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Background and Purpose-Recently, a new radiotherapy delivery technique has become clinically available -Volumetric Modulated Arc Therapy (VMAT). VMAT is the delivery of IMRT while the gantry is in motion using dynamic leaf motion. The perceived benefit of VMAT over IMRT is a reduction in delivery time. In this study, VMAT was compared directly with IMRT for a series of prostate cases.Materials and Methods-For ten patients, a biologically optimised seven field IMRT plan was compared with a biologically optimised VMAT plan using the same planning objectives. The Pinnacle RTPS was used. The resultant target and organ at risk dose volume histograms (DVHs) were compared. The normal tissue complication probability (NTCP) for the IMRT and VMAT plans was calculated for three model parameter sets. The delivery efficiency and time for the IMRT and VMAT plans was compared.Results-The VMAT plans resulted in a statistically significant reduction in the rectal V25Gy parameter of 8.2% on average over the IMRT plans. For one of the NTCP parameter sets the VMAT plans had a statistically significant lower rectal NTCP. These reductions in rectal dose were achieved using 18.6% fewer monitor units and a delivery time reduction of up to 69%.Conclusion-VMAT plans resulted in reductions in rectal doses for all ten patients in the study. This was achieved with significant reductions in delivery time and monitor units. Given the target coverage was equivalent, the VMAT plans were superior.

Results confirm anatomical dependence of specific GI toxicities. They provide an atlas summarizing dose-histogram effects and derived constraints as functions of anatomical region, dose, toxicity, and endpoint for informing future radiation therapy planning.