From Simulation Studies To Clinical Measurements: Challenges, Limitations, And Future Outlook In Proton Acoustic Range Verification

Proton therapy is gaining increased popularity in the last decade. However, full utilization of its utility to precisely localize the Bragg peak to the tumor volume during a patient treatment is challenging because of range uncertainties. Proton acoustic imaging promises a non-invasive and real-time treatment monitoring system to aid localization of the Bragg peak. The purpose of this work is to report our investigations of proton acoustic imaging, including computer simulations as well as preliminary experimental studies at clinical facilities. The ultimate achievable accuracy, sensitivity, and clinical translation challenges are discussed. Our simulation model included multiple clinical proton beam scenarios. Both pristine and spread-out Bragg peak were simulated. The generated acoustic pulse due to pressure rise was estimated followed by the wave propagation using finite element model. Since the ionoacoustic pulse is highly dependent on the proton pulse width and energy, multiple pulse widths were studied. Based on the received signal spectrum at piezoelectric ultrasound transducer with consideration of random thermal noise, maximum spatial resolution of the proton-acoustic imaging modality was calculated. The simulation studies defined the design specifications of the system to detect proton acoustic signal from Hitachi and Mevion clinical machines. A 500 KHz hydrophone with 100 dB amplification was set up in a water tank placed in front of the proton nozzle. The acquisition was synchronized by a trigger signal provided by the machine. The data was digitized and sent to the computer for real-time processing using our 40 MHz data acquisition system. The minimum number of protons detectable by the proton acoustic technique was on the order of 10×10ˆ6 per pulse, with 30–800 mGy dose per pulse at the Bragg peak. Wider pulses produced signal with lower acoustic frequencies, with 10 μs pulses producing signals with frequency less than 100 kHz. As the proton beam pulse width increases, the minimum number of protons for producing detectable acoustic signal increased exponentially. Our experimental measurements at clinical facilities support the simulation studies. The proton-acoustic process was investigated using both computer simulation and experimental measurements. Using our model, we have established the minimal detection limit for proton-acoustic range validation for a variety of pulse parameters. These limits correspond to a best case scenario with a single large detector. Our study indicated practical proton-acoustic range verification can be feasible with a pulse shorter than 10 μs, 5×10ˆ6 protons/pulse, 50 nA beam current and a highly sensitive ultrasonic transducer. The translational challenges into current clinical machines include proper magnetic shielding of the measurement equipment, providing a clean trigger signal from the machine, providing a shorter proton beam pulse and higher dose per pulse.