Technical note: Application of an optical hydrophone to ionoacoustic range detection in a tissue‐mimicking agar phantom

Background Ionoacoustics is a promising approach to reduce the range uncertainty in proton therapy. A miniature‐sized optical hydrophone (OH) was used as a measuring device to detect weak ionoacoustic signals with a high signal‐to‐noise ratio in water. However, further development is necessary to prevent wave distortion because of nearby acoustic impedance discontinuities while detection is conducted on the patient's skin. Purpose A prototype of the probe head attached to an OH was fabricated and the required dimensions were experimentally investigated using a 100‐MeV proton beam from a fixed‐field alternating gradient accelerator and k‐Wave simulations. The beam range of the proton in a tissue‐mimicking phantom was estimated by measuring γ‐waves and spherical ionoacoustic waves with resonant frequency (SPIRE). Methods Four sizes of probe heads were fabricated from agar blocks for the OH. Using the prototype, the γ‐wave was detected at distal and lateral positions to the Bragg peak on the phantom surface for proton beams delivered at seven positions. For SPIRE, independent measurements were performed at distal on‐ and off‐axis positions. The range positions were estimated by solving the linear equation using the sensitive matrix for the γ‐wave and linear fitting of the correlation curve for SPIRE; they were compared with those measured using a film. Results The first peak of the γ‐wave was undistorted with the 3 × 3 × 3‐cm3 probe head used at the on‐axis and 3‐cm off‐axis positions. The range positions estimated by the γ‐wave agreed with the film‐based range in the depth direction (the maximum deviation was 0.7 mm), although a 0.6–2.1 mm deviation was observed in the lateral direction. For SPIRE, the deviation was