THGEM-Based Air Ionization Chamber for Online Dose Monitoring in Conventional and FLASH Radiotherapy.

Abstract

The aim of this work was to develop an online dose monitoring device that can be applied to both conventional radiotherapy and ultra-high dose rate radiotherapy. Thick Gas Electron Multiplier (THGEM) technology, with its uniform microstructure characteristics, emerges as an ideal solution to these challenges. This study introduces an innovative approach by covering both THGEM electrodes with Mylar conductive films, creating independent micro-ionization chambers in each microhole that effectively reduce charge density issues in high dose rate environments. Experimental results demonstrate that the developed THGEM air ionization chamber (THGEM-ADIC) exhibits nearly identical linear response to the standard PTW Farmer 30013 ionization chamber over a dose range of 1-100 Gy in conventional radiotherapy settings, with dose deviations within ±3% and a stable operational plateau of approximately 400 V. Accuracy at clinical single-field doses typical of IMRT or fractionated treatments remains to be investigated. More importantly, benefiting from its unique microstructure design, THGEM-ADIC has rapid signal response, enabling direct measurement of Flash-RT accelerator macro-pulse structures. In ultra-high dose rate working environments, the detector maintains an operational plateau of about 200 V and demonstrates a dose response with linearity exceeding 99% compared to EBT3 film dosimetry measurements, even at extreme dose rates up to 250 Gy/s. However, a dose-rate dependence was observed between 50 and 250 Gy/s under UHDR conditions, which should be taken into account for accurate dosimetry at ultra-high dose rates. Collectively, these results indicate that THGEM-ADIC, with its microstructure design and rapid response capabilities, not only provides measurement accuracy consistent with traditional ionization chambers in conventional radiotherapy but also enables precise dose monitoring under ultra-high dose rate Flash radiotherapy conditions, offering an important dosimetric tool for the clinical translation of Flash-RT technology.