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Curtis Wilgenbusch
February Case Study
February 28, 2014
Volumetric Modulated Arc Therapy (VMAT) for Prostate Cancer
History of Present Illness: Patient EC is an 82 year-old male with a history of an elevated
prostate specific antigen (PSA) dating back to 2006. EC underwent his first prostate biopsy in
2006. At the time of that biopsy, his PSA level was 4.6 nanograms (ng)/milliliter (mL). The
biopsy was negative. EC was then followed closely through the years. His PSA originally
decreased to 2.2 ng/mL in 2007 but then gradually increased until he was sent for another biopsy
in August of 2011 when his PSA reached 5.6 ng/mL. This biopsy revealed that 4 of the 14 cores
were positive for adenocarcinoma of the prostate gland. The Gleason score at that time was 6.
EC opted for active surveillance. The next documented PSA in July 2013 was 2.7 ng/mL. The
patient then underwent his third set of prostate biopsies in August 2013. The biopsy revealed
one of 12 cores positive for Gleason 7, involving 30% of the right mid apex. The patient was
staged as T1cN0M0 with intermediate adenocarcinoma of the prostate.
The patient was referred to the radiation oncology department and met with the radiation
oncologist in January 2014 to discuss treatment options. Due to the patient's age and multiple
comorbidities, the radiation oncologist did not believe that short-term androgen ablation would
benefit the patient. The radiation oncologist recommended external beam irradiation of the
prostate with the use of intensity-modulated radiation therapy (IMRT) and image-guided
radiation therapy (IGRT).
Past Medical History: EC has a past medical history of asthma, thrombocytopenia, anemia,
epilepsy, umbilical hernias, left eye blindness, right eye cataract, abdominal adhesions, and small
bowel obstruction. The patient also has a history of gastric lymphoma, which was treated with
chemotherapy in 1993. The patient did not receive radiation therapy for the gastric lymphoma.
Social History: EC is originally from Cuba. The patient was in the insurance business but
retired a few years ago. The patient is married with two children. EC quit smoking 45 years ago
but smoked two packs per day for 25 years. He is a recovering alcoholic who started drinking
alcohol at the age of 12 and quit at the age of 37.
Medications: EC takes the following medications: Flomax, Ketoconazole, Dilantin, Zantac,
Symbicort, and topical 5-Fluorouracil (FU) for the treatment of keratoses.
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Diagnostic Imaging: EC had a transrectal prostate ultrasound (TRUS) performed in August
2013. Although there were areas of hyperdense echoes seen that were suggestive of calculi in
the capsule, the TRUS did not reveal any hypoechogenic areas suggestive of cancer. The
seminal vesicles and bladder were normal. An ultrasound guided biopsy was performed on the
abnormal appearing areas in the base, mid-gland, and apex bilaterally. Results of the biopsy
revealed adenocarcinoma of the prostate with a Gleason score of 7. The prostatic tissue involved
by tumor was roughly 30%. After agreeing with the recommendation for external beam
radiation the radiation oncologist sent the patient for a total body bone scan in January 2014.
The bone scan did not reveal any evidence of bony metastases.
Radiation Oncologist Recommendations: After a review of EC's diagnostic studies, biopsy
report, and pathology, the radiation oncologist recommended that EC receive external beam
radiation to the prostate. The recommendation included definitive IMRT/IGRT using a
volumetric modulated arc therapy (VMAT) technique. Before the introduction of IMRT, the
doses delivered via 3-dimensional (3-D) conformal radiotherapy to the organs at risk (OR) were
much greater. Higher doses to the OR such as the rectum and bladder increase the likelihood of
grade ≥ 2 toxicity.1 It has been shown that IMRT can dramatically reduce the doses to small
bowel, rectum, bladder, and pelvic bones.2 However, one of the downsides to static field IMRT is
the length of time it takes to deliver the complete treatment for several static fields. The VMAT
technique has been shown to dramatically reduce the treatment time, which has many benefits
including the reduction of intrafraction motion.3 In a study by Quan et al,4 the quality of the
VMAT plans were superior to the 8-field IMRT plans used at their institution when comparing
dose to the rectum and treatment delivery efficiency.
