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

Abstract

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.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.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.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 < 6 us) pulses would increase efficiency and could enable online verification before 1 Gy is delivered. Table 1. Time and range shifts between measured and simulated data compared to initial pressure per proton pulse and cumulative dose within the planned treatment spot. Beamlets from layer 2 that stopped in soft tissue in boldface.