Emulating Clinical Workflow of Scintillator Array Dosimetry for FLASH Pencil-Beam Scanning Proton Therapy

The emergence of ultra-high dose rate (UHDR) pencil-beam scanning (PBS) proton therapy and associated clinical trials has outpaced the capabilities of in vivo treatment validation systems. Current dosimetry tools are limited in spatiotemporal resolution and are insufficient for clinical FLASH treatment monitoring. This study aimed to demonstrate the ability to monitor UHDR proton therapy in an emulated clinical in vivo setting using a novel scintillation imaging system. We demonstrate the system's ability and evaluate the accuracy of measuring two-dimensional dose and dose rate maps with verification against treatment plan projection.

A novel optical dosimetry imaging system composed of a 1 kHz intensified complementary metal-oxide semiconductor camera, stereo-vision system, and a deformable scintillator array was deployed at a clinical proton therapy center for end-to-end validation. A conventional lung PBS plan was modified to simulate FLASH treatment at 99 nA and 250 MeV. Validation was performed using an anthropomorphic chest phantom through cumulative dose comparison with gafchromic film, spot position analysis, and PBS dose rate area verification. Clinical workflow was evaluated for impact on treatment time and compatibility to prepare the system for large animal and clinical studies.

Scintillation imaging showed high gamma passing rates comparing the cumulative dose distribution to film (92.5% at 1%/1 mm, >99.9% at 2%/2 mm). Spot monitoring achieved submillimeter accuracy (0.32 ± 0.19 mm deviation). Planned and measured surface dose rates showed excellent agreement, with a 0.71% difference in coverage at 40 Gy/s. Time added to treatment workflow was limited to array placement, contributing ∼1 minute. Postbeam array activation showed negligible additional exposure (671 nSv/h).

This study presents the first practical implementation of scintillator array dosimetry for UHDR PBS proton therapy, enabling real-time dose and dose rate monitoring over complex geometries. The system offers a novel approach to in vivo treatment validation with clinical compatibility, providing unique metrics for UHDR proton beam dynamics.