The determination of a dose deposited in reference medium due to (p,n) reaction occurring during proton therapy

Abstract

Aim

The aim of the investigation was to determine the undesirable dose coming from neutrons produced in reactions (p,n) in irradiated tissues represented by water.

Background

Production of neutrons in the system of beam collimators and in irradiated tissues is the undesirable phenomenon related to the application of protons in radiotherapy. It makes that proton beams are contaminated by neutrons and patients receive the undesirable neutron dose.

Materials and methods

The investigation was based on the Monte Carlo simulations (GEANT4 code). The calculations were performed for five energies of protons: 50[[ce:hsp sp="0.25"/]]MeV, 55[[ce:hsp sp="0.25"/]]MeV, 60[[ce:hsp sp="0.25"/]]MeV, 65[[ce:hsp sp="0.25"/]]MeV and 75[[ce:hsp sp="0.25"/]]MeV. The neutron doses were calculated on the basis of the neutron fluence and neutron energy spectra derived from simulations and by means of the neutron fluence–dose conversion coefficients taken from the ICRP dosimetry protocol no. 74 for the antero-posterior irradiation geometry.

Results

The obtained neutron doses are much less than the proton ones. They do not exceed 0.1%, 0.4%, 0.5%, 0.6% and 0.7% of the total dose at a given depth for the primary protons with energy of 50[[ce:hsp sp="0.25"/]]MeV, 55[[ce:hsp sp="0.25"/]]MeV, 60[[ce:hsp sp="0.25"/]]MeV, 65[[ce:hsp sp="0.25"/]]MeV and 70[[ce:hsp sp="0.25"/]]MeV, respectively.

Conclusions

The neutron production takes place mainly along the central axis of the beam. The maximum neutron dose appears at about a half of the depth of the maximum proton dose (Bragg peak), i.e. in the volume of a healthy tissue. The doses of neutrons produced in the irradiated medium (water) are about two orders of magnitude less than the proton doses for the considered range of energy of protons.