SU‐E‐T‐884: Dosimetric Evaluation of Arc Techniques in Stereotactic Radiosurgery

Abstract

Purpose: Conventionally, conformal arcs (called dynamic in BrainLab) are used in cranial stereotactic radiosurgery (SRS). This study dosimetrically evaluates the volumetric modulated arc therapy (VMAT) and VMAT‐conformal hybrid plans against conformal‐arc plans. In hybrid plans, various numbers of conformal arcs in a conformal plan are replaced with VMAT‐arcs.Methods: Ten brain‐cancer patients previously treated with SRS using conformal arcs were retrospectively studied with various numbers of VMAT‐arcs replacing conformal arcs. Pure VMAT plans of different numbers of arcs generated using the same angle and arc length of all or part of the conformal arcs were compared with the original and hybrid plans. SmartArc was used for VMAT planning for a Novalis accelerator. The conformity index (CI), defined as the ratio of the isodose volume to the isodose covered target volume, gradient index (GI), defined as the ratio of the 30%‐isodose volume (V30) to the 100%‐isodose volume, mean dose in target were studied.Results: With one of the conformal arcs replaced with a VMAT‐arc, the average CI improved from 1.28±0.11 of the conformal plans to 1.22±0.11 while GI degraded from 6.04±1.10 to 10.10±4.90. The CI and GI changed little with the number of arcs being replaced. Pure VMAT plans demonstrated better CI when the number of arcs was greater than 2. For plans of 3 VMAT arcs, the CI and GI were 1.22±0.16 and 9.54±2.95 respectively. GI improves with more arcs, but never gets better than conformal‐arc plans. The degraded GI in plans with VMAT‐arcs is because of the absence of beam‐margin limit in SmartArc planning. The target mean dose was about the same. Conclusions: VMAT and conformal‐VMAT hybrid plans usually offer better target conformity but compromise dose fall‐off in normal tissues. The number of arcs in VMAT plans is recommended to be greater than 2.