A Simulation of Ion-Acoustic Signals After Proton FLASH Radiation From a Synchrocyclotron

Abstract

<h3>Purpose/Objective(s)</h3> FLASH radiation therapy (FLASH-RT) is an emerging treatment modality that promises to increase the therapeutic index of radiotherapy. FLASH-RT consists of delivering treatment doses of radiation within an extremely short irradiation time. Proton therapy can produce the very high instantaneous dose rate necessary for proton FLASH-RT using a synchrocyclotron. Additionally, the maximum dose of proton FLASH-RT that is tuned to release at a precise depth could generate an ion-acoustic signal strong enough to reach the body surface overcoming all the interferences and attenuation. To translate proton FLASH-RT to the clinic, precise dosimetry and dose administration are critically important. One wants to verify that the high dose of radiation is delivered to the right location. Ion-acoustic imaging is an attractive modality for Bragg peak range verification for proton FLASH-RT. However, standard ultrasound transducers, which typically operate in the 3.5-7.5 MHz range, are not tuned to capture the ion-acoustic signal from proton FLASH-RT, which could range from 10-100 kHz. Novel ultrasound transducers with lower resonance frequencies are needed to make ion-acoustic imaging for proton FLASH-RT a reality. In this study, we hypothesize that simulating ion-acoustic signals imaging with parameters derived from an actual proton FLASH-RT experiment can determine the target frequency response of a potential ion-acoustic transducer. <h3>Materials/Methods</h3> We simulated the propagation of an ion-acoustic mechanical wave through an anatomical phantom using the k-wave toolbox. The proton FLASH synchrocyclotron, whose parameters we were simulating, can deliver a dose of 22,000 cGy in 750 pulses of ∼10 us in duration. Therefore, we simulated one pulse using a gaussian whose FWHM was 10 us and whose integral summed to 29.333 cGy. The size of the beamlet was about 1cm in diameter. The dose was converted to pressure using the following equation: δp = ΓρD, where D is the dose delivered, Γ is the dimensionless Grüneisen parameter, and ρ is target density. For the simulation, D = 29.333 cGy, Γ = 0.1, and ρ = 1000. The anatomical phantom was a slice of an abdominal CT scan whose Hounsfield units were converted to density and speed of sound using a hounsfield2density function. The dose was delivered to the center of the phantom. Ultrasonic sensors were placed along the surface of the phantom. <h3>Results</h3> The average frequency response of all the sensors was obtained for the simulation. The resonance frequency of the average frequency response was 38.8 kHz. <h3>Conclusion</h3> In this study, we demonstrate that the k-wave toolbox can be used to simulate the propagation of the ion-acoustic mechanical wave generated from proton FLASH-RT. This simulation can be used to guide the development of novel ultrasonic transducers. The results of this simulation indicate that ultrasonic transducers with lower resonance frequencies are needed.