Thermoacoustic Range Verification During Pencil Beam Delivery of a Clinical Plan to an Abdominal Imaging Phantom

Purpose/Objective(s) The purpose of this phantom study is to demonstrate that thermoacoustic range verification could be performed clinically. Thermoacoustic emissions generated in an anatomical multimodality imaging phantom during delivery of a clinical plan are compared to simulated emissions to estimate range shifts compared to the treatment plan. Materials/Methods A single-field 12-layer proton pencil beam scanning (PBS) treatment plan prescribing 6 Gy/fraction was delivered to a triple modality (CT, MRI, and US) abdominal imaging phantom made of hydrogels (CIRS 057a). A superconducting synchrocyclotron delivered ∼2 cGy/pulse at an average dose rate of approximately 20 Gy/sec, with 0.5% duty cycle. High-dose spots received approximately 40 cGy. Data was acquired by four acoustic receivers rigidly affixed to a linear ultrasound array. Acoustic receivers (transducer + amplifier) tuned to this application provided 15-25 dB amplification relative to 1 mV/Pa over 10-100 kHz. Receivers 1-2 were located distal to the treatment volume, whereas 3-4 were lateral. Receivers’ room coordinates were computed relative to the ultrasound image plane after co-registration to the planning CT volume. For each prescribed beamlet, a MCsquare Monte Carlo simulation of energy density provided initial pressure from which thermoacoustic emissions were computed using k-Wave. Emissions from beams that stopped in soft tissue were bandlimited below 100 kHz∼15 mm. To overcome the diffraction limit, range shifts were computed from time shifts between simulated and measured emissions. Results Shifts were small for high-dose beamlets that stopped in soft tissue. Signals acquired by channels 1-2 yielded shifts of -0.2 ± 0.7 mm relative to Monte Carlo simulations for high dose spots (∼40 cGy) in the second layer. Additionally, for beam energy ≥125 MeV, thermoacoustic emissions qualitatively tracked lateral motion of pristine beams in a layered gelatin phantom, and time shifts induced by changing phantom layers were self-consistent within nanoseconds. Conclusion Thermoacoustic range verification is feasible for a hypofractionated protocol delivering 6 Gy/fraction at a conventional dose rate. Improving receive sensitivity by another 20 dB is WIP and could enable adaptive planning for 2 Gy fractionation or online verification before an entire hypofractionated dose is delivered at a conventional dose rate. SNR is proportional to dose/pulse so FLASH delivery with short (FWHM