Establishing a framework for dosimetry in ultra-short pulsed high dose rate “FLASH” electron radiations.

Over 50% of all cancer patients receive radiotherapy during their treatment. The tumor can be eradicated by a focused beam of ionizing radiation. The efficacy of this treatment, however, is limited by the toxicity of the organs surrounding the tumor. In 2014 it was discovered that the healthy tissue toxicity could drastically be reduced, with conservation of the anti-tumoral effect, by delivering the dose in milliseconds rather than minutes. These ultra-high dose rates (UHDR) needed to obtain the so-called FLASH effect pose dosimetric challenges as most conventional dosimeters saturate.
The aim of my PhD was to provide the tools for a dosimetric framework for UHDR electron radiotherapy. During my research I investigated working dosimeters for real-time and time-resolved point dosimetry, and for passive and time-resolved 2D dosimetry. I assessed the average dose rate, dose per pulse (for pulse durations < 4 ms), pulse repetition frequency (PRF), instantaneous dose rate and energy (in)dependence of these candidate dosimeters.
I collaborated with different research groups for the work on real-time UHDR point dosimeters. During these collaborations, we showed the UHDR suitability of the flashDiamond up to 20 Gy per pulse, a SiC thin membrane dosimeter up to 2 Gy per pulse and an ultra-thin ionization chamber up to 10 Gy per pulse. In addition, a theoretical description of a gas-filled ionization chamber was presented that would allow operation up to a dose per pulse of 40 Gy. Finally, we performed a proof-of-principle study for a novel method to determine the ion recombination correction factor in UHDR, based on artificial intelligence.
I manufactured and characterized point scintillator dosimeters for time-resolved UHDR point dosimetry. The in-house made point scintillators had sufficient temporal resolution to measure the pulse structure of the beam (up to a PRF of at least 245 Hz), but showed saturation effects at a dose per pulse > 1 Gy. The commercial Hyperscint-RP200 system showed no saturation and superior stem signal removal, but could only measure individual pulses for PRF < 12 Hz.
An optically stimulated luminescence sheet was investigated as solution for passive 2D dosimetry for UHDR. The system showed suitable for doses per pulse up to at least 4.3 Gy and superior ease of use compared to radiochromic film. However, it also showed to be susceptible to physical damage, resulting in systematic errors and need for recalibration.
Finally, a scintillating sheet-camera setup was investigated for time resolved 2D dosimetry in UHDR. It showed suitable for a dose per pulse up to at least 1.8 Gy. The temporal resolution showed sufficient to measure the pulsed beam structure, however, triggering was needed to measure all relevant pulses. The use of this setup in a preclinical setting was tested and found adequate. However, the specific sheets used in the preclinical setup showed a characterizable dose dependence that needed correction.
This work provides the basis to develop a dosimetric framework for UHDR electron beams. It presents potential tools for real-time and time-resolved point dosimetry, and for passive and time-resolved 2D dosimetry.