Beam Monitors for Tomorrow: The Challenges of Electron and Photon FLASH RT

Abstract

The emergent FLASH RadioTherapy (RT) uses ultrahigh dose-rate irradiation (up to 10 7 Gy/s instantaneous dose-rate in each µs pulse) to deliver a single high dose of irradiation in a very short time (<200 ms). Pre-clinical studies at ultrahigh dose-rates recently showed an increased ratio between tumoricidal effect and normal tissue toxicity (therapeutic index), compared to conventional RT at standard Gy/min dose-rates. If confirmed by biological in vivo validations, this could represent a breakthrough in cancer treatment. However, the reliability and the accuracy of experimental studies are nowadays limited by the lack of detectors able to measure online the beam fluence at FLASH dose-rates. The behavior of standard beam monitors (gas-filled ionization chambers) is compromised by the volume recombination caused by the amount of charges created per unit volume and unit time, due to the large dose-rate. Moreover, due to the lack of proper monitoring devices and to the uncertainties of its future applications, very few facilities are able to deliver at present FLASH irradiations. In this contribution, we report about the physical and technological challenges of monitoring high and ultra-high dose-rates with electrons and photon beams, starting from the pre-clinical and clinical constraints for new devices. Based on the extensive experience in silicon detectors for monitoring applications in RT with external beams, the work then investigates silicon sensors as a possible option to tackle such extreme requirements and a rugged thin and large (e.g., 10 × 10 cm 2) flat detector (silicon-based sensor + readout electronics) is therefore outlined. This study aims at presenting the FLASH-RT dosimetry problem and analyzing the possibilities for a silicon sensor to be employed as sensing device for several FLASH scenarios, including some ideas on the readout part. However, more detailed simulations and studies are demanded to delineate more precisely the technical choices to be undertaken in order to tackle the clinical accuracy required on the beam fluence, typically a few %, during photon and electron high and ultra-high irradiations, the required minimal perturbation of the beam and the high level of radiation resistance.

Design, realization, and characterization of a novel diamond detector prototype for FLASH radiotherapy dosimetry

“…As far as solid‐state detectors are concerned, silicon diodes exhibit response nonlinearities as a function of the DPP. 25 , 39 In addition, the effect of radiation damage is to be considered as well, further limiting the suitability of such detectors in FLASH‐RT. 11 The PTW 60019 diamond detector, developed by some of the authors at Rome Tor Vergata University in cooperation with PTW‐Freiburg, is well assessed in conventional RT dosimetry, due to its near tissue equivalence, radiation hardness, and good spatial resolution.…”

“…However, the use of this dosimetric system in clinical environments is often impractical. As far as solid‐state detectors are concerned, silicon diodes exhibit response nonlinearities as a function of the DPP 25,39 . In addition, the effect of radiation damage is to be considered as well, further limiting the suitability of such detectors in FLASH‐RT 11 .…”

Application of a novel diamond detector for commissioning of FLASH radiotherapy electron beams

“…8,25,27 This results in an additional uncertainty in the dose rate determination due to possible fluctuations of the accelerator output. On the other hand, no commercial real-time response dosimetric systems are available yet for UH-DPP applications, due to recombination, saturation, and nonlinearity effects typically observed in the response of ionization chamber (IC) 15,24,26,[28][29][30][31] and solid-state detectors such as silicon and diamondbased diodes. 8,15,20,31 Novel detector prototypes are being designed and characterized by several research groups, [30][31][32] whose properties are expected to overcome these limitations.…”

“…On the other hand, no commercial real-time response dosimetric systems are available yet for UH-DPP applications, due to recombination, saturation, and nonlinearity effects typically observed in the response of ionization chamber (IC) 15,24,26,[28][29][30][31] and solid-state detectors such as silicon and diamondbased diodes. 8,15,20,31 Novel detector prototypes are being designed and characterized by several research groups, [30][31][32] whose properties are expected to overcome these limitations. In particular, a diamond Schottky diode detector prototype was recently proposed, 33 the so-called flashDiamond (fD), specifically designed for UH-DPP and UH-DR applications.…”

Translational Research in FLASH Radiotherapy—From Radiobiological Mechanisms to In Vivo Results

“…Jorge et al validated TLD, alanine pellets and films for absolute dosimetry against an Advanced Markus ionization chamber [ 37 ]. Vignati et al modelled the response of silicon dosimeters to investigate their potential for FLASH therapy dosimetry [ 38 ]. Oraiqat et al investigated the use of ionizing radiation acoustic imaging to provide real time 3D patient dosimetry [ 39 ].…”