Design and performance evaluation of a novel hybrid ionisation chamber and transmission calorimeter for dose rate independent beam monitoring in UHDR-RT

Journal article

Background. Ultra-High Dose Rate Radiotherapy (UHDR-RT) is an emerging cancer treatment that delivers radiation at dose rates exceeding 40 Gy/s. While UHDR-RT offers potential therapeutic benefits, accurate dosimetry presents significant challenges. Ionisation chambers, the standard for clinical dosimetry as reference and monitor dosimeters, experience significant ion recombination at high dose rates, reducing measurement accuracy and increasing uncertainty when corrections are applied. In contrast, calorimeters are dose rate independent and potentially more reliable for accurate UHDR-RT dosimetry. However, their slow thermal response limits their use in dynamic beam monitoring. Purpose. This study investigates whether a hybrid dosimeter, combining an ionisation chamber and two transmission calorimeters, can effectively provide dose-rate independent beam monitoring for both UHDR-RT and conventional radiotherapy (CONV-RT). The goal is to leverage the fast response of ionisation chambers for routine use, while maintaining the dose rate independence of calorimetry for periodic calibrations, to develop a prototype dose rate independent beam monitor. Methods. The hybrid dosimeter was designed and evaluated using the finite element modelling software, COMSOL, to assess the interactions between the ionisation and calorimeter components. Unwanted electrical and thermal effects were studied, including their impact on electric field distribution, charge collection efficiency, and self-heating effects on calorimetry. After construction, the device was tested at the FLASH Core Facility, University of Oxford, in a 6 MeV UHDR-RT beam. Experimental investigations were conducted under various beam conditions to evaluate performance. Results. The hybrid dosimeter demonstrated a linear response in both calorimeter cores and the ionisation chamber. Minimal electrical interference was observed in the proximal calorimeter core due to the ionisation process, though this effect was deemed negligible. The ion collection efficiency was measured to be approximately 3% during the highest dose delivery of 6 Gy/pulse (5.4 MGy/s instantaneous dose rate). A Jaffé plot method derived ion recombination factor for a 2 Gy/pulse (1.8 MGy/s instantaneous dose rate) measurement was determined to be 7.84 ± 2.35 (k = 1), with a calorimeter derived correction determined to be at least 5.22 ± 0.49 (k = 1). Conclusions. This study successfully demonstrates the feasibility of combining ionisation and calorimetry into a single hybrid dosimeter. The device offers the potential for online beam monitoring with dose rate independence, making it a promising candidate for UHDR-RT dosimetry and further development for clinical use.

Authors: Flynn, S; Habgood, R; Bass, G; Subiel, A; Thomas, R

• Publication status: Published
• Peer review status: Peer reviewed
• Publisher: IOP Publishing
• Journal: Journal of Instrumentation
• Volume: 21
• Issue: 06
• Pages: P06001
• Article number: P06001
• Publication date: 2026-06-01
• Acceptance date: 2026-03-26
• DOI: 10.1088/1748-0221/21/06/p06001
• EISSN: 1748-0221
• ISSN: 1748-0221
• Language: English
• Keywords: Dosimetry concepts and apparatus; Radiation monitoring