Characterization of 250 MeV Protons from the Varian ProBeam PBS System for FLASH Radiation Therapy

Design and characterization of a novel scintillator array for UHDR PBS proton therapy surface dosimetry

BACKGROUND

Ultrahigh dose rate (UHDR) proton therapy has shown promise in normal tissue sparing by enhancing the therapeutic ratio through a method termed the FLASH effect. As in all radiotherapy, accurate in vivo dosimetry is crucial for quality assurance of safe and efficient treatment delivery. However, this remains a challenge for UHDR as existing dosimetry systems lack the spatial and temporal resolution required to verify dose and dose rate in complex anatomical regions, especially for pencil beam scanning (PBS) proton therapy.

PURPOSE

This study aims to develop and evaluate a novel 3D surface dosimetry method for UHDR PBS proton therapy using high-speed imaging of a scintillator array, coupled with stereovision to provide real-time, high-resolution surface dose monitoring during treatment. The spatial, temporal, and dosimetric components of the proposed system are validated via imaging of a custom QA phantom and are compared against a gafchromic film reading of the same field delivered onto a flat surface.

METHODS

A freely deformable multielement scintillator array was designed with a single element pitch of 7.5 mm and interelement gap of 0.5 mm. Scintillation linearity with dose was evaluated along with the variation in scintillator response with increasing imaging and irradiation angles. Water-equivalent thickness (WET) testing was conducted to evaluate beam attenuation at two energy levels. Scintillation emission in response to dose delivery was imaged at 1000 Hz using a high frame rate camera (BeamSite Ultra, DoseOptics LLC) and the array position was monitored via a 2-camera stereovision system. Imaging system setup was validated using a custom 3D QA phantom to assess spatial accuracy and guide systematic setup correction. Stereovision properties of each array element were used to guide angular emission correction, and geometric transformation to beams-eye-view (BEV). Kernel-based residual spot fitting was applied to derive cumulative dose maps which were then compared to the flat film dose profile of a 5 × 5 cm UHDR PBS delivery using 3%/2 mm gamma analysis. PBS and maximum dose rate maps were also calculated.

RESULTS

System setup achieved an average localization error of 0.62 mm, surpassing the typical 1+ mm threshold used in clinical practice. Intensity correction based on angular information was applied and yielded a cumulative spot dose uncertainty of ∼1% (5.428 mGy). The processed dose map was compared to film via gamma analysis with 3%/2 mm criteria and showed a 99.9% passing rate, indicating high agreement between the planned and measured dose profiles. The WET of the scintillator array was measured to be 1.1 mm, minimizing its impact on dose distribution.

CONCLUSION

The novel scintillator array system provides accurate, real-time surface dose monitoring with high spatial and temporal resolution, making it a promising tool for in vivo dosimetry in UHDR proton therapy. Future work will focus on optimizing the system and expanding its application to other modalities, such as photon and electron therapy.

Whole Abdominal Pencil Beam Scanned Proton FLASH Increases Acute Lethality

PURPOSE

Ultrahigh dose-rate FLASH radiation therapy has emerged as a modality that promises to reduce normal tissue toxicity while maintaining tumor control. Previous studies of gastrointestinal toxicity using passively scattered FLASH proton therapy (PRT) have, however, yielded mixed results, suggesting that the requirements for gastrointestinal sparing by FLASH are an open question. Furthermore, the more clinically relevant pencil beam scanned (PBS) FLASH PRT has not yet been assessed in this context, despite differences in the spatiotemporal dose-rate distributions compared with passively scattered PRT. Here, to our knowledge, we provide the first report on the effects of PBS FLASH PRT on acute gastrointestinal injury in mice after whole abdominal irradiation.

METHODS AND MATERIALS

Whole abdominal irradiation was performed on C57BL/6J mice using the entrance channel of the Bragg curve of a 250 MeV PBS proton beam at field-averaged dose rates of 0.6 Gy/s for conventional (CONV) and 80 to 100 Gy/s for FLASH PRT. A 2D strip ionization chamber array was used to measure the dose and dose rate for each mouse. Survival was assessed at 14 Gy. Intestines were harvested and processed as Swiss rolls for analysis using a novel artificial intelligence-based crypt assay to quantify crypt regeneration 4 days after irradiation.

RESULTS

Survival was significantly reduced after 14 Gy FLASH PRT compared with CONV (P < .001). Our artificial intelligence-based crypt assays demonstrated no significant difference in intestinal crypts/cm or crypt depth between groups 4 days after irradiation. Furthermore, we found no significant difference in 5-ethynyl-2'-deoxyuridine+ cells/crypt or Olfactomedin4+ intestinal stem cells with FLASH relative to CONV PRT.

CONCLUSIONS

Overall, our data demonstrate significantly impaired survival after abdominal PBS FLASH PRT without apparent differences in intestinal histology 4 days after irradiation.

Feasibility of Synchrotron-Based Ultra-High Dose Rate (UHDR) Proton Irradiation with Pencil Beam Scanning for FLASH Research

BACKGROUND

This study aims to present the feasibility of developing a synchrotron-based proton ultra-high dose rate (UHDR) pencil beam scanning (PBS) system.

METHODS

The RF extraction power in the synchrotron system was increased to generate 142.4 MeV pulsed proton beams for UHDR irradiation at ~100 nA beam current. The charge per spill was measured using a Faraday cup. The spill length and microscopic time structure of each spill was measured with a 2D strip transmission ion chamber. The measured UHDR beam fluence was used to derive the spot dwell time for pencil beam scanning. Absolute dose distributions at various depths and spot spacings were measured using Gafchromic films in a solid-water phantom.

RESULTS

For proton UHDR beams at 142.4 MeV, the maximum charge per spill is 4.96 ± 0.10 nC with a maximum spill length of 50 ms. This translates to an average beam current of approximately 100 nA during each spill. Using a 2 × 2 spot delivery pattern, the delivered dose per spill at 5 cm and 13.5 cm depth is 36.3 Gy (726.3 Gy/s) and 56.2 Gy (1124.0 Gy/s), respectively.

CONCLUSIONS

The synchrotron-based proton therapy system has the capability to deliver pulsed proton UHDR PBS beams. The maximum deliverable dose and field size per pulse are limited by the spill length and extraction charge.