High-resolution scintillation imaging for detecting delivery errors in preclinical ultrahigh dose rate proton beam
Megan Clark1, Roman Vasyltsiv1, David Gladstone1
1Medical Physics, Radiation Onology, Stanford University, Palo Alto, California, USA.
Background:
Ultrahigh dose rate (UHDR) pencil beam scanning (PBS) proton therapy represents an emerging treatment modality that potentially reduces normal-tissue toxicities, termed the FLASH effect. Despite rapid clinical translation, accurate delivery of dose and dose rate is critical, and current quality assurance and beam monitoring methods have limited capabilities for detecting subtle delivery errors in both spatial and temporal domains. The limitations of existing dosimetry tools make robust characterization and validation of treatment plans during patient-specific quality assurance (PSQA) challenging.
Purpose:
To test a previously validated, high-resolution scintillation imaging dosimetry (SID) system for detecting clinically relevant deviations in proton beam spot position and intensity during UHDR particle beam therapy. The secondary aim of this study was to investigate the impact of spot position and intensity errors on dose rate, emphasizing the need for high spatial and temporal resolution detection systems.
Methods:
Treatment plans were created for a Varian ProBeam system operating in FLASH mode. Two types of plans were developed, delivered, and imaged: a uniform diamond-shaped and a more complex plan derived from a stereotactic lung treatment protocol. A high-speed (4500 frames per second) imaging system was used to capture temporally and spatially resolved data of light output from a scintillator at isocenter during both types of UHDR PBS deliveries. Image processing was performed in MATLAB, calculating relevant treatment parameters such as cumulative dose, dose per spot, temporal dose deposition throughout the treatment field (dose rate mapping), and dose/dose rate area histograms.
Results:
The imaging system was able to detect deviations in spot position as low as 0.5 ± 0.3 mm and in intensity as low as 7 cGy per spot. Imaging different preclinical treatment fields demonstrated the impact of potential treatment errors on dose rates and dose rate area histograms, with deviations in spot position of 3 mm demonstrating an impact of ± 8% dose rate variations, for example. The spatial and temporal aspects of UHDR PBS deliveries were investigated, highlighting the importance of current high-resolution detectors.
Conclusions:
High-resolution scintillation imaging effectively identifies simulated beam delivery errors in UHDR proton therapy, revealing critical relationships between spatial accuracy and dose rate distribution.
