A simulation study of ionizing radiation acoustic imaging (iRAI) as a real‐time dosimetric technique for ultra‐high dose rate radiotherapy (UHDR‐RT)

Abstract

Electron-based ultra-high dose rate radiation therapy (UHDR-RT), also known as Flash-RT, has shown the ability to improve the therapeutic index in comparison to conventional radiotherapy (CONV-RT) through increased sparing of normal tissue. However, the extremely high dose rates in UHDR-RT have raised the need for accurate real-time dosimetry tools. This work aims to demonstrate the potential of the emerging technology of Ionized Radiation Acoustic Imaging (iRAI) through simulation studies and investigate its characteristics as a promising relative in vivo dosimetric tool for UHDR-RT. Methods: The detection of induced acoustic waves following a single UHDR pulse of a modified 6 MeV 21EX Varian Clinac in a uniform porcine gelatin phantom that is brain-tissue equivalent was simulated for an ideal ultrasound transducer. The full 3D dose distributions in the phantom for a 1 × 1 cm 2 field were simulated using EGSnrc (BEAMnrc∖DOSXYZnrc) Monte Carlo (MC) codes. The relative dosimetry simulations were verified with dose experimental measurements using Gafchromic films. The spatial dose distribution was converted into an initial pressure source spatial distribution using the medium-dependent dose-pressure relation. The MATLAB-based toolbox k-Wave was then used to model the propagation of acoustic waves through the phantom and perform time-reversal (TR)-based imaging reconstruction. The effect of the various linear accelerator (linac) operating parameters, including linac pulse duration and pulse repetition rate (frequency), were investigated as well. Results:The MC dose simulation results agreed with the film measurement results, specifically at the central beam region up to 80% dose within approximately 5% relative error for the central profile region and a local relative error of <6% for percentage dose depth. IRAI-based FWHM of the radiation beam was within approximately 3 mm relative to the MC-simulated beam FWHM at the beam entrance. The real-time pressure signal change agreed with the dose changes proving the capability of the iRAI for predicting the beam position. IRAI was tested through 3D simulations of its response to be based on the temporal changes in the linac operating parameters on a dose per pulse basis as expected theoretically from the pressure-dose proportionality.The pressure signal amplitude obtained through 2D simulations was proportional to the dose per pulse. The instantaneous pressure signal amplitude decreases as the linac pulse duration increases, as predicted from the pressure wave generation

Image guidance for FLASH radiotherapy

“…It is worth noting that these images are formed from single Linac pulses, demonstrating the potential to use this technique for pulse‐to‐pulse dosimetry and beam localization (since US images can be taken in between Linac pulses to give real‐time anatomical feedback), which will be vital for FLASH‐RT. However, the application of this system in clinical FLASH‐RT may benefit from further optimization of the acquisition system as suggested by recent Monte Carlo simulations 75 …”

“…However, the application of this system in clinical FLASH-RT may benefit from further optimization of the acquisition system as suggested by recent Monte Carlo simulations. 75 Depending on the pulsing time (micro)structure and peak current, iRAI promises to be also an attractive method to monitor the Bragg peak position (if stopped within the patient) and reconstruct the delivered dose in FLASH particle therapy. 47,76 In fact, despite the still ongoing developments in terms of optimal detector technology (especially regarding sensitivity and bandwidth) and the current limitations to proof -of -concept studies primarily at standard dose rates from commercially available synchrocyclotrons or artificially pulsed cyclotrons and synchrotrons with controller circuits, 77 the prospects of ultra-high dose rate delivery for FLASH-RT are expected to favor detectability of iRAI signals with protons and light ions at deeper tissues, potentially also on a single pulse or microstructure basis in high-frequency accelerators.…”

4D in vivo dosimetry for a FLASH electron beam using radiation-induced acoustic imaging

“…Based on the foregoing, simulation studies have already indicated the potential of RAI in capturing images of FLASH beams, applicable to both protons (Kim et al 2023) and electrons (Ba Sunbul et al 2021). Notably, recent experimental research has showcased the use of RAI with two linear arrays in an orthogonal setup for detecting the edges of FLASH electron beams up to 25 cGy, proving its feasibility (Oraiqat et al 2020).…”