ESTRO 2024 - Abstract Book

S4936

Physics - Quality assurance and auditing

ESTRO 2024

3068

Digital Poster

How Precisely to Deliver Pencil Beam Scanned Proton FLASH? Robustness of Dose Rate to AAPM TG224

Jack Aylward

University of Manchester, Division of Cancer Sciences, Manchester, United Kingdom. The Christie NHS Foundation Trust, Christie Medical Physics and Engineering, Manchester, United Kingdom. UDaresbury Laboratory, Cockcroft Institute, Warrington, United Kingdom

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.