Target specific collimation within a VMAT trajectory delivery for stereotactic radiosurgery

Volumetric modulated arc therapy (VMAT) is the standard of care for stereotactic treatments. During a VMAT delivery, using a single collimator angle throughout a continuous arc may be suboptimal due to variations in the target's shape in the Beam Eye's view (BEV). This work proposes a VMAT optimization method which segments a single continuous arc to allow for varying collimator angle to achieve an efficient delivery and Monitor Unit (MU) utilization. Thirteen retrospective stereotactic radiosurgery (SRS) and stereotactic radiotherapy (SRT) patient plans with irregularly shaped targets were used in this study. The targets were mainly cavity and meningioma due to the irregular shapes. These cases were planned originally using the Eclipse treatment planning system (TPS). These plans were then replanned with rotated collimators at specific points in each VMAT arc, where the shape of the target was deemed to have changed extensively in the BEV. The start and end gantry angles of the trajectory followed the initially approved clinical plan. The resulting plans were compared with the original clinical plans using the mean aperture ratio, total plan monitor unit (MU), total beam delivery time, patient specific quality assurance (PSQA) results and the plan dosimetric metrics. These plans were delivered using Truebeam STx with HDMLCs and PSQA was measured using the Sun Nuclear SRS MapCheck with the gamma passing rate (GPR) criteria of 3%/2 mm, 2%/2 mm, 2%/1 mm, and 1%/1 mm. The replans achieved a 15.1% reduction in MUs while maintaining the target coverage. The organ-at-risk (OAR) doses showed no significant improvement when using this approach. The beam-on time for SRS plans decreased by 20% compared the clinically approved plan, while the SRT plans showed a 5% time savings. The gamma passing rate (GPR) yielded at least 99% for 2%/1 mm, and 100% for 2%/2 mm for both the clinically approved plans and the replans, respectively. There was no statistically significant difference in GPRs across all criteria and in the plan dosimetric metrics for target and normal tissues. These results show that the proposed optimization approach improves delivery efficiency in terms of both delivery time and MU reduction.