The Quasi-Adiabatic Graphite Calorimeter for Absolute Dosimetry in Ultra-High Dose Rate (FLASH) X-Ray Radiotherapy.

Abstract

FLASH radiotherapy using high-energy X-rays combines ultra-high dose-rate irradiation with the physical characteristics of high-energy X-ray beams, achieving a significant reduction in normal tissue biological damage while maintaining sufficient tissue penetration, thereby presenting great potential for clinical translation. However, the absence of a traceable absolute dosimetry method for FLASH X-ray beams remains a substantial limitation to its clinical implementation. This study aims to establish a quasi-adiabatic water-controlled probe-type graphite calorimeter for the absolute measurement of absorbed dose to water in the 10 MV FLASH X-ray beam, to address the current lack of a traceable dosimetry standard for X-ray FLASH radiotherapy. A probe-type graphite calorimeter was developed, employing thermally stabilised water as the thermal control medium to precisely regulate the thermal equilibrium of the graphite core. This quasi-adiabatic system is designed to facilitate accurate absolute dose measurements under ultra-high dose rate conditions. The results indicate that for a single irradiation with a total dose exceeding 2 Gy, the mean Type A relative uncertainty, determined from five repeated measurements using the sample standard deviation, is less than 0.2%. By deriving the necessary correction factors for determining the absolute dose (i.e., in Gy) of FLASH photon radiotherapy, the uncertainty in water-absorbed dose measurement is determined to be 1.0% (1σ). This study develops a probe-type graphite calorimeter for the absolute measurement of absorbed dose to water in 10 MV FLASH X-ray beams. The system is designed to address the current lack of a traceable dosimetric standard for X-ray FLASH radiotherapy, thereby supporting its clinical translation and application.