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Louise Francis

May Case Study

May 27, 2012

Volumetric Modulated Arc Therapy of Adenocarcinoma of the Prostate

History of Present Illness: WS is a 57 year old African-American inmate who on a routine

physical in 2010, was found to have an abnormal prostate-specific antigen (PSA) level. PSA is a

protein produce by cells of the prostate gland. The PSA test measures the level of PSA in the

blood and along with a digital rectal exam (DRE) has been approved by the U.S. Food and Drug

Administration (FDA) to help detect prostate cancer in men 50 years of age and older.1

Traditionally, most doctors consider a PSA level below 4.0 ng/mL as normal. WS was reported

to have a PSA level of 9.2 ng/mL and was subsequently scheduled to undergo a prostate core

biopsy. The patient underwent an 8-core prostatic biopsy on December 14, 2010 which

demonstrated a Gleason score of 3+3=6 in 5/8 cores with volumetric disease ranging from 10% -

90%. Gleason scores provide an effective measure of prostate cancer based on the appearance of

cancer cells when viewed under a microscope by a pathologist.2 The Gleason score is the

summary of primary and secondary grades with a total score ranging from 2 (1+1) to a 10 (5+5).

A lower score is indicative of less aggressive disease. WS was pathologically staged as having

adenocarcinoma of the prostate stage T1cNxMx.

The patient was further evaluated by a urologist who recommended surgical resection of the

walnut shaped prostate gland, however this option was refused by the patient. WS was then

referred to radiation oncology for treatment consultation. At the time of consultation the patient

stated that he normally urinated 3 times during the night and had normal daytime voiding habits.

He denied hesitancy, dysuria, hematuria, bowel or bladder incontinence, or any changes in his

urine or bowel movements. The radiation oncologist reviewed the patient’s history and explained

the risks and benefits associated with prostate irradiation. WS expressed an understanding of the

risks and benefits of prostate irradiation and elected to proceed with radiation therapy treatments.

Past Medical History: WS has a past medical history significant for peripheral vascular disease

with claudication in the lower extremities. He had a left to right femoral- femoral bypass in 2007,

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recanalization of the right superficial femoral artery and angioplasty of the right superficial

femoral artery. In addition, WS had a right femoral artery occlusion in February 2011.

Additional surgical history includes an appendectomy during childhood and repair of an undated

stab wound. The patient also has a history of hypertension and gastroesophageal reflux disease,

both of which are controlled by medication.

Family/Social History: WS is currently single and incarcerated. He has a 42-pack-year history

of smoking, but quit in 2010 when his correctional facility became smoke-free. The patient has a

history of alcohol and marijuana abuse. There is no known history of cancer in his family.

Medications and Allergies: The patient has no known drug allergies. His current medications

include aspirin, Plavix™ , hydrochlorothiazide, lisinopril, omeprazole, pentoxifylline, simvastatin,

and a multivitamin.

Diagnostic Imaging: WS had an arteriogram done in 2011 that aided in the successful

recanalization of the right lower extremity. During the procedure the patient also has ultrasound

guided punctures of the left carotid femoral artery and femoral-femoral bypass. There were no

diagnostic imaging studies as part of his prostate cancer staging workup.

Radiation Oncologist Recommendations: The radiation oncologist reviewed current findings

with WS and offered a course of high dose conformal external beam radiation therapy. The

radiation oncologist discussed the placement of Calypso markers in the prostate to facilitate

intensity modulated radiation therapy (IMRT) and aid in the daily execution of external beam

treatments. The radiation oncologist proposed initial treatment to the prostate and seminal

vesicles of 2Gy/fraction for 23 fractions to a dose of 46Gy. Additional dose to the prostate and

periprostatic tissue would then be delivered at 2Gy/fraction for 16 fractions to a total dose of

78Gy. WS was in agreement with the radiation oncologist’s recommendation and was given an

appointment to return for Calypso marker placement within the prostate.

Simulation: Prior to simulation, Calypso markers were placed in WS’s prostate to aid with daily

treatment setup. The patient was placed supine on a simulation table that was fitted with stirrups.

WS’s feet were placed in the stirrups revealing direct access to his perineum and rectal area. An

ultrasound probe was placed in the patient’s rectum to visualize the prostate during marker

placement. Using needle guidance, the radiation oncologist proceeded to place three Calypso

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markers into the patient’s prostate. Markers were placed in the prostate apex, right base and left

base. Following marker placement, the patient was given an appointment time to return in one

week for simulation. The one week interval was intended to ensure that all prostate swelling

from marker placement had ample time to subside.

