ESTRO 2024 - Abstract Book

S4637

Physics - Optimisation, algorithms and applications for ion beam treatment planning

ESTR0 2024

1 Paul Scherrer Institute, Center for Proton Therapy, Villigen PSI, Switzerland. 2 ETH Zurich, Physics Department, Zurich, Switzerland. 3 Vrije Universiteit Amsterdam, Department of Radiation Oncology, Amsterdam, Netherlands. 4 University Hospital Zurich, Department of Radiation Oncology, Zurich, Switzerland. 5 University Hospital Bern, Department of Radiation Oncology, Bern, Switzerland

Purpose/Objective:

Recent preclinical animal trials exploring the FLASH effect encounter a significant challenge: attaining exceptionally high beam currents in Cyclotron-based proton therapy machines. This necessitates relying on 230/250 MeV beams with substantial excess penetration. One approach to achieving conformal dose distribution with Flash dose rates is to employ a 230/250 MeV beam in conjunction with a patient-specific ridge filter. However, to cover the majority of tumors, typically require maximum energies within the 140-170 MeV range. Consequently, utilizing a 230/250 MeV beam in combination with a patient-specific ridge filter results in an increased beam size and an additional neutron dose to patients. Our primary objective is to develop a cyclotron-based proton therapy beam line capable of delivering ultra-high dose rates for all clinical energy levels while utilizing a patient-specific ridge filter designed for the specific maximum energy required to cover the target, thus keeping the beam size minimal and reducing neutron exposure to patients while achieving conformal dose distribution. In our new beamline design, we have incorporated momentum cooling in conjunction with an energy selection system that utilizes a wedge. This innovative approach effectively reduces momentum spread and minimizes beam losses, ultimately enabling the attainment of ultra-high dose rates at the isocenter, as detailed in Maradia et al.'s study in Nature Physics (2023) [1]. To evaluate the performance of our beamline, we conducted comprehensive Monte Carlo simulations and calculates beam size and maximum beam current for each energy beams. We also fine-tuned treatment plans using spot reduction techniques, drawing from the research of Maradia et al. in 2022 [2]. Our treatment plans took into consideration the use of patient-specific ridge filters, ensuring the maximum energy required to cover tumors in various scenarios, such as treatments for Lung, Liver, and Nasal Cavity conditions. We presumed that the ridge filter is positioned directly adjacent to the patient's body. The beam sizes for each pencil beam following the placement of the ridge filter are determined by analytically calculating the multiple Coulomb scattering for the specific thickness of the ridge filter. We also assumed the minimum spot weight of 14 Mega protons. Material/Methods:

Results:

Our Monte Carlo simulations have revealed that at the isocenter, beam currents can reach a maximum of 100 nA for 70 MeV beams and 550 nA for 230 MeV beams, utilizing 800 nA from the cyclotron. This represents an almost hundredfold increase in beam current compared to other commercial facilities. Notably, we have maintained parity in beam size with other commercially available systems. Our compact beamline covers a scanning area of 40x40 cm^2. The treatment plans implemented with our beamline parameters have demonstrated comparable quality to clinical plans. By employing ultra-high dose rates in conjunction with patient-specific ridge filters and plans with reasonable spot sizes, we've achieved a minimum Dose Average Dose Rate (DADR) per field of 1409 Gy/s for Liver (PTV: 600 cm^3),