PSI, Villigen PSI, Switzerland
ETH, Zurich, Switzerland
University of Zurich, University Hospital, Zurich, Switzerland
KRO, Bern, Switzerland
In proton therapy, FLASH-RT, irradiation at ultra-high dose rates (>40 Gy/s) that can minimize radiation-induced harm to healthy tissue without reducing its ability to treat tumors, is a topic of great interest. However, in cyclotron-based proton therapy facilities, losses caused by the energy degradation process reduce the transmission to less than 1% for low energies, making it difficult to achieve high dose rates over the clinical range (70-230 MeV). We will demonstrate how an already existing clinical beamline can be converted into a FLASH beamline by beam optic changes only. To achieve maximum transmission, we have developed a new optics that transports the undegraded 250 MeV beam from the cyclotron to the isocenter. However, this has asymmetric emittance in the transverse planes, leading to gantry angle-dependent beam characteristics at the patient. Particle transport has been simulated with MINT (in-house matrix multiplication transport program with Monte Carlo simulations for scattering effects) and benchmarked with beam profile measurements. We used the method of σ matrix matching (M. Benedikt et al. 1997) to achieve gantry angle-independent optics. MINT simulations and beam profile measurements showed a good agreement, and with FLASH optics, we experimentally achieved almost 90% transmission at the patient, translating to a maximum current of 720 nA (>9000 Gy/s). Further, we demonstrate that using the matrix matching optimization criteria together with fine-tuning of the magnets, we could achieve gantry angle-independent beam profiles at the patient location. In conclusion, we demonstrated how an already existing cyclotron-based proton gantry can be adapted to achieve ultra-high dose rates at 250 MeV, enabling investigations of FLASH radiotherapy with protons. Since most of the modifications are performed on the beam optics, it is entirely transparent to clinical operations, making the method transferable to other facilities.
PSI, Villigen PSI, Switzerland
In proton therapy (PT), high dose rates could allow efficient utilization of motion mitigation techniques for moving targets, and potentially enhance normal tissue sparing due to the FLASH effect. Cyclotrons are currently the most common accelerator for PT, accounting for two-thirds of the total installations. However, for cyclotron-based facilities, high dose rates are difficult to reach for low-energy beams, which are generated by passing a high-energy beam through an energy degrader and an energy selection system (ESS); due to scattering and range straggling in the degrader, the emittance and energy/momentum spread increase significantly, incurring large losses from the cyclotron to the patient position. To solve these problems, we propose two approaches: a) transporting the maximum acceptable emittance in both transverse planes (using asymmetric collimators and/or scattering foil); b) an ESS with a wedge (instead of slits), reducing the momentum spread of the beam without significant beam losses. We demonstrate in simulation that low-energy beam transmission can be increased up to a factor of 60 using these approaches compared to the currently used beamline and ESS. This concept is key to enhance the potential of proton therapy by increasing the possibilities to treat new indications in current and future proton therapy facilities while reducing the cost.