Purpose: 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.
Methods: A single-field 12-layer proton pencil beam scanning (PBS) treatment plan created in Pinnacle prescribing 6 Gy/fraction was delivered by a superconducting synchrocyclotron to a triple modality (CT, MRI, and US) abdominal imaging phantom. Data was acquired by four acoustic receivers rigidly affixed to a linear ultrasound array. Acoustic receivers (transducer + amplifier) were 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 Monte Carlo simulation of the energy density provided initial pressure from which simulated emissions were computed. To overcome the diffraction limit, range estimates were computed by comparing measured to simulated 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: Acoustic receivers tuned to spectra of thermoacoustic emissions may enable range verification during proton therapy. Improving receive sensitivity by another 20 dB could enable range verification during delivery of a standard 2 Gy fraction.
Funding Support, Disclosures, and Conflict of Interest: Partial funding from NIH-NCI SBIR #R43CA243764. Dr. Patch founded Acoustic Range Estimates and holds US patent #10,758,127. Mr. Nguyen was employed by A.R.E. Dr. Lambert is employed by The Rutherford Cancer Centers. Drs. Janssens and Labarbe are employed by IBA. Dr. Lynch and Mr. Ono are employed by CIRS.