At the time of simulation the patient was positioned on the computed tomography (CT) couch

atop a full egg crate sponge. A toe ring was placed around his feet and his hands were positioned

on his upper abdomen. The patient was scanned using a Philips Big Bore multislice CT unit with

a scan slice thickness of 3 mm. The scanned area included the upper abdomen to the level of the

mid femurs. The radiation therapist placed the isocenter within the prostate between the

Calypso® markers in anterior and lateral views of the pelvis (Figure 1). The patient was given

anterior and lateral tattoos with instructions to return to begin his treatment.

Anatomical Contouring: The small bowel, prostate, rectum, right and left femoral heads,

bladder, seminal vesicles, and penile bulb were contoured by the radiation oncology resident.

The medical dosimetrist combined the prostate and seminal vesicles into one structure called

clinical target volume (CTV1). For the initial treatment, planning target volume (PTV1_46Gy)

was created by expanding CTV1 7 mm in all directions except posteriorly. Posteriorly this

volume was expanded 3 mm per the radiation oncologist’s request. For the boost treatment the

prostate represented CTV2. PTV2_32Gy was created by expanding CTV2 7mm in all directions

except posteriorly. Posteriorly the volume was expanded 3mm per the oncologist’s request. All

CTV to PTV expansions were done by the medical dosimetrists. It was the radiation

oncologist’s belief that the posterior expansion could be less because of the use of the Calypso®

markers, thus sparing tumor dose to a portion of the rectum. The medical dosimetrist constructed

a 2.3 cm thick ring around PTV1_46Gy and PTV2_32Gy volumes measuring 7mm away from

each volumes surface to assist in creating a formal dose distribution (Figures 2 and 3). The

medical dosimetrist also constructed a posterior rectum contour to assist in limiting the dose to

the posterior half of the rectum. All contours were reviewed by the radiation oncologist and

adjusted when necessary to limit overlap with normal structures. Upon contour approval, the

medical dosimetrist was given a prescription and a prostate planning form listing treatment

planning objectives requested by the radiation oncologist.

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Treatment Planning: The simulation dataset was sent to medical dosimetry and imported into

the Philips ADAC Pinnacle treatment planning system (TPS) version 9.0. The isocenter, set

during simulation, was placed within the prostate between the Calypso® markers. The Calypso®

markers are beacons used to track prostate motion during daily treatment by detecting the

slightest tumor movement and enabling the patient to be repositioned if necessary.3 The three

beacons were contoured by the medical dosimetrist. The initial prescription the expanded

prostate and seminal vesicle was 46Gy with a boost of 16Gy to follow to the prostate expanded

volume. The medical dosimetrist reviewed the contours and treatment objectives for possible

dose conflicts. The medical dosimetrist expressed concern to the radiation oncologist regarding

the penile bulb dose due to its close proximity to the PTV volumes. It was decided by the

physician that PTV volume coverage took priority over the penile bulb dose constraint.

The radiation oncologist requested that WS be treated with volumetric modulated arc therapy

(VMAT) instead of fixed field IMRT. VMAT is a form of IMRT delivery that is capable of

delivering dose to the PTV in a single gantry rotation.4 A full rotational range of beam angles

coupled with dose-rate and gantry speed modulation, provides the potential for achieving higher

dose conformity to the PTV and tighter constraint on organs at risk (OR) limits.4 The medical

dosimetrist assigned the TPS to dynamic arc mode and elected to use a single arc to treat

PTV1_46Gy. The gantry start position was set at 220° and the gantry stop angle was set at 140°

with rotation in a clockwise direction (Figure 4). The chosen treatment energy was 6MV.

PTV1_46Gy was assigned a minimum dose volume objective of 46.4 Gy and a maximum dose

volume objective of 49.7 Gy by the medical dosimetrist to ensure full prescription dose coverage

of PTV1_46Gy and limiting allowable dose beyond 46 Gy within the volume. All PTV and OR

constraints for the initial prostate arc were entered in the inverse planning window of the TPS.

The dose volume constraints for the rectum were: 23 Gy to 50%, 33 Gy to 30% 41.3 Gy to 20%

and 45.1 Gy to 5%. The dose volume constraints for the bladder were: 23 Gy to 50%, 36 Gy to

30% and 41.3 Gy to 20%. The small bowel dose volume constraint was 30 Gy to less that 1%.

Both the left and right femoral head dose volume constraints were not to exceed 29.5 Gy to 1%

and the posterior rectum was limited to 25.6 Gy to 1%.The penile bulb dose volume constraint

was 17.7 Gy to 15%, but meeting this constraint would compromise coverage on the

PTV1_46Gy, therefore it was exceeded. A dose conforming ring around PTV1_46Gy was given

a maximum allowable dose of 36 Gy to assist in conforming the 46 Gy isodose line to the

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PTV1_46Gy volume. After calculating, the medical dosimetrist evaluated the plan and slightly

adjusted the dose constraints thereby producing an acceptable initial plan to 46 Gy (Figures 5

and 6).

