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.
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
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.