Impact of Dose Rate Settings on Delivery Accuracy In Dynamic Swing Arc for Prostate SBRT
Abstract
Purpose
Dynamic Swing Arc (DSA) irradiation with the OXRAY radiotherapy system integrates simultaneous gantry rotation and ring motion, enabling high-dose-rate delivery for prostate stereotactic body radiotherapy (SBRT). Although the system can deliver 6 MV flattening-filter-free (FFF) beams up to 1150 MU/min, large and rapid dose rate modulations may challenge the mechanical response and dosimetric accuracy. This study evaluated the impact of varying minimum dose rate settings on dosimetric accuracy, delivery efficiency, and organ-at-risk (OAR) sparing for DSA-based prostate SBRT.
Methods
Three prostate cancer cases were planned using a 36.25 Gy/5 fx DSA technique prescribed to PTV D95. Four minimum dose rate settings were compared: 250, 450, 650, and 850 MU/min (all with a maximum of 1150 MU/min). The dosimetric quality was assessed using standard clinical metrics. Delivery accuracy was quantified using gamma analysis (2 mm/3 %, 10% threshold) with a high-resolution 2D detector array. Treatment log files were analyzed to compare the planned and delivered dose rate modulation and cumulative monitor units (MU).
Results
Higher minimum dose rate conditions significantly reduced the beam-on time while maintaining comparable PTV coverage. However, the intermediate OAR doses increased moderately with higher minimum dose rates, particularly at 850 MU/min. Gamma pass rates improved with increased minimum dose rate: the lowest setting (250 MU/min) exhibited the lowest pass rate (mean ~63%), whereas 650 and 850 MU/min settings consistently exceeded 95% gamma passing rate. Log analysis revealed that lower minimum dose rates were associated with frequent rapid dose rate adjustments, which correlated with increased delivery deviation.
Conclusion
For DSA-based prostate SBRT on the OXRAY platform, a minimum dose rate of 650 MU/min balances efficient delivery with high dosimetric fidelity and acceptable OAR sparing. Lower minimum dose rates may compromise delivery accuracy because of mechanical dose rate modulation limitations.