Influence of beam efficiency through the patient-specific collimator on secondary neutron dose equivalent in double scattering and uniform scanning modes of proton therapy

Purpose: Conventional proton therapy facilities use double scattering nozzles, which are optimized for delivery of a few fixed field sizes. Similarly, uniform scanning nozzles are commissioned for a limited number of field sizes. However, cases invariably occur where the treatment field is significantly different from these fixed field sizes. The purpose of this work was to determine the impact of the radiation field conformity to the patient-specific collimator on the secondary neutrondose equivalent. Methods: Using a WENDI-II neutron detector, the authors experimentally investigated how the neutrondose equivalent at a particular point of interest varied with different collimator sizes, while the beam spreading was kept constant. The measurements were performed for different modes of dosedelivery in proton therapy, all of which are available at the Midwest Proton Radiotherapy Institute (MPRI): Double scattering, uniform scanning delivering rectangular fields, and uniform scanning delivering circular fields. The authors also studied how the neutrondose equivalent changes when one changes the amplitudes of the scanned field for a fixed collimator size. Results: The secondary neutrondose equivalent was found to decrease linearly with the collimator area for all methods of dosedelivery. The relative values of the neutrondose equivalent for a collimator with a 5 cm diameter opening using 88 MeV protons were 1.0 for the double scattering field, 0.76 for rectangular uniform field, and 0.6 for the circular uniform field. Furthermore, when a single circle wobbling was optimized for delivery of a uniform field 5 cm in diameter, the secondary neutrondose equivalent was reduced by a factor of 6 compared to the double scattering nozzle. Additionally, when the collimator size was kept constant, the neutrondose equivalent at the given point of interest increased linearly with the area of the scanned proton beam. Conclusions: The results of these experiments suggest that the patient-specific collimator is a significant contributor to the secondary neutrondose equivalent to a distant organ at risk. Improving conformity of the radiation field to the patient-specific collimator can significantly reduce secondary neutrondose equivalent to the patient. Therefore, it is important to increase the number of available generic field sizes in double scattering systems as well as in uniform scanning nozzles.