Interplay Effect During Lung SBRT With VMAT Delivered by Flattening Filter–Free Beams

Abstract

Volumetric modulated arc therapy (VMAT) delivered by flattening filter free (FFF) beams can significantly reduce treatment time for stereotactic body radiation therapy (SBRT). However, high dose rate raises the concern regarding the impact of the interplay effect between MLC leaf sequences and target motion. We have utilized four-dimensional (4D) dose calculations to investigate the interplay effect on the delivered dose in lung SBRT cases treated using FFF beams. Ten lung cancer patients with tumor motion amplitude of 10-30 mm were investigated. For each patient, a respiratory-correlated 4D computed tomography data set was acquired. Single arc SBRT VMAT plans were generated using the treatment planning system module for 10 MV FFF beam on a linear accelerator. The delivered dose was estimated at two dose rates: conventional dose rate of 400 MU/min and the maximum FFF dose rate of 2300 MU/min. Based on the dose rate, beam-on timing and respiratory cycle, the planned segment MUs were assigned to different respiratory phases to simulate MLC interplay effect. The fractional dose was evaluated in 10 scenarios corresponding to the treatment starting at 0%, 10%, 20%, 90% respiratory phases, respectively. For each scenario the 4D dose was evaluated on the CT at end of exhalation. We also calculated the 4D dose with no interplay effect where the MUs for each segment were equally distributed over all breathing phases. Following parameters were used to analyze the interplay effect in single fraction: standard deviation (SD) of the minimum dose Dmin to the GTV; dose deviation (DE) for GTV between 4D dose and the original planning dose; DE for GTV between 4D doses with and without interplay effects. For all patients, dose delivered at 2300 MU/min FFF mode resulted in larger interplay effects between MLC and tumor motion than that delivered at 400 MU/min. SD for Dmin of GTV was 1.7% of the prescription dose for 2300 MU/min, and 0.15% for 400 MU/min. Compared to the original planning dose, maximum DE of GTV dose delivered at 2300 MU/min was 4.9%, and the DE was less than 3% for 94% of GTV. Compared to the 4D dose calculated without interplay effect, the maximum DE was 3.0%, and the DE is less than 2% for over 99% of GTV. Lung SBRT VMAT treatment delivered by high dose rate FFF beams is more susceptible to the MLC interplay effect compared with that delivered at conventional dose rate. However the dosimetric impact of interplay on the target volume is unlikely to be clinically significant.