Validation of the relative insensitivity of volumetric-modulated arc therapy (VMAT) plan quality to gantry space resolution

Volumetric-modulated arc therapy (VMAT) is an efficient form of radiotherapy used to deliver intensity-modulated radiotherapy beams. The aim of this study was to investigate the relative insensitivity of VMAT plan quality to gantry angle spacing (GS). Most previous VMAT planning and dosimetric work for GS resolution has been conducted for single arc VMAT. In this work, a quantitative comparison of dose-volume indices (DIs) was made for partial-, single- and double-arc VMAT plans optimized at 2 degrees, 3 degrees and 4 degrees GS, representing a large variation in deliverable multileaf collimator segments. VMAT plans of six prostate cancer and six head-and-neck cancer patients were simulated for an Elekta SynergySA (R) Linac (Elekta Ltd, Crawley, UK), using the SmartArc (TM) module of PinnacleA(3) TPS, (version 9.2, Philips Healthcare). All optimization techniques generated clinically acceptable VMAT plans, except for the single-arc for the head-and-neck cancer patients. Plan quality was assessed by comparing the DIs for the planning target volume, organs at risk and normal tissue. A GS of 2 degrees, with finest resolution and consequently highest intensity modulation, was considered to be the reference, and this was compared with GS 3 degrees and 4 degrees. The differences between the majority of reference DIs and compared DIs were < 2%. The metrics, such as treatment plan optimization time and pretreatment (phantom) dosimetric calculation time, supported the use of a GS of 4 degrees. The ArcCHECK (TM) phantom-measured dosimetric agreement verifications resulted in a > 95.0% passing rate, using the criteria for gamma (3%, 3 mm). In conclusion, a GS of 4 degrees is an optimal choice for minimal usage of planning resources without compromise of plan quality.