A novel intensity compensation method to achieve energy independent beam intensity at the patient location for cyclotron based proton therapy facilities

Abstract In cyclotron-based proton therapy facilities, an energy selection system is typically used to lower the beam energy from the fixed value provided by the accelerator (250/230 MeV) to the one needed for the treatment (230-70 MeV). Such a system has the drawback of increase beam emittance and introducing an energy-dependent beam current at the patient location, resulting in energy dependent beam intensity ratios of about 103 between high and low energies. This complicates treatment delivery and challenges patient safety systems. As such, we propose the use of a dual energy degrader method that can reduce beam intensity for high-energy beams. The first degrader is made of high Z material and the second is made of low Z material and are placed next to each other. For high energies (190-230 MeV), we use only the first degrader to increase beam emittance after the degrader and thus lose intensity in the emittance selection collimators. For intermediate energy beams (110-190 MeV) we use the combination of both degraders, whereas for low energy beams (70-110 MeV), only the second degrader limits the increase in emittance. With this approach, energy-independent beam intensities at the patient location can be achieved, whilst localizing beam losses around the degrader.