Beam Directed Radiotherapy - methods and principles

Beam Directed Radiothe

rapy – Principle

s and practice

Moderator: Dr F D Patel , Department of Radiotherapy, PGIMER

Definition

Exact Calculations Beam Directing devices

Advance Planning

Beam directed radiotherapyBeam directed radiotherapy


Need for Beam directionHomogenousHomogenous Tumor Dose LowLow normal tissue dose

Best therapeutic ratio


StepsPositioning

Immobilization

Localization

Field SelectionDose distribution

Calculations

Verification

Execution


Positioning• Patient positioning is the most vital

and often the most NEGLECTEDNEGLECTED part of beam direction:

• Good patient position is ALWAYS:– Stable.– Comfortable.– Minimizes movements.– Reproducible.


Examples


Standard Positions

• MC used body position.• Also most comfortable.• Best and quickest for

setup.• Minimizes errors due to

miscommunication.

• Best for treating posterior structures like spine

• In some obese patients setup improved as the back is flat and less mobile.

Supine

Prone


Positioning aids• Help to maintain patients in non

standard positions.• These positions necessary to

maximize therapeutic ratio.• Accessories allow manipulation of

the non rigid human body to allow a comfortable, reproducible and stable position.


Positioning aids…

Pituitary Board

Prone Support

3 way support


Breast Boards• Disadvantages:

– Possibility of skin reactions in the infra mammary folds

– Access to CT scanners hampered

• Solutions:– Thermoplastic

brassieres.– Breast rings.– Prone treatment

support.

• Allow comfortable arm up support ► brings arms out of the way of lateral beams.

• Positions patient so that the breast / sternum is horizontal ► avoiding angulation of the collimator.

• Pulls breast down into a better position by the pull of gravity.


Breast boards…

Modern Breast Board

Indexed Arm supports

Indexed wrist support

Head rest

Carbon fiber tilt board

Wedge to prevent sliding


Arm Support

• Also known as the T bar.

• Allows the arm to be positioned laterally when treating the thorax using lateral beams.


Belly boards & leg immobilizer`


Mould making


Mould making : Contd..


Mould making : Contd..


Thermoplastics• Thermoplastics are

long polymers with few cross links.

• They also possess a “plastic memory” - tendency to revert to normal flat shape when reheated


Thermoplastics : Principle


Foam systems• Made of polyurethane• Advantages:

– Ability to cut treatment portals into foam.

– Mark treatment fields on the foam.

– Rigid and holds shape.

• Disadvantages:– Chance of spillage– Environmental hazard

during disposal


Vacuum bags

• Consist of polystyrene beads that are locked in position with vacuum.

• Can be reused.• However like former immobilization not perfect.


Bite Blocks• A simple yet elegant

design to immobilize the head.

• A dental impression mouthpiece used.

• The impression is attached to the base plate and is indexed.

• Head position recorded with 3 numbers.


SRS devices• Sterotactic frames.• Gill Thomas

Cosman System.• TALON® Systems –

NOMOS corp.


Localization• The target volume and critical normal

tissues are delineated with respect with respect toto patient’s external surface contour.

• What to localize?– Tumor– Organ

• Methods?– Clinical examination– Imaging


Why Localize?• Irradiate the tumor and spare the

normal tissue.• Allow calculations and beam

balancing.• Define radiation portals.• Use the beam directing devices.


Clinical localization• Advantages:

– Available everywhere. – Cheapest and quickest(?).– Needs little additional equipment.

• Disadvantages:– Error prone in the wrong hands.– Accessible areas required.– Volumetric data not easily obtained.

• Clinical localization is mandatory despite advanced imaging – need to know what to image!


Imaging Localization• Imaging:

– X-rays:• Plain • Contrast Studies

– CT scans– MRI scans – USG scans– PET scan– Fusion imaging

• Type of study selected depends on:– Precision desired.– Cost considerations– Time considerations– Labour

considerations


X rays• The most common and

cheapest modality available.

• However 2-D data acquired only.

• Orthogonal films can be used with appropriate contrast enhancement for localization in 3 dimensions.


Estimation of depth• From data gained by localization

studies:– CT / MRI – Accurate data– Lateral height method– Tube shift method

• Depth estimation necessary for:– Calculations– Selection of beam energy


Lateral height method

d

d

H1 + H2

2d =

H1H2


Tube shift method• Image shift and tube shift are

interrelated WHEN the tube to target distance remains constant.

• Goal: To obtain a graph of different object heights against the tube shift.

• Serial measurements of image shift measured (for same tube to film distance) while varying the height of the markers above the table.


