In scattering proton therapy, the beam incidence, i.e. the patient’s orientation with respect to the beam axis, can significantly influence stray neutron doses although it is almost not documented in the literature.
MCNPX calculations were carried out to estimate stray neutron doses to 25 healthy organs of a 10-year-old female phantom treated for an intracranial tumor. Two beam incidences were considered in this article, namely a superior (SUP) field and a right lateral (RLAT) field. For both fields, a parametric study was performed varying proton beam energy, modulation width, collimator aperture and thickness, compensator thickness and air gap size.
Using a standard beam line configuration for a craniopharyngioma treatment, neutron absorbed doses per therapeutic dose of 63 μGy Gy−1 and 149 μGy Gy−1 were found at the heart for the SUP and the RLAT fields, respectively. This dose discrepancy was explained by the different patient’s orientations leading to changes in the distance between organs and the final collimator where external neutrons are mainly produced. Moreover, investigations on neutron spectral fluence at the heart showed that the number of neutrons was 2.5 times higher for the RLAT field compared against the SUP field. Finally, the influence of some irradiation parameters on neutron doses was found to be different according to the beam incidence.
Beam incidence was thus found to induce large variations in stray neutron doses, proving that this parameter could be optimized to enhance the radiation protection of the patient.