Enhancing Delivery Efficiency on the MR-Linac: A Comprehensive Evaluation of Prostate SBRT using VMAT

Abstract

PurposeLong treatment sessions are a limitation within MRIgART. This work aims for significantly enhancing the delivery efficiency on the MR-linac by introducing dedicated optimization and delivery techniques for VMAT. VMAT plan and delivery quality during MRIgART is compared to step-and-shoot IMRT for prostate SBRT. Methods and MaterialsTen prostate patients previously treated on a 1.5T MR-linac were retrospectively replanned to 36.25 Gy in five fractions using step-and-shoot IMRT and the clinical Hyperion optimizer within Monaco (Hyp-IMRT), the same optimizer with a VMAT technique (Hyp-VMAT), and a research-based optimizer with VMAT (OFL+PGD-VMAT). The plans were then adapted onto each daily MRI dataset using two different optimization strategies to evaluate the ATP workflow: “optimize weights” (IMRT-Weights and VMAT-Weights) and “optimize shapes” (IMRT-Shapes and VMAT-Shapes). Treatment efficiency was evaluated by measuring optimization time, delivery time, and total time (optimization + delivery). Plan quality was assessed by evaluating OAR sparing. Ten patient plans were measured using a modified linac control system to assess delivery accuracy via a gamma analysis (2%/2mm). Delivery efficiency was calculated as average dose rate divided by maximum dose rate. ResultsFor Hyp-VMAT and OFL+PGD-VMAT the total time was reduced by 124 ± 140 seconds (p = .020) and 459 ± 110 seconds (p < 0.001), respectively as compared to the clinical Hyp-IMRT group. Speed enhancements were also measured for ATP with reductions in total time of 404 ± 55 (p<0.001) for VMAT-Weights as compared to the clinical IMRT-Shapes group. Bladder and rectum DVH points were within 1.3 % or 0.8 cc for each group. All VMAT plans had gamma passing rates greater than 96 %. The delivery efficiency of VMAT plans was 89.7 ± 2.7 % compared to 50.0 ± 2.2 % for clinical IMRT. ConclusionsIncorporating VMAT into MRIgART will significantly reduce treatment session times while maintaining equivalent plan quality.