Sci—Sat AM(1): Planning — 03: Volumetric Modulated Arc Therapy (VMAT) Planning Strategies for Treating Target Volumes of Varying Complexity

The ability to modulate dose rate, gantry speed, and field aperture simultaneously while the Linac gantry rotates has made volumetric modulated arc therapy (VMAT) an attractive radiotherapy option. The goal of this study was to quantify differences in treatment delivery efficiency between VMAT and step and shoot intensity modulated radiotherapy (IMRT) plans of equal dosimetric quality. VMAT and 7 beam IMRT plans were generated using identical optimization objectives in Pinnacle for treatment sites of varying complexity including low risk prostate (n=5), high risk prostate (n=5, with nodal target), brainstem glioma (n=5), and head and neck cancer (HNC, n=5). A single arc was used as a starting point for all treatment sites and modified if planning objectives were not met. Due to the simple geometry of localized prostate treatments, a single arc sufficed. For the high risk prostate and brainstem gliomas, the addition of a 90 degree partial arc, delivered anteriorly with a different collimator angle, reduced dose to OARs compared to a single arc. In HNC cases, two full arcs were required to achieve VMAT plans comparable to IMRT. Delivery times were measured and included both beam‐on and gantry rotation time between subsequent beams / arcs. Time savings of 50–85% and monitor unit savings of 10–40% (91–423 per fraction) over IMRT were observed in all VMAT plans. VMAT improved delivery efficiency in simple and more complex treatment sites with similar dosimetric quality as IMRT. While additional arcs may diminish savings, they can improve dose sculpting around OAR adjacent to targets. One of the authors (KM) has research collaboration agreement with Philips Medical Systems and with Elekta Inc.