(P141) Dosimetric Comparison of Volumetric-Modulated Arc Therapy With Step-and-Shoot Intensity-Modulated Radiation Therapy for Prostate Cancer

Eric A. Mellon, MD, PhD, Khosrow Javedan, PhD, Tobin J. Strom, MD, Eduardo G. Moros, PhD, Matthew C. Biagioli, MD, Daniel C. Fernandez, MD, PhD, Richard B. Wilder, MD; H. Lee Moffitt Cancer Center

Purpose: Step-and-shoot intensity-modulated radiation therapy (IMRT) and variable-dose-rate volumetric modulated arc therapy (VMAT) are two common treatment techniques for the definitive treatment of prostate cancer. While VMAT is generally understood to be more rapidly delivered, minimizing intrafraction prostate motion, there is disagreement about whether VMAT produces more favorable planning target volume (PTV) coverage while delivering lower doses to bladder, rectum, and femoral heads. The disagreement may be partially explained by the inclusion of a wide variety of PTVs in prior studies, such as prostate ± seminal vesicles ± pelvic lymph nodes, as well as various prescribed doses and technical details of early precommercial implementations. In order to compare these two techniques in modern practice, we analyzed two cohorts of patients treated at our institution who were matched for PTV, prescribed dose, and patient characteristics.

Methods: We studied 32 patients who received 8,100 cGy in 45 daily fractions to the prostate and proximal 1 cm of the seminal vesicles using VMAT (n = 22) or seven-field, step-and-shoot IMRT (n = 10) for intermediate-risk or high-risk prostate cancer between July 2010 and April 2013. Image guidance utilized daily pretreatment kV imaging of 3–4 gold fiducials. Treatment planning was by the Philips Pinnacle system. In 20/22 (91%) patients, VMAT was delivered with two 356-degree arcs on Varian Trilogy or TrueBeam linear accelerators. Acute toxicity was assessed using Common Terminology Criteria for Adverse Events version 3 (CTCAE v3) criteria.

Results: VMAT reduced median radiation delivery time from 4.5 to 2.5 minutes (P = .02). There was no significant difference in PTV volumes between the VMAT and IMRT groups (P = .76). VMAT plans were more conformal, based on a conformity index (P = .04), and PTV coverage was more homogenous (ie, fewer hot and cold spots), based on a homogeneity index (P < .001). There was a slight improvement in the amount of rectum receiving 60 Gy or more (VMAT 18 cc vs IMRT 24 cc; P = .046). However, there was no difference between the two groups with regard to rectal V65, V70, and V75; bladder V65, V70, V75, and V80; or femoral heads V33. No grade 2 or higher acute diarrhea was observed in either group. No grade 3 or higher cystitis was observed in either group, and rates of grade 2 cystitis were not statistically different (VMAT 32% vs IMRT 60%; P = .13).

Conclusions: Two-arc VMAT resulted in shorter treatment times, as well as more conformal and more homogeneous treatment plans than seven-field, step-and-shoot IMRT for prostate cancer. Decreased treatment time results in less of an opportunity for intrafraction prostate motion during radiation delivery, thereby reducing the likelihood of a geographic miss.