Toward real‐time, volumetric dosimetry for FLASH‐capable clinical synchrocyclotrons using protoacoustic imaging

Abstract Background Radiation delivery with ultra‐high dose rate (FLASH) radiotherapy (RT) holds promise for improving treatment outcomes and reducing side effects but poses challenges in radiation delivery accuracy due to its ultra‐high dose rates. This necessitates the development of novel imaging and verification technologies tailored to these conditions. Purpose Our study explores the effectiveness of proton‐induced acoustic imaging (PAI) in tracking the Bragg peak in three dimensions and in real time during FLASH proton irradiations, offering a method for volumetric beam imaging at both conventional and FLASH dose rates. Methods We developed a three‐dimensional (3D) PAI technique using a 256‐element ultrasound detector array for FLASH dose rate proton beams. In the study, we tested protoacoustic signal with a beamline of a FLASH‐capable synchrocyclotron, setting the distal 90% of the Bragg peak around 35 mm away from the ultrasound array. This configuration allowed us to assess various total proton radiation doses, maintaining a consistent beam output of 21 pC/pulse. We also explored a spectrum of dose rates, from 15 Gy/s up to a FLASH rate of 48 Gy/s, by administering a set number of pulses. Furthermore, we implemented a three‐dot scanning beam approach to observe the distinct movements of individual Bragg peaks using PAI. All these procedures utilized a proton beam energy of 180 MeV to achieve the maximum possible dose rate. Results Our findings indicate a strong linear relationship between protoacoustic signal amplitudes and delivered doses ( R 2 = 0.9997), with a consistent fit across different dose rates. The technique successfully provided 3D renderings of Bragg peaks at FLASH rates, validated through absolute Gamma index values. Conclusions The protoacoustic system demonstrates effectiveness in 3D visualization and tracking of the Bragg peak during FLASH proton therapy, representing a notable advancement in proton therapy quality assurance. This method promises enhancements in protoacoustic image guidance and real‐time dosimetry, paving the way for more accurate and effective treatments in ultra‐high dose rate therapy environments.