A dosimetric comparison of volumetric modulated arc therapy with step-and-shoot intensity modulated radiation therapy for prostate cancer

Purpose: To compare variable dose-rate volumetric modulated arc therapy (VMAT) with 7-field, step-and-shoot intensity modulated radiation therapy (IMRT) in prostate cancer patients treated with a consistent planning target volume (PTV) to a uniform total radiation therapy dose.

Methods and materials: We studied 32 patients who received 8100 cGy in 45 daily fractions to their prostate and proximal 1 cm of the seminal vesicles using variable dose rate VMAT (n = 22) or 7-field, step-and-shoot IMRT (n = 10) for intermediate-risk or high-risk prostate cancer between July 2010 and April 2013. In 90% of patients, VMAT was delivered with 2 arcs. To have an unbiased comparison of plan quality, patients who were treated with VMAT were also planned with IMRT and vice versa. Each patient served as his own control for the comparison.

Results: VMAT reduced median radiation beam-on time from 4.3 to 3.4 minutes (P = .03). There was no statistically significant difference in PTV volumes between the VMAT and step-and-shoot IMRT groups (P = .76). VMAT dose distributions were more homogeneous (P = .003). There was no difference between groups with regard to rectal V60, V65, V70, V75, bladder V65, V70, V75, V80, or femoral heads V33.

Conclusions: Two-arc VMAT resulted in shorter beam-on times and more homogenous dose distributions than 7-field, step-and-shoot IMRT for prostate cancer. With decreased beam-on time, the intrafraction motion during irradiation is reduced, thus improving confidence that the delivered dose distribution agrees with the plan.