Protoacoustic range verification by direct comparison with beam range measurements using a tissue-equivalent polymer gel

Background and purpose

Protoacoustics offers a promising method for in vivo range verification in proton therapy. To experimentally assess the protoacoustic range accuracy, conventional tissue-mimicking phantoms (TMPs) often require Monte Carlo (MC) simulations to estimate the ground truth. However, limited knowledge of material properties and/or insufficient fine-tuning of dose models can introduce range errors in MC simulations. This study aims to propose a TMP capable of measuring the dose distribution concurrently with the protoacoustic measurement, thereby circumventing the need for MC simulations and their associated uncertainties.

Methods

Normoxic N-vinylpyrrolidone-based polymer gel dosimeters were employed as TMPs. Two gel dosimeters of varying sizes were irradiated with a 226.5 MeV proton beam from a clinical synchrocyclotron. Post-irradiation, magnetic resonance imaging was used to determine the beam range within the gels. Concurrently, protoacoustic ranges were measured using an optical hydrophone, employing both a time-of-arrival (TOA) method and an acoustic simulation-based approach. The ranges obtained from the gel dosimeters were then compared with those from the protoacoustic methods.

Results

The range differences between the gel dosimeter measurements and both protoacoustic methods were within 1 mm for both gel sizes. In contrast, the maximum deviation between the protoacoustic range and the MC simulation-derived range was 2.1 mm, potentially attributable to inaccuracies in the assumed material properties within the MC simulation.

Conclusions

Polymer gel dosimeters provide a direct method for validating protoacoustic beam range measurements in clinical proton beams, circumventing the reliance on MC simulations and their associated uncertainties.