How precisely to deliver pencil beam scanned proton FLASH? Robustness of dose rate to AAPM TG224

Aylward, Jack

Abstract

Purpose/Objective:
The American Association of Physicists in Medicine (AAPM) Task Group Report 224: Comprehensive Proton Therapy Machine Quality Assurance gives recommendations for the delivery of Pencil Beam Scanned (PBS) proton radiotherapy. The tolerances given in the report have been determined based on the literature and consensus data demonstrating the effect of delivery deviations on associated dose distributions. The appropriateness of these recommendations have not yet been considered for FLASH applications where the effect on the dose rate distribution is also relevant. This work assesses the suitability of these recommendations for a FLASH application, where the robustness of the dose rate to each voxel in a 3D volume should be maintained.
Material/Methods:
A PBS proton model was created using a 245 MeV proton DICOM dose taken from EclipseTM TPS. A regular square 30 x 30 mm scanning pattern was created using Python V3.11 (defined as the nominal scenario). Simulated errors in dose output, spot position, spot size, proton range, as well as beam pause and current fluctuations were introduced, each of the same magnitude of AAPM TG 224 tolerances where appropriate (defined as the error scenario). Dose Averaged Dose Rate (DADR) [1] and PBS Dose Rate (PBSDR) [2] with a 2.5% threshold, were determined and compared for both the nominal and error scenarios. Maximum dose rate difference was reported for both the
transmission (plateau) and Bragg Peak section of the distribution for each error scenario. Qualitative analysis of the dose rate distributions was also carried out, and dose rate volume histograms were calculated to assess the effect of the errors.
Results:
The dose rate in the bragg peak is robust to errors in output and spot position. The dose rate in the transmission (plateau) region is robust to errors in output and proton range. A spot position error resulted in a dose difference of 3.1% in the bragg peak and 9.3% in the transmission region, and a DADR difference of 4.4% and 10.0% in the bragg peak and transmission regions respectively (fig 1). A proton range error resulted in a DADR difference of 0.7% and 44.3% in the transmission and bragg peak regions respectively. Spot size, beam pause, and current fluctuations causes dose rate deviations in both the bragg peak and plateau regions of the dose rate distribution.
Conclusion:
The suitability of a number of AAPM TG 224 recommendations have been assessed for both Bragg Peak and transmission Ultra-High Dose Rate proton PBS deliveries. This work informs the precision requirements for PBS proton FLASH.

• Presentation Conference Type: Conference Abstract
• Conference Name: European Society of Radiation Oncology Congress
• Start Date: May 3, 2024
• End Date: May 7, 2024
• Acceptance Date: May 1, 2024
• Online Publication Date: Aug 30, 2024
• Publication Date: 2024-05
• Deposit Date: Sep 10, 2024
• Journal: Radiotherapy and Oncology
• Print ISSN: 0167-8140
• Publisher: Elsevier
• Peer Reviewed: Peer Reviewed
• Volume: 194
• Issue: Supplement 1
• Pages: S4936-S4938
• DOI: https://doi.org/10.1016/s0167-8140%2824%2903138-4
• Public URL: https://uwe-repository.worktribe.com/output/12875815