the 2% tolerance, suggested by the ICRU 42. It is also noteworthy that the point at which the

treatment planning system differed the most from the measured values in every case was the

angle with the highest measured attenuation for each energy and field size combination.

All the treatment couches show a significant change in the surface dose delivered to

patients which could be detrimental for the ability of treatments to avoid external radiation

skin burns. Traditionally, higher energy beams are chosen for their ability to spare skin dose,

while still administering effective dose to target regions below the skin. The attenuation

caused by treatment couches can undermine this reasoning and practice since while a higher

energy beam should spare skin superficial skin tissue, if the couch attenuates the beam such

35

that it is already delivering 80 to 90% of its maximum delivered dose, the skin is no longer

spared a high radiation dose; therein defeating a major use of megavolt energy beams.

Furthermore, it should be noted that for some cases the interference of the treatment

couch in the beam, the depth at which the treatment dose delivered its maximum dose was

also reduced. In a clinical setting this could cause maximum dose to be delivered outside or

off set in the planned target volume, for small treatment volumes. This change in dmax lends

itself to the possibility of inefficient, if not ineffective treatment procedures. If the effect of

the treatment couch is not accounted for in treatment planning, patients may be required to

undergo more radiation in order for the same treatment efficiency produced through the

accounting for treatment couch interference in planning. The principles of ALARA require

that prescribers strive for efficiency and effectiveness without exposing patients to

unreasonable radiation.

Now while the treatment couch may not produce ‘unreasonable’ increases in the skin

dose exposure and severely alter the dose distribution through the treatment area, it is

possible that the couch could cause unnecessary changes in dose delivery. Future research

should be done to evaluate how a redefinition of density and Hounsfield units used in the

Eclipse treatment planning system could compensate for the differences between planned and

delivered dose. Data presented by Wagner and Vorwerk suggests that these changes could

perfect dose planning. At the very least, it should be recognized that multi-field treatment

techniques need to take treatment couch attenuations into consideration during the treatment-

planning period.

3.1 CONCLUSION Treatment of patients through a treatment couch or even a section of a treatment

couch can alter the dose received by that patient in a clinical setting. The largest effect the

treatment couch may occur in a clinical setting, may take the form of the increase in surface

dose when the beam intersects the treatment couch surface. In cases such as the Varian

EXACT couch this can approximately double the surface dose, relative to the maximum

delivered dose, on the skin of the patient and increase the risk of external negative reactions.

This interference by the treatment couch is most prevalent for lower treatment energies, when

using smaller field sizes.

36

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