VHEE dosimetry in a FLASH: ultrahigh dose rate dosimetry and real-time beam monitoring for very high energy electron FLASH radiotherapy

Thesis

Very High Energy Electrons (VHEE) are a promising radiotherapy modality due to their increased penetration, reduced sensitivity to inhomogeneities, and delivery via scanning or focusing. VHEE beams at ultrahigh dose rates (UHDR) could be beneficial for treating deep-seated tumours using the FLASH effect, which selectively spares healthy tissues while maintaining effective tumour control. One of the main challenges in making VHEE FLASH treatment clinically viable is real-time dosimetry and beam monitoring, as ionisation chambers exhibit non-linear responses at UHDR due to recombination effects. This research addresses this challenge through the characterisation of VHEE interactions using Monte Carlo (MC) simulations, film dosimetry at the CLEAR Facility, and by developing a novel fibre array beam monitor.

Using TOPAS MC simulations, the interactions of VHEE beams were characterised. The dose distributions and resulting secondary particles generated from these interactions were evaluated to determine feasible in vivo dose verification methods for VHEE UHDR beams.

A radiochromic film dosimetry protocol was developed for VHEE FLASH experiments at the CLEAR Facility to ensure accurate dose measurements. Various Gaussian beam size determination methods were compared. Charge measurements using an integrated current transformer were correlated with dose-area-product measurements on radiochromic films for both UHDR and conventional irradiations. Radiochromic film measurements were also compared to those made with other passive dosimeters to ensure accuracy and reliability.

A novel optical fibre beam monitor was developed for real-time beam profile and dose monitoring at UHDR with VHEE beams. Consisting of silica fibres and a CMOS camera, the monitor was tested and characterised at the CLEAR Facility. A linear response with dose rate was demonstrated alongside accurate beam profile measurements for Gaussian and uniform beams. This shows real potential as a solution to address the critical need for accurate beam monitoring with VHEE FLASH radiotherapy.

Authors: Bateman, JJ

• Alternative title: Very high energy electron ultrahigh dose rate dosimetry and beam monitoring
• DOI: 10.5287/ora-e2zzvm42o
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: ultrahigh dose rate dosimetry; radiotherapy; FLASH effect; Monte Carlo simulations
• Subjects: Medical physics; Radiation dosimetry; Electron accelerators; Radiotherapy; Linear accelerators in medicine