The Plan (prescription): The treatment plan recommended by the radiation oncologist was high
dose irradiation of the prostate with IMRT/IGRT utilizing the VMAT technique. This plan was
designed to deliver a total of 7740 centigray (cGy) to the planning target volume (PTV) at 180
cGy per fraction, for a total of 43 fractions. The PTV included the prostate gland plus a 0.8 cm
uniform margin, with the exception of the posterior aspect of the gland. The posterior margin
was set at 0.5 cm due to the close proximity of the rectum. These margins were included to
account for uncertainties such as motion and setup errors.
Patient Setup / Immobilization: The Philips Brilliance Big Bore CT/simulator (Sim) was used
for the simulation. The patient was placed in the supine position with his hips and legs
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immobilized in a Civco Vac-Lok cradle (Figure 1). The hands were placed on the chest holding
a blue ring. A urethrogram was performed to help visualize the apex of the prostate as there was
no magnetic resonance imaging (MRI) study to fuse with the planning CT. Once the patient was
immobilized properly, a CT scan of the abdomen and pelvis using 0.3 cm axial slices was
performed to obtain a treatment planning data set. The radiation oncologist was called to the
simulator to draw the prostate gland. The isocenter was placed in the center of the target volume
drawn by the radiation oncologist and reference marks were put on the patient using the LAP
laser system in the CT/Sim room (Figure 2).
Anatomical Contouring: The CT data set was exported to the Varian Eclipse 11.0 treatment
planning system (TPS). The medical dosimetrist contoured the left and right femoral heads,
pubic symphysis, bladder, and rectum. A structure that included the bladder minus the PTV plus
an additional 2 mm margin and a structure that included the rectum minus the PTV plus an
additional 2 mm margin were created for planning purposes only. The radiation oncologist
contoured the prostate during the CT/Sim. All of the contours were then reviewed and
subsequently approved by the radiation oncologist.
Beam Isocenter / Arrangement: A Varian TrueBeam STX was used to plan this patient. The
medical dosimetrist placed the beam isocenter in the geometrical center of the prostate (Figures
3-9). The VMAT plan consisted of 2 full arcs: the first arc rotated clockwise from 181.0 degrees
to 179.0 degrees in the Varian International Electrotechnical (IEC) 61217 scale and the second
arc rotated counter-clockwise from 179.0 degrees to 181.0 degrees. The collimator angle for the
first arc was set at 30.0 degrees (Figure 8). The collimator angle for the second arc was set at
330.0 degrees (Figure 9). The couch angle was set at 0.0 degrees. The energy used for each arc
was 6 megavolts (MV). Once the treatment objectives were entered into the TPS, the field sizes
and multileaf collimator (MLC) positions were automatically set to deliver the optimal target
coverage, while sparing the OR and the normal tissue. The monitor units (MU) for the first and
second arc fields were 297 and 334, respectively.
Treatment Planning: Treatment planning was performed using Eclipse 11.0. The radiation
oncologist outlined the treatment prescription and the dose constraints to the OR. The patient
received a total of 7740 cGy in 43 fractions at 180 cGy per fraction. The objectives for the target
were a maximum and minimum dose, which corresponded to the treatment prescription. The
goal was to achieve 95% coverage of the target volume with 100% of the prescribed dose. The
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objectives for the bladder were a maximum volume of 15% to receive 6500 cGy and a maximum
volume of 40% to receive 4000 cGy. The objectives for the rectum were a maximum volume of
15% to receive 6500 cGy and a maximum volume of 35% to receive 4000 cGy. After several
iterations, the TPS achieved an acceptable dose distribution, which included sufficient coverage
of the target while maintaining acceptable doses to the OR (Figures 5-7, 10). The plan was
designed for treatment on a Varian TrueBeam STX with 120 leaf high-definition MLC (HDMLC). The energy used for both arcs was 6 MV. The maximum dose rate of 600 MU per
minute was chosen. The angular resolution value was set at 5 degrees. The plan was normalized
to 100% at the primary reference point after being reviewed and approved by the radiation
oncologist.
