A dosimetric comparison between single-arc and double-arc prostate VMAT plans

This study compared the dosimetry between prostate plans using the single-arc and double-arc technique in volumetric modulated arc therapy (VMAT). Single-arc and double-arc prostate VMAT plans were created in five patients with prostate volumes ranging from 39.0-68.1 cm . 6 MV photon beams generated by a Varian 21EX linac with 120-leaf multileaf collimator, and prescription dose of 78 Gy with 39 fractions were used. Dose-volume histograms of the target and critical organs, dose criteria such as the planning target volume (PTV) D99%, rectum D30% and rectum D50%, and dose profiles along the anterior-posterior and lateral directions crossing the isocenter were determined using the Eclipse treatment planning system with RapidArc optimization. It is found that the double-arc technique provided a better PTV coverage and rectal sparing compared to the single-arc for all patients. For dose profiles crossing the isocenter, it is seen that the dose in the anterior-posterior direction was shifted to the lateral, when the single-arc technique was replaced by the double-arc. This dosimetric change made the dose in the rectal position shift laterally to the femoral head, resulting in a lower rectal dose. The calculated monitor units (MUs) were 471-600 for the single-arc plans and 587-730 for the double-arc. Although the double-arc plans on average required more MUs of about 20%, and almost twice the treatment time compared to the single-arc, it is worthwhile to be considered particularly in some cases, where the dose criteria are difficult to achieve using the single-arc technique.