Purpose: To evaluate the impact of radiosensitizers on ion-induced acoustic emissions (ionoacoustics) and assess the accuracy of ionoacoustic range verification for carbon ion therapy at clinical synchrotrons.
Methods: Acoustic emission resulting from the absorption of optical photons produced from scintillation and absorbed in optical dyes or photosensitizers (similar to photodynamic therapy) were investigated. Alternatively, the local enhancement of the energy deposition in presence of gold nanoparticles (radiosensitizers) was also evaluated. FLUKA Monte Carlo simulations were performed to model a monoenergetic carbon ion beams (215 MeV/u) in water in presence of scintillation materials or gold nanoparticles. The resulting acoustic emission was simulated using k-Wave, considering the temporal microstructures of carbon ions delivered by synchrotrons. The interference pattern (beats) between the direct ionoacoutic signal from the Bragg peak (BP) and the pressure produced due to radiosensitizers at the location where the ion beam enters the targeted volume was analyzed in the frequency-domain. The BP position relative to the tumor entrance was estimated from the beat frequency accounting for the speed-of-sound in the target.
Results: Assuming sufficiently high light yield in the typical range of scintillation decay time, the absorption of optical photons as used in photodynamic therapy can enhance the ionoacoustic emission, e.g., light emission at wavelength of 300 nm with a light yield of 10³ photons/MeV increases the signal amplitude by 50%. Alternatively, low concentration (<1wt.%) of high-density materials like gold nanoparticles significantly enhance the ionoacoustic emission (>100 % increase). The additional pressure generated at the entrance of the target in presence of radiosensitizers allows for accurate BP localization (<1% error relative to the 9.8 cm range) for ion beam at synchrotrons.
Conclusion: This simulation study shows the potential benefit of radiosensitizers on ionoacoustics-based range verification to enable accurate BP localization for carbon ion beams delivered by synchrotron accelerators.