(Sunday, 7/12/2020) [Eastern Time (GMT-4)]
Room: AAPM ePoster Library
Purpose: To measure the ionoacoustic signal generated by a proton beam from the fixed-field alternating gradient accelerator (FFA) and assess the proton range accuracy by comparing depth dose measurement using the Bragg peak (BP) chamber.
Methods: A 100 MeV proton beam from the FFA (Kyoto University, Japan) was incident on a water phantom; its range measured by the PTW34070 BP chamber (PTW, Freiburg, Germany) was 77.6 ± 0.5 mm. The number of protons constituting the pulse was ~1.0 × 108, which is comparable to that constituting a spot in a clinical setting. The pulse width was ~30 ns (1s). An SU-V391 hydrophone (Olympus, Waltham, MA, USA) was placed 20 mm downstream of the BP. Signals were amplified using a 46 dB amplifier and stored using a digital oscilloscope at a sampling rate of 50 MS/s after averaging 50 events. A low-pass filter was applied. To estimate the BP’s absolute position, we measured and used time differences between the following three characteristic points in the waveforms: (1) a negative peak of the direct wave from the BP, (2) a negative peak of the reflected wave at the water–acrylic wall boundary, and (3) a positive peak of the reflected wave at the acrylic wall–air boundary.
Results: The BP’s absolute position when using the time differences of (2)–(1) and (3)–(1) was 78.16 ± 0.01 and 78.14 ± 0.01 mm, respectively. Both values agreed with the BP’s position measured by the BP chamber. However, the signal-to-noise ratio (SNR) was close to 1, and averaging was necessary to detect peaks.
Conclusion: The range value determined through ionoacoustic measurement agreed well with the value estimated from depth dose measurement to better than 1 mm. However, to apply this technique in a clinical setting, the SNR needs to be improved.
Funding Support, Disclosures, and Conflict of Interest: Taisuke Takayanagi is paid by Hitachi Ltd.
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