First Retrospective QA of FAST-01 Clinical Treatment Fields Using High-Speed Quantitative Scintillation Imaging.

Abstract

Purpose: To retrospectively validate the dose and dose rates delivered in XXX clinical trial fields via sub-millimeter spatial and <0.25 ms temporal resolution scintillation imaging.

Methods: An ultra-fast intensified CMOS camera (4.5-12 kHz sampling rate) imaged the light response of a scintillator sheet at treatment isocenter and irradiated by pencil beam scanning proton fields, including XXX clinical fields. Dose and dose rate linearity studies were performed, followed by camera calibration, via EBT3 film. An EDGE diode detector was placed directly under the scintillator at the surface and at 5cm depth, for comparison with the imaging data and log files. Frame-by-frame analysis of image stacks yielded dose and dose-rate maps for each delivery at the surface. Using the percent depth dose curves and 3D spot profiles, the surface images were projected to 5 cm depth for comparison with a secondary diode and log file recordings.

Results: Camera response was linear with dose (R² =0.9998) and beam current (R²=0.9883) from 2-12Gy and 20-210nA, respectively. Gamma analysis of the cumulative dose maps at 3%/2mm indicated a mean passing rate of 100% compared with film. Total irradiation time agreed with the log file recordings with an average deviation of 0.20±0.07ms. At the surface, the average imaged dose rate across the seven fields was 114±1Gy/s, agreeing with the diode within 1±1%. The dose at 5cm depth from the projected images (mean 8.4 Gy) agreed with the reported dose (log files) within 0.13±0.03 Gy (2±1%). The average dose rate from projected images at 5 cm depth was 62±1 Gy/s which agreed with the reported and diode values within 2±3%.

Conclusion: This study provides the first independent validation of dose and dose rate for clinical proton XXX fields at unprecedented spatiotemporal resolution. Owing to the non-trivial dose rate distributions in PBS fields, such direct 2D dose rate mapping will be important in future pre-treatment plan quality assurance.