Thermoacoustic range verification during conventional dose rate delivery by a synchrocyclotron

Background: Minimization of range uncertainties is critical for precise and effective proton therapy. Thermoacoustic range verification is a promising non-invasive technique for pinpointing the Bragg peak location in proton therapy. Purpose: Verification of a prototype device for thermoacoustic range verification with minimal disruption to a clinical workflow for proton therapy delivered by a synchrocyclotron. Materials/Methods: 5–8 pC/pulse were delivered to a gelatin ultrasound quality-assurance phantom by a Mevion Hyperscan S250i system in 200–300 pulses/spot. Spots were laterally separated by 25 mm; energy layers were separated in depth by 10 MeV. Thermoacoustic and nuclear emissions were captured by a wireless thermoacoustic range verifier, with a radiation detector on board. Dose maps for individual beamlets were exported from RayStation 12A and used to compute initial values for finite element acoustic simulation, assuming Gaussian beam current with σ = 0.95 µs. The plan was re-delivered with varying thicknesses of solid water equivalent phantom by Sun Nuclear placed on the snout. Range shifts and total dose were recomputed and loaded into RayStation for comparison to the original plan. To assess applicability to clinical plans, analysis was performed by averaging N = 16 pulses for which the radiation detector signal exceeded 50% of its maximum. Finite element results were bandpass filtered. Results: Using only N = 16 thermoacoustic pulses yielded best- and worst-case errors of (Formula presented.) and (Formula presented.), averaged over all spots. Averaged over all spots only in a single layer, best- and worst-case errors were 0.0 (Formula presented.) 1.3 mm and −1.3 (Formula presented.) 1.6 mm. Conclusions: A wireless range verification research device that can be setup in less than 10 min provided millimeter range accuracy in an ultrasound quality assurance phantom.