Sub-Arc Collimator Angle Optimization for VMAT Planning of Multiple Metastases Brain SRS Treatments

Abstract

To investigate the impact of collimator angle optimization in multiple metastases VMAT SRS treatments with respect to treatment delivery efficiency and OAR doses. We hypothesize that optimized collimator angle selection will improve treatment delivery efficiency (i.e. reduce required monitor units) while preserving similar OAR dose levels relative to plans in which the collimator angle has not been optimized. Using a previously published collimator angle optimization algorithm, 11 VMAT cases with 3 metastases (n=7) and 4 metastases (n=4) were planned using a standard 4 arc template (1 full axial arc and 3 partial arcs with couch rotations of 45, 315 and 90 degrees). A total of 10 sub arcs were distributed among the 4 arcs with optimized collimator angles (4 sub-arcs in the full axial arc, 2 sub-arcs in each of 3 partial arcs) . These cases were also planned using the 4 arc template with a single, standardized collimator angle per arc. The primary planning objective was to achieve 99.5% volume covered with the prescription dose (24 Gy to all targets in this study). The secondary objective was to meet the maximum OAR dose constraints as per QUANTEC; otherwise as low as achievable. Total monitor units were reduced on average by 15% for both the 3 met (p = 0.017) and 4 met cases (p = 0.057) using the collimator optimization method. This corresponded to reductions of 1132 and 1239 MU on average for the 3 and 4 met cases respectively. There were no statistically significant differences in the maximum doses received by any contoured OAR (brainstem, chiasm, eyes, optic nerves, optic tracks) (3 met: p > 0.46, 4 met p > 0.2). Although V12Gy was lower with optimized collimator angle, the results were not significant (3 met: p = 0.46, 4 met: p = 0.2). Increased treatment delivery efficiency is of significant interest in high dose per fraction treatments in which intrafraction motion could be particularly deleterious. The MU reductions achieved in this study translate into beam on time reductions on the order of a minute (depending on the dose rate used to deliver the treatment).