Tube shift principles

Marker

d2

y

f

S

Tumor

x1

x2

d1


Calculation

d1

f

yyd2

x1

x2

x1

S=

d1

f – d1

S

x2

S=

d2

f – d2

yy = d2 – d1

= fx2 + S

x2 -x1 + S

x1

TumorMarker


CT scans• Provides electron density

data which can be directly used by the TPS.

• Volumetric reconstruction possible.

• Good image resolution - better where bony anatomy is to be evaluated.

• The image is a gray scale representation of the CT numbers – related to the attenuation coefficients.

• Hounsfield units = (μtissue – μwater) x 1000/ (μwater)

253 265 235

125 125 112

56 450 156

135 158 247

269 300 65

36 123 598


CT scan perquisites• Flat table top • Large diameter scan aperture

(≥ 70 cm).• Positioning, leveling and

immobilization done in the treatment position.

• Adequate internal contrast – external landmarks to be delineated too.

• Preferably images to be transferred electronically to preserve electron density data.


MRI scans• Advantages:

– Imaging in multiple planes without formatting.– Greater tissue contrast – essential for proper

target delineation in brain and head and neck– No ionizing radiation involved.

• Disadvantages:– Lower spatial resolution– Longer scan times– Inability to image calcification or bones.


Fusion Imaging• Includes PET – CT

imaging and Fusion MRI.

• Allows “biological modulation” of radiation therapy.

• Technology still in it’s infancy – (?) The future of radiotherapy.


Patient Contouring• Contour is the representation of

external body outline.• Methods:

– Plaster of Paris– Lead wire– Thermoplastic contouring material– Flurographic method– CT/MRI


Contour Plotter


Radiation Field

• Types:– Geometrical: Area DEFINED by the light beam at any

given depth as projected from the point of origin of the beam.

– Physical: Area encompassed by the 50% isodose curve at the isocenter. In LINACs often defined at the 80% isodose.


Single Field• Criteria for

acceptability:1. Dose distribution to

be uniform (±5%)2. Maximum dose to

tissues in beam ≤ 110%.

3. Critical structures don’t receive dose exceeding their normal tolerance.

• Situations used:– Skin tumors– CSI– Supraclavicular

region– Palliative treatments


2 Field techniques• Can be :

– Parallel opposed– Angled

• Perpendicular• Oblique

– Wedged pair

• Advantages:– Simplicity– Reproducibility– Less chance of

geometrical miss– Homogenous dose

• Dose homogeneity depends on:– Patient thickness– Beam energy– Beam “flatness”


Multiple fields• Used to obtain a “conformal” dose

distribution in the modern radiotherapy techniques.

• Disadvantages:– Integral dose increases– Certain beam angles are prohibited due to

proximity of critical structures.– Setup accuracy better with parallel

opposed arrangement.


Dose distribution analysis• Done manually or in the TPS.• Manual distribution gives a hands on

idea of what to expect with dose distributions.

• Inefficient and time consuming.• Pros:

– Cheap– Universally available– Adequate for most clinical situations.


Calculations• Techniques:

– SSD technique (PDD method)– SAD technique– Clarkson’s technique– Computerized


Prescription• Mandatory statements:

– Dose to be delivered.– Number of fractions– Number of fractions per week


SSD technique• PDD is the ratio of

the absorbed dose at any point at depth d to that at a reference depth d0.

• D0 is the position of the peak absorbed dose.

• Dmax is the peak absorbed dose at the central axis.

Total Tumor dose

Number of fieldsx

Number of #s

=T

Incident dose =

T x 100

PDD

Time =ID

Output


SAD Technique• Uses doses normalized at isocenter for

calculation.• In this technique the impact of setup

variations is minimized.• Dose homogeneity is better with the

SAD technique.• Setup is easier but manual planning

not possible / difficult.


SAD calculations

Total Tumor dose

Number of fieldsx

Number of #s

=T

Incident dose =

T x 100

TMR/TARTime =

IDOutput


TAR vs. SSD• TAR = Tissue Air

Ratio• TAR introduced by

Jones for rotation therapy.

• Allows calculation of dose at isocenter WITHOUT correcting for varying SSDs.

• TAR is the ratio of dose at a point in the phantom to the dose in free space at the same point (Dq /D0)

Dq D0


TAR• TAR removes the influence of SSD as it

is a ratio of two doses at the SAME point.

• However like PDD the TAR also varies with:– Energy– Depth– Field Size– Field Shape


Verification• Can be done using:

– Portal Films– Electronic Portal images– Cone Beam CT mounted on treatment

machines (IGRT).• Portal Films:

– Cheapest.– Legal necessity(?)– But have several disadvantages.


