1

Louise Francis

July Case Study

July 22, 2012

Volumetric Modulated Arc Therapy of Adenocarcinoma of the Prostate

History of Present Illness: ST is an 81-year-old Caucasian male with a history of chronic

prostatitis. He has had numerous prostate ultrasounds and biopsies with consistently negative

results. In 2009, it was noted by his urologist that ST’s prostate specific antigen (PSA) level had

increased from 1.9 ng/mL to 3.2 ng/mL. The PSA test measures the level of PSA in the

bloodstream and aids in detecting prostate cancer in men.1 A prostate biopsy was ordered and

again yielded negative results for cancer. Traditionally, most doctors consider a PSA level below

4.0 ng/mL to be normal. In June 2009, ST’s PSA level had increased to 4.7ng/mL and

subsequently to 6.96 ng/mL on a follow-up PSA test. Another prostate biopsy was ordered

revealing several cores of the right posterior prostate lobe to be positive for adenocarcinoma.

Biopsy results indicated that ST had 10% involvement of the right base core, 85% involvement

of the right mid core, 50% involvement of the right apex core, 30% involvement of the right

lateral base core and 40% involvement of the right mid lateral core. Perineural involvement was

also noted. The patient was pathologically staged as having adenocarcinoma of the prostate stage

T1cN0M0, with a Gleason score of 3+4=7. A Gleason score provides an effective measure of

prostate cancer based on the appearance of cancer cells when viewed under a microscope by a

pathologist.2 The score is the sum of the primary and secondary grades with a total score ranging

from 2 (1+1) to 10 (5 +5). A lower Gleason score is indicative of a less aggressive form of

prostate cancer. The urologist explained the PSA and Gleason test results and counseled the

patient on possible treatment options including hormonal treatment and external beam radiation

therapy. The patient expressed an interest in proceeding with hormonal and radiotherapy

treatments. He was started on a 3-month Lupron treatment regimen by the urologist and

subsequently referred to radiation oncology.

At the time of his radiation therapy consultation, ST reported that he had initially experienced

some difficulties with persistent perineal discomfort and dysuria following his prostate biopsy,

which has since greatly improved. He denied hematuria, bowel and bladder incontinence, or any

2

changes in his bladder or bowel movements. The radiation oncologist reviewed the patient’s

history and explained the risk and benefits associated with prostate irradiation. The radiation

oncologist also explained the need for placement of Calypso transponders in the prostate to aid

with daily treatment setup. ST expressed an understanding of the risks and benefits of prostate

irradiation and elected to follow the radiation oncologist’s recommendations.

Past Medical/Surgical History: ST has a past medical history significant for coronary artery

disease and coronary artery stent placement for which he had coronary bypass surgery. In

addition, he has had complications with arrhythmia and currently has a pacemaker. The patient

has a surgical history that includes open-heart surgery, cholecystectomy, hernia repair and a

colon resection for diverticulitis. ST reports having had two transurethral resections of the

prostate (TURP) procedures in his fifties to relieve chronic prostate issues.

Family/Social History: ST is currently retired and married. He has a past smoking history and

currently drinks alcohol frequently. The patient denies a family history of prostate and kidney

cancer or any other type of kidney disorders.

Medications and Allergies: The patient currently takes a list of medications that include the

following: Zocor, Plavix, Klonopin, Mexiletine HCL, Proscar, aspirin, Fosamax, Seroquel and

Nortriptyline. The patient was also started on a 3-month Lupron therapy regimen in September

2009. ST is allergic to penicillin, Cephalexin and Clindamycin.

Diagnostic Imaging: ST has a history of several prostate ultrasounds used for the management

of chronic prostatitis. As part of his prostate cancer workup, he underwent a computed

tomography (CT) exam of the chest, abdomen and pelvis, as well as a bone scan. Both exams

were negative for evidence of metastatic disease.

