Individual pulse monitoring and dose control system for pre-clinical implementation of FLASH-RT

Abstract

Objective. Existing ultra-high dose rate (UHDR) electron sources lack dose rate independent dosimeters and a calibrated dose control system for accurate delivery. In this study, we aim to develop a custom single-pulse dose monitoring and a real-time dose-based control system for a FLASH enabled clinical linear accelerator (Linac). Approach. A commercially available point scintillator detector was coupled to a gated integrating amplifier and a real-time controller for dose monitoring and feedback control loop. The controller was programmed to integrate dose for each radiation pulse and stop the radiation beam when the prescribed dose was delivered. Additionally, the scintillator was mounted in a solid water phantom and placed underneath mice skin for in vivo dose monitoring. The scintillator was characterized in terms of its radiation stability, mean dose-rate ( D ̇ m ), and dose per pulse (D p) dependence. Main results. The D p exhibited a consistent ramp-up period across ∼4–5 pulse. The plastic scintillator was shown to be linear with D ̇ m (40–380 Gy s−1) and D p (0.3–1.3 Gy Pulse−1) to within +/− 3%. However, the plastic scintillator was subject to significant radiation damage (16%/kGy) for the initial 1 kGy and would need to be calibrated frequently. Pulse-counting control was accurately implemented with one-to-one correspondence between the intended and the actual delivered pulses. The dose-based control was sufficient to gate on any pulse of the Linac. In vivo dosimetry monitoring with a 1 cm circular cut-out revealed that during the ramp-up period, the average D p was ∼0.045 ± 0.004 Gy Pulse−1, whereas after the ramp-up it stabilized at 0.65 ± 0.01 Gy Pulse−1. Significance. The tools presented in this study can be used to determine the beam parameter space pertinent to the FLASH effect. Additionally, this study is the first instance of real-time dose-based control for a modified Linac at ultra-high dose rates, which provides insight into the tool required for future clinical translation of FLASH-RT.

Characterization of a diode dosimeter for UHDR FLASH radiotherapy

“…18 A W1 point scintillator detector was coupled to a gated integrating amplifier and a real-time controller for dose monitoring and feedback control loop. 24 The controller was programmed to integrate dose per pulse when the sync pulse measurement exceeded a threshold and provide feedback to the LINAC when the prescribed dose was delivered, which stopped the beam after delivery of desired dose through a reed relay connected to a realtime position management gating switchbox (Varian Inc, CA). For comparison of the EDGE Detector to W1 and radiochromic film, the experimental setup shown in Figure 1a was used.…”

“…The calibration of the W1 scintillation was based on the manufacturer's suggestions for determining calibration coefficients (gain and Cherenkov to light ratio [CLR]). 24 This setup was used to measure the detectors' response with respect to varying mean dose rate, dose, dose per pulse, and per pulse beam output. The mean dose rate was varied by changing the repetition rate of the LINAC, while the dose per pulse was varied by changing the sourceto-surface distance.…”

“…The dose was varied by utilizing the beam stopping mechanism integrated with the W1 scintillator. 24 For comparison of the EDGE Detector to IC the setup shown in Figure 1b was used, again the EDGE and IC measured output simultaneously during each delivery from the LINAC. The IC was in the bremsstrahlung tail (17 cm depth) of the electron beam to mitigate potential ion recombination effects from UHDR beams.…”

“…22 In this study, an EDGE Detector (Sun Nuclear Corp, Melbourne, FL) with a modified electrometer was characterized in UHDR electron beams with regards to dose linearity, mean dose rate dependence, and dose per pulse dependence of the detector response. A W1 scintillator detector 23 studied 23,24 at conventional and UHDR conditions respectively (Standard Imaging, Mid-dleton, WI), and EBT-XD Gafchromic film studied 25 at UHDR conditions (Ashland Advanced Materials, Bridgewater NJ), have been demonstrated to be dose rate independent in previous studies, and an IC (Exradin A28, Standard Imaging, Middleton, WI) served as reference.The IC was positioned in the bremsstrahlung tail of the beam to circumvent its dose rate dependence. The beam's depth and temporal profiles were then characterized with the EDGE Detector and compared with film and W1 detectors, respectively.…”

Technical note: Measurement of the bunch structure of a clinical proton beam using a SiPM coupled to a plastic scintillator with an optical fiber

“…To ensure the clinical observation of the FLASH effect, its reproducibility and translation, beam parameters must be thoroughly measured in vivo. A recent work performed by Ahsraf et al 50 using a point scintillator detector has demonstrated this possibility in a murine model, by measuring the number of pulses and the dose per pulse. Moreover, if the exit dose is measured, the procedure becomes minimally invasive.…”