Room: AAPM ePoster Library
Purposes: To measure resonant ionoacoustic waves generated from spherical metal markers and to explore the possible use of this phenomenon as an online range verification tool. Metal spherical markers used for patient positioning act as a strong pressure source when irradiated with proton beams; the markers briefly act as an acoustic transmitter. However, this phenomenon still has not been investigated experimentally.
Methods: A beam experiment was done in the fixed-field alternating gradient accelerator at Kyoto University, Japan; this facility can produce the pulsed beams that are required to generate resonant waves. A spherical gold marker (diameter, 2 mm) was set in a water phantom and proton pencil beams were irradiated while the range was modulated from 5.95 to 7.80 g/cm² using range shifters. Commercially available hydrophones which are not specialized for spherical wave measurements require averaging to identify resonant waves. Hence, to improve measurement sensitivity, a hydrophone and an amplifier specialized for the measurement of spherical waves with the resonance frequency were developed.
Results: Specific high-frequency waves were observed in one pulse irradiation. The beam intensity was about 1.2×108 particles/pulse. This is equivalent to the clinical dose per spot in the spot scanning irradiation method. Measured wave frequency was 1.53 MHz, and this was close to the value (1.62 MHz) simulated by MATLAB k-Wave. Moreover, amplitude of the resonant waves correlated with the residual beam range at the marker.
Conclusion: Resonant waves were observed in clinical beam conditions. The residual beam range at the marker could be obtained in real time from the in-situ acoustic measurement if the correlation coefficient between the amplitude of resonant waves and the residual beam range at the marker was estimated before treatment. It was concluded that the measurement of resonant ionoacoustic waves generated from metal markers is useful for particle beam range verification.
Funding Support, Disclosures, and Conflict of Interest: Author Taisuke Takayanagi is employee of Hitachi, Ltd., Tokyo, Japan.