Port film disadvantages• Factors leading to poor image

contrast:– High beam energy (> 10 MV)– Large source size ( Cobalt)– Large patient thickness (> 20 cm)

• Slow acquisition times.• Image enhancement not possible.• Storage problems.


Electronic Portal Imaging• Video based EPIDS• Fiber optic systems• Matrix liquid ion

chambers• Solid state detectors• Amorphous Si

technology*


Electronic Portal Imaging


Advantages of EPIDs• Allow real time verification

of patient setup.• Acquisition times short.• Multiple images possible.• Reasonable image quality.• Software assisted image

enhancement.• Online corrections

possible.


Disadvantages of EPIDs• Cost of equipment.• Added service and software update

requirements.• Fragility of the equipment – Si matrix

deteriorates with time and exposure.


Cone Beam CT• Incorporates a

special CT scanner on the LINAC.

• Useful to obtain 3 D real time images of the patient.

• Can use kilovoltage or megavoltage CT

• Allows IGRT.


Beam direction devicesThe main beam direction devices are:

– Collimators– Front pointer / SSD indicator– Back Pointer– Pin and arc– Isocentric mounting– Lasers


Collimators• Collimators provide beams of desired

shape and size.• Types:

– Fixed / Master collimator.– Movable / Treatment collimator.


Fixed Collimators• Protects the patient from bulk of the

radiation.• Dictates the maximum field size for

the machine.• Maximum beam size is when exposure

at periphery is 50% of that of the center.

• In megavoltage radiotherapy beam angle used is 20°.


Master Collimator : Design

20°

• In megavoltage x ray machines beam energy is maximum in forward direction.

20°

• Beam energy is equal in telecurie sources so primary collimators are spherical.


Movable Collimators• Define the required field size and

shape.• Placed below the master collimators

results in trimming of the penumbra.• Types:

– Applicators– Jaws / Movable diaphragms


Applicators: Design

Metal Plate with hole

Lead Sheet

Box

Plastic Cap


Applicators • Advantages:

– Indicate size and shape of beam.

– Distance maintained.– Direction shown.– Plastic ends allow

compression.– Compression allows

immobilization.– Penumbra

minimized.

• Disadvantages:– Useful for low

energy only.– Separate sizes and

shapes required.– Costly.– Shapes may change

due frequent handling.


Jaws• Handling of heavy weight

not required.• Skin sparing effect

retained.• Jaws moved mechanically

– accurately.

Jaw border lies along the line radiating from

focal spot


Jaws: Disadvantages

Disadvantages RemedySize and shape of field remain unknown

Light beam shining through the jaws

Patient to source distance unknown

SSD indicator used.

Compression not possible

A Perspex box may be applied to the head


Front & Back Pointers


Front Pointer/ SSD indicator• Detachable device to measure the SSD

and align the beam axis.• Designed so that it may be swung out

of the beam path during treatment.


Back Pointer• The pointer can be moved in the sleeve.• A nipple is used to allow compression.• The arrow lies along the central ray.


Limitations• Requires skin marks – inherently

unreliable.• Back pointer is unreliable when

compression is desired.• Both front and back points must be

accessible.• Accurate localization of tumor center

is mandatory.


Pin & Arc

Pin

Arc

Bubble


Pin & Arc: Principle


Pin & Arc : Methodd

D

d DD

This is the isocenter


Advantages of Pin & Arc• Allows Isocentric treatment of

– Deep tumors.– Eccentric tumors.

• Can be used with compression e.g. in treating deep seated tumors.

• Can be used for manual verification of Isocentric placement of machines


Isocentric Mounting• First used by Flanders and Newberg of

Hammersmith Hospital for early linear accelerators.

• The axis of rotation of the three structures:– Gantry– Collimator– Couch

coincide at a point known as the Isocenter.


Principle of Isocentric mounting


Why Isocentric Mounting?• Enhances accuracy.• Allows faster setup and is more

accurate than older non isocentrically mounted machines.

• Makes setup transfer easy from the simulator to the treatment machine.


Lasers• LASER = Light Amplification Of

stimulated Emission Of Radiation• Typically 3 pairs are provided with

the machine and intersect at the isocenter.

• Also define:– Beam Entry– Beam Exit


Lasers• Other uses:

– Checking the isocenter– Reproducing the setup on the simulator at

the treatment couch.

• Fallacies:– Accurate setup depends on proper

alignment of the lasers themselves– Lasers known to move frequent

adjustments needed.


How to setup with LASER

2. Align the fiduciary marks with the laser

system

3. Move the couch by to bring the

planned isocenter to the machine

isocenter

1. Note the coordinates of the isocenter

4. Verify the Setup

5. Treat


ConclusionTeam Work Precision

Quality Assurance

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Beam Directed Radiotherapy - methods and principles