Thermoacoustic range verification during pencil beam delivery of a clinical plan to an abdominal imaging phantom

•Fast (4-6 us) and stress-confined, delivery enables thermoacoustic range verification.•Submillimeter range accuracy was achieved for beamlets that delivered ~0.5 Gy to soft tissue.•Estimates are accurate relative to ultrasound images soundspeed inhomogeneity•Synchrocyclotrons deliver stress-confined proton pulses.•Acoustic ringing indicated beam traversed bone; metallic implants will also cause ringing.