Purpose: Ultra-high dose rate (UHDR) proton therapy for FLASH delivered with a cyclotron-based system utilizing a degrader and magnetic energy selection system (ESS) is limited by the transport efficiency of the ESS to only the highest energy, in most cases ~230MeV. Treatment using the Bragg peak and UHDR for lesions shallower than the maximum range (~32cm in water) must then incorporate a range shifter in the treatment room which increases patient exposure to secondary neutron and photon radiation. We use Monte Carlo (MC) methods to investigate the excess secondary dose when employing a range shifter during UHDR proton therapy.
Methods: Two pencil beams with nominal energies of 196.5MeV and 230MeV are individually simulated with TOPAS/GEANT4 MC. At the 230MeV, a 6.78cm range shifter of polymethyl methacrylate is placed in the beam line with a 30cm air gap from a water phantom. This shifts the range of 230MeV protons (32.77cm) to the range of 196.5MeV protons (25.05cm). Neutron and photon physical dose and fluence at different depths are compared between the two configurations.
Results: An increase in both neutron and photon dose is observed for the configuration of the 230MeV proton beam with the range shifter. The increase in the secondary neutron and photon contamination are the most significant at the surface. It reduces to an average of 19%+-3% increase for the neutron contribution and 12.4%+-0.4% for the photon contribution compared to the 196.5MeV proton beam. The increased in secondary neutrons exhibits significant energy dependency at the surface.
Conclusion: Use of an energy degrader to shift the range of a high energy beam used in UHDR proton therapy results in a significant increase of excess neutron and photon dose. This study provides insight on the necessity to consider secondary neutrons and photons when employing a range shifter in FLASH proton therapy.