Radiation Oncologist Recommendations: The radiation oncologist reviewed current findings

with ST and offered a course of intensity modulated radiation therapy (IMRT) to the prostate.

IMRT would be used to create a highly conformal radiotherapy plan that would limit dose to the

surrounding organs at risk (OR) such as the bladder, rectum, and small bowel. The radiation

oncologist discussed the placement of Calypso transponders in the prostate to aid in daily

positioning for IMRT. The Calypso transponders are beacons used to track prostate motion

during daily treatment by detecting the slightest tumor movement and enabling the patient to be

3

repositioned if necessary.3 The radiation oncologist proposed an initial course of 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. ST was in agreement with the radiation oncologist’s recommendation and

was given an appointment to return for Calypso transponder placement within the prostate.

Simulation: Prior to simulation, Calypso transponders were placed in ST’s prostate to aid with

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

stirrups. ST’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 clearly visualize the prostate

during transponder placement. Using needle guidance, the radiation oncologist proceeded to

place three Calypso transponders into the patient’s prostate. Transponders were placed in the

prostate apex, right base and left base. Following transponder 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 transponder placement would have ample time to subside.

At the time of simulation, the patient was positioned on the CT couch atop a full egg crate

sponge. A toe ring was placed around his feet for immobilization and his hands were positioned

together on his upper abdomen. The patient was scanned by a radiation therapist using a Philips

Big Bore multi-slice 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 then placed the isocenter

within the prostate between the Calypso transponders in anterior and lateral views of the pelvis

per the radiation oncologist’s request (Figure 1). At the completion of simulation, ST was given

anterior and lateral treatment setup tattoos and instructed to return in one week to begin his

radiation 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. The boost plan target volume, PTV2_32Gy, was created by

4

expanding CTV2 7 mm in all directions except posteriorly. Posteriorly the volume was expanded

3 mm per the radiation oncologist’s request. All CTV to PTV expansions were done by the

medical dosimetrist. It was the radiation oncologist’s belief that the posterior volume expansion

could be less because of the use of the Calypso transponders, 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 7 mm away from each volume’s surface to

assist in creating a conformal 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 appropriate 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.

Treatment Planning: The simulation dataset was imported into the Philips ADAC Pinnacle

treatment planning system (TPS) version 9.0. The isocenter, set per the radiation oncologist’s

instruction during simulation, had been placed within the prostate between the Calypso

transponders. The three Calypso transponders were contoured by the medical dosimetrist. The

initial prescription would treat the expanded prostate and seminal vesicles volume to 46Gy,

followed by a boost of 16Gy 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 radiation oncologist that PTV volume

coverage took priority over the penile bulb dose constraint.

The radiation oncologist requested that ST 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 position was set at

140° with rotation in a clockwise direction (Figure 4). The chosen treatment energy was 6MV.

5

Planning objectives and OR constraints were added to the IMRT objective section in the TPS.

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

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

of PTV1_46Gy and limiting allowable dose beyond 46Gy within the volume. The dose volume

constraints for the rectum were: 23Gy to 50%, 33Gy to 30%, 41.3Gy to 20%, and 45.1Gy to 5%.

The dose volume constraints for the bladder were: 23Gy to 50%, 36Gy to 30%, and 41.3Gy to

20%. The small bowel dose volume constraint was 30Gy to less that 1%. Both the left and right

femoral head dose volume constraints were not to exceed 29.5Gy to 1% and the posterior rectum

was limited to 25.6Gy to 1%. The penile bulb dose volume constraint was listed as 17.7Gy to

15% on the prostate planning form, but meeting this constraint would compromise coverage on

the PTV1_46Gy. After consulting with the radiation oncologist, the penile bulb dose volume

constraint was adjusted to 46Gy to 1% allowing for adequate coverage to PTV_46Gy. A dose

conforming ring around PTV1_46Gy was given a maximum allowable dose of 36Gy to assist in

conforming the 46Gy isodose line to the PTV1_46Gy volume. After calculating, the medical

dosimetrist evaluated the plan and slightly adjusted the dose volume constraints to produce an

initial plan isodose distribution and dose volume histogram (DVH) to 46Gy (Figures 5 thru 7)

that met the radiation oncologist’s approval.

