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

To retrospectively validate the dose and dose rates delivered in FAST-01 clinical trial fields via submillimeter spatial and <0.25 ms temporal resolution scintillation imaging.

An ultrafast intensified CMOS camera (4.5-12 kHz sampling rate) imaged the light response of a scintillator sheet at the treatment isocenter and irradiated by pencil beam scanning proton fields, including FAST-01 clinical fields. Dose and dose rate linearity studies were performed, followed by camera calibration via gafchromic (external beam therapy 3) film. An EDGE diode detector was placed directly under the scintillator at the surface and at a 5 cm 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 3-dimensional spot profiles, the surface images were projected to a 5 cm depth for comparison with a secondary diode and log file recordings.

Camera response was linear with dose (R² = 0.9998) and beam current (R² = 0.9883) from 2 to 12 Gy and 20 to 210 nA, respectively. Gamma analysis of the cumulative dose maps at 3%/2 mm 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 ± 0.07 ms. At the surface, the average imaged dose rate across the 7 fields was 114 ± 1 Gy/s, agreeing with the diode within 1% ± 1%. The dose at a 5 cm 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 a 5 cm depth was 62 ± 1 Gy/s, which agreed with the reported and diode values within 2% ± 3%.

This study provides the first independent validation of dose and dose rate for clinical proton FAST-01 fields at unprecedented spatiotemporal resolution. Owing to the nontrivial dose rate distributions in pencil beam scanning fields, such direct 2-dimensional dose rate mapping will be important in future pretreatment plan quality assurance.