Quality Assurance/Physics Check: A quality assurance (QA) plan was delivered to a Sun
Nuclear ArcCHECK phantom as well as a Sun Nuclear MapPhan device and subsequently
evaluated by Sun Nuclear's SNC Patient software. The measured plan was then compared with
the expected plan that was exported by the Eclipse TPS and was within tolerance. The treatment
plan and QA measurements were then reviewed by the radiation oncologist and a medical
physicist as a final check before treatment began.
Conclusion: According to the American Cancer Society, there will be an estimated 233,000
new cases of prostate cancer in 2014.5 They also estimate that there will be 29,480 deaths related
to prostate cancer, which is second only to lung and bronchus cancers.5 Although prostate cancer
is very common, it can be well controlled with high doses of radiation. Before the introduction
of IMRT, 3-D conformal therapy was able to successfully deliver a high dose of radiation to the
target but fell short in limiting the doses to the OR. Intensity-modulated radiation therapy
delivered utilizing the VMAT technique has been able to deliver high doses to the target, limit
the doses to the OR, and reduce the total treatment time. There were several challenges with this
case such as reproducibility of bladder, rectal filling, and finding the right optimization
objectives. The planning structures created by subtracting the PTV plus a margin assisted in
overcoming the challenges of delivering the prescribed dose to the target while minimizing the
dose to the normal structures. Overall, the VMAT technique was an excellent option for
achieving the planned objectives.
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References
1. Sveistrup J, Rosenschold P, Deasy J, Hun Oh J et al. Improvement in toxicity in high risk
prostate cancer patients treated with image-guided intensity-modulated radiotherapy compared to
3D conformal radiotherapy without daily image guidance. Radiat Oncol. 2014; 9(1):44.
doi:10.1186/1748-717X-9-44
2. Kopp R, Duff M, Catalfamo F, Shah D, Rajecki M, Ahmad K. VMAT vs. 7-field IMRT:
Assessing the dosimetric parameters of prostate cancer treatment with a 292-patient sample. Med
Dosim. 2011;(36):365-372. doi:10.1016/j.meddos.2010.09.004
3. Palma D, Vollans E, James K, Nakano S et al. Volumetric modulated arc therapy for delivery of
prostate radiotherapy: comparison with intensity-modulated radiotherapy and three-dimensional
conformal radiotherapy. Int J Radiat Oncol Biol Phys. 2008;72(4):996-1001.
doi:10.1016/j.ijrobp.2008.02.047
4. Quan E, Xiaoqiang L, Yupeng L, Xiaochun W et al. A comprehensive comparison of IMRT and
VMAT plan quality for prostate cancer treatment. Int J Radiat Oncol Biol Phys.
2012;83(4):1169-1178. doi:10.1016/j.ijrobp.2011.09.015
5. American Cancer Society. Cancer Facts and Figures 2014. Atlanta: American Cancer Society;
2014.
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Figures
Figure 1. Image of leg cradle system used. Image courtesy of
http://www.civco.com/ro/products/Vac-Lok-Positioning-Cushions.htm.
Figure 2. LAP laser system in the CT/Sim. Image courtesy of www.lap-laser.com.
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Figure 3. Anterior-Posterior (AP) Beams Eye View of Treatment Isocenter
Figure 4. Right Lateral Beams Eye View of Treatment Isocenter
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Figure 5. Axial view of treatment isocenter. Green isodose line is 100%, blue isodose is 98%,
and purple isodose line is 50%.
Figure 6. Coronal view of treatment isocenter. Green isodose line is 100%, blue isodose is 98%,
and purple isodose line is 50%.
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Figure 7. Sagittal view of treatment isocenter. Green isodose line is 100%, blue isodose is 98%,
and purple isodose line is 50%.
Figure 8. Beams Eye View of the first arc field.
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Figure 9. Beams Eye View of the second arc Field.
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Figure 10. Dose Volume Histogram (DVH).