The medical dosimetrist proceeded to plan the prostate arc boost to deliver an additional 32Gy 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 32Gy and a maximum dose volume objective of 34.6Gy. The dose volume

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

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

20%. The small bowel dose volume constraint was 15Gy to 1%. Both the left and right femurs

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

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

The penile bulb dose volume constraint was adjusted to 32Gy to 0.5% of the penile bulb to

ensure adequate PTV2_32Gy coverage. A dose conforming ring around PTV2_32Gy was given

a maximum allowable dose of 25.6Gy to assist in conforming the 32Gy isodose line to the

PTV2_32Gy volume. After calculation the medical dosimetrist evaluated the prostate boost plan

and boost DVH to ensure that the radiation oncologist’s boost planning objectives were met

6

(Figures 8 thru 10). The initial and boost VMAT prostate plans were then combined to form a

composite plan. The composite plan was review by the medical dosimetrist to ensure that all

overall dose constraints were not exceeded. The composite plan was then reviewed by the

radiation oncologist who was in agreement with the composite isodose line distribution and the

composite DVH values (Figures 11 thru 13).

All treatment fields were checked by a physicist for accuracy. The physics 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 14 and 15).

Conclusion: This case was chosen because VMAT planning is relatively new to my facility. I

wanted to obtain a better understanding of the differences between IMRT fixed-field and VMAT

arc field planning. Several studies suggest an enhanced treatment efficacy with VMAT

radiotherapy compared to fixed-field IMRT.4 From a planning perspective, I found that adding

additional pseudo structures was needed in order to optimize the VMAT plan to the same level as

a fixed-field IMRT plan. Overall, the VMAT prostate plan looks very similar to a fixed-field

prostate plan. The VMAT prostate plan monitor units were typically 50-60% less than a

comparable fixed-field IMRT prostate plan. This translates into the patient being in the treatment

room for a shorter period of time and the possibly of treating a greater number of patients per day

because of the decreased time per patient VMAT plans allow.

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

important to evaluate the plan as a composite. ST’s prostate composite plan dose constraints

were achieved, excluding the penile bulb, making the plan clinically acceptable to the radiation

oncologist. 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.

7

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

8

Figure 2: Dose constraining ring for PTV1_46Gy.

Figure 3: Dose constraining ring for PTV2_32Gy.

9

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

10

Figure 5: Transverse isodose distribution for initial arc field for PTV1_46Gy.

Figure 6: Sagittal and coronal isodose distribution for initial arc field for PTV1_46Gy.

11

Figure 7: Initial arc field PTV1_46Gy DVH.

12

Figure 8: Transverse isodose distribution for boost arc field PTV2_32Gy.

Figure 9: Sagittal and coronal isodose distribution for boost arc field PTV2_32Gy.

13

Figure 10: Boost arc field PTV2_32Gy DVH.

14

Figure 11: Composite isodose line distribution.

Figure 12: Composite isodose line distribution.

15

Figure 13: Composite DVH for initial and boost arc fields.

16

Figure 14: Initial arc field point dose verification.

17

Figure 15: Boost arc field point dose verification.

18

References

1. Zelefsky M, Valicenti R, Hunt M, Perez C. Low risk prostate cancer. In: Perez C, Brady

L, Halperin E, eds. Principles and Practice of Radiation Oncology. 5th ed. Philadelphia,

PA: Lippincott, Williams and Wilkins; 2008:1447-1449.

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. Calypso: accurate, precise, real-time tracking. Calypso Web site.

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

018%20Calypso%20Product%20Bro%20M5.pdf. Published 2009. Accessed July 17,

2012.

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

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

372.