Implementation of novel measurement‐based patient‐specific QA for pencil beam scanning proton FLASH radiotherapy

Abstract

Background Several studies have shown pencil beam scanning (PBS) proton therapy is a feasible and safe modality to deliver conformal and ultra‐high dose rate (UHDR) FLASH radiation therapy. However, it would be challenging and burdensome to conduct the quality assurance (QA) of the dose rate along with conventional patient‐specific QA (psQA). Purpose To demonstrate a novel measurement‐based psQA program for UHDR PBS proton transmission FLASH radiotherapy (FLASH‐RT) using a high spatiotemporal resolution 2D strip ionization chamber array (SICA). Methods The SICA is a newly designed open‐air strip‐segmented parallel plate ionization chamber, which is capable of measuring spot position and profile through 2 mm‐spacing‐strip electrodes at a 20 kHz sampling rate (50 μs per event) and has been characterized to exhibit excellent dose and dose rate linearity under UHDR conditions. A SICA‐based delivery log was collected for each irradiation containing the measured position, size, dwell time, and delivered MU for each planned spot. Such spot‐level information was compared with the corresponding quantities in the treatment planning system (TPS). The dose and dose rate distributions were reconstructed on patient CT using the measured SICA log and compared to the planned values in volume histograms and 3D gamma analysis. Furthermore, the 2D dose and dose rate measurements were compared with the TPS calculations of the same depth. In addition, simulations using different machine‐delivery uncertainties were performed, and QA tolerances were deduced. Results A transmission proton plan of 250 MeV for a lung lesion was planned and measured in a dedicated ProBeam research beamline (Varian Medical System) with a nozzle beam current between 100 to 215 nA. The worst gamma passing rates for dose and dose rate of the 2D SICA measurements (four fields) compared to TPS prediction (3%/3 mm criterion) were 96.6% and 98.8%, respectively, whereas the SICA‐log reconstructed 3D dose distribution achieved a gamma passing rate of 99.1% (2%/2 mm criterion) compared to TPS. The deviations between SICA measured log, and TPS were within 0.3 ms for spot dwell time with a mean difference of 0.069 ± 0.11 s, within 0.2 mm for spot position with a mean difference of −0.016 ± 0.03 mm in the x‐direction, and −0.036 ± 0.059 mm in the y‐direction, and within 3% for delivered spot MUs. Volume histogram metric of dose (D95) and dose rate (V40Gy/s) showed minimal differences, within less than 1%. Conclusions This work is the first to describe and validate an all‐in‐one measurement‐based psQA framework that can fulfill the goals of validating the dose rate accuracy in addition to dosimetric accuracy for proton PBS transmission FLASH‐RT. The successful implementation of this novel QA program can provide future clinical practice with more confidence in the FLASH application.

A comprehensive pre‐clinical treatment quality assurance program using unique spot patterns for proton pencil beam scanning FLASH radiotherapy

“…This study presented high consistent (variations < 0.2 ms) spot dwell time and transition time over a long‐term observation period. The impact of scanning speed variations on dose rate was also found to be relatively minimal by Huang et al., 44 in which a 10% discrepancy in scanning speed results in a 1% effect on the resultant dose rate for typical clinical plans.…”

Proton 3D dose measurement with a multi-layer strip ionization chamber (MLSIC) device

“…The ProBeam system delivers proton spots in quasi-continuous mode (∼72 MHz frequency). The 'raster mode' scanning leads to ambiguous separation between individual spots (beam stays on between planned spots) (Huang et al 2023). As a result, the spot 'spills' between the discrete spots in the clinical plan.…”

FLASH instead of proton arc therapy is a more promising advancement for the next generation proton radiotherapy

“…18,19,83 With substantial resources and time devoted to refining the FLASH irradiation system, quality assurance devices, and radiation biology models, the path toward clinical implementation is being actively pursued. [87][88][89][90] Proton FLASH therapy may gain momentum as more clinical evidence and biological data are accumulated. These two developments are not mutually exclusive, and their coexistence can bring a new era of fast, efficient, precise, and effective personalized therapy.…”

“…Proton FLASH therapy has shown promising results, and ongoing investigations aim to address the remaining challenges 18,19,83 . With substantial resources and time devoted to refining the FLASH irradiation system, quality assurance devices, and radiation biology models, the path toward clinical implementation is being actively pursued 87–90 …”