The medical dosimetrist proceeded to plan the prostate arc boost to deliver an additional 32 Gy

to the expanded prostate volume. A clockwise arc with the same arc angles and 6 MV beam

energy were utilized for the boost arc plan. PTV2_32Gy was assigned a minimum dose volume

objective of 32 Gy and a maximum dose volume objective 34.6 Gy. The dose volume constraints

for the rectum were: 16 Gy to 50%, 23 Gy to 30% 28.7 Gy to 20% and 31.3 Gy to 5%. The dose

volume constraints for the bladder were: 16 Gy to 50%, 25.6 Gy to 30%, and 28.7 Gy to 20%.

The small bowel dose volume constraint was 15Gy to 1%. Both the left and right femurs were

not to exceed 20 Gy to 1% of their volume. The posterior rectum was given a dose volume

constraint of 22.5 Gy to 2% and the penile bulb was not to exceed 12.3 Gy to 15% of its volume.

This proved to not be feasible and the penile bulb does was exceeded. A dose conforming ring

around PTV1_32Gy was given a maximum allowable dose of 25.6 Gy to assist in conforming

the 32 Gy isodose line to the PTV1_32Gy volume. After calculation the medical dosimetrist

evaluated the prostate boost plan and deemed it acceptable (Figures 7 and 8). The initial and

boost VMAT prostate plans were then combined to form a composite plan. This was first

evaluated by the medical dosimetrist to ensure that all overall dose constraints were not

exceeded. Upon approval, the composite plan was reviewed by the radiation oncologist who was

in agreement with the isodose line distribution and the dose volume histogram (DVH) values

(Figures 9 and 10). All treatment fields were checked by a physicist for accuracy. The second

check utilized an arc IMRT point dose verification that was expected to agree with the ADAC

Pinnacle plan within 5%. Each arc received an independent physics second check (Figures 11

and 12).

Conclusion: Several studies suggest an enhanced treatment efficacy with VMAT radiotherapy

compared to fixed-field IMRT.4 With arc therapy being relatively new to my facility, I was

interested in testing the validity of this claim. From a planning perspective, additional pseudo

structures were needed to make the VMAT plan as conformal as a comparable fixed-field IMRT

plan. Overall the plan looked very similar to what I would expect a fixed-field plan to be. I

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believe that I will develop a better assessment of VMAT versus fixed-field IMRT planning with

experience.

When planning sequential plans as opposed to simultaneous integrated boost (SIB) plans, it is

important to evaluate the plan as a composite. SIB plans allow for better dose conformity and

better control of peripheral dose distribution. A sequentially planned composite will not be as

conformal as an SIB plan due to the lack of control over the peripheral doses one the sequential

plans are combined. The prostate composite plan constraints were met, excluding the penile bulb,

making the plan clinically acceptable. It is important to receive feedback from the radiation

oncologist when dose constraints conflict with tumor coverage. In some instances, sacrificing

tumor coverage may take precedence over exceeding an OR dose volume objective.

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Figure 1: Treatment isocenter placed with in the prostate between the Calypso® markers.

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Figure 2: Dose constraining ring for PTV1_46Gy.

Figure 3: Dose constraining ring for PTV2_32Gy.

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Figure 4: Arc clockwise gantry rotation start and stop angles.

Figure 5: Isodose distribution for initial arc field for PTV1_46Gy.

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Figure 6: Initial arc field PTV1_46Gy DVH.

Figure 7: Isodose distribution for boost arc field PTV2_32Gy.

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Figure 8: Boost arc field PTV2_32Gy DVH.

Figure 9: Composite isodose line distribution.

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Figure 10: Composite DVH for initial and boost arc fields.

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Figure 11: Initial arc field point dose verification.

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Figure 12: Boost arc field point dose verification.

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References

1. Prostate-specific antigen (PSA) test. National Cancer Institute Web site.

http://www.cancer.gov/cancertopics/factsheet/detection/PSA. Updated March 18, 2009.

Accessed May 22, 2012.

2. Kuban D, Trad M. Male reproductive and genitourinary tumors. In: Washington C,

Leaver D, eds. Principles and Practice of Radiation Therapy. St. Louis, MO: Mosby

Elsevier; 2010:826.

3. The convergence of precision and accuracy. Calypso Web site.

http://www.calypsomedical.com/sites/default/files/CAL-

018%20Calypso%20Product%20Bro%20M5.pdf. Accessed May 27, 2012.

4. Kopp RW, Duff MT vs. 7 field-imrt: assessing the dosimetric parameters of prostate

cancer treatment with a 292-patient sample. Med Dosimetry. 2011;36(4):365-372.