Sensitivity of different dose scoring methods on organ-specific neutron dose calculations in proton therapy

Scattered doses, e.g. neutron doses in proton therapy, are of concern in radiation therapy. Although measured data are the gold standard, Monte Carlo simulations allow a more realistic consideration of patient anatomy via whole-body phantoms. When calculating neutron doses with Monte Carlo techniques, the dose can be scored in different ways because neutrons deposit dose indirectly. The purpose of this study was to assess the differences in neutron dose predictions when using different dose scoring methods. Two methods were tested. In the first method, the organ dose was calculated by accumulating dose from each individual dose deposition event with a particle-specific radiation weighting factor applied. Alternatively, we applied a method where the calculation was done by averaging the dose over the total number of events irrespective of particle type and applying average neutron radiation weighting factors. In addition, we assessed the sensitivity of different neutron quality factor assignments based on two recommendations by the International Commission on Radiological Protection (ICRP). We found that the scoring procedure can lead to differences in the organ equivalent dose of about 25%. As to the ICRP definition of neutron quality factors, the most recent recommendation results in about 10% higher organ doses.