gms | German Medical Science

Purpose: Proton beam losses in the treatment nozzle generate secondary neutrons, which bring unwanted out of field dose during treatments. The purpose of this study is to develop an analytic method for estimating neutron dose to a particular organ at risk during proton therapy.

Methods: Based upon radiation shielding calculation methods proposed by Sullivan, we have developed an analytical model for converting the proton beam losses in the nozzle components and in the treatment volume into the secondary neutron dose at a point of interest. Using MCNPx Monte Carlo code we have benchmarked the neutron dose rates generated by proton beam stopped at various media.

Results: The Monte Carlo calculations suggest that the neutron production in water laterally to the incident beam direction is about factor of five smaller than in iron or copper. This indicates that the neutron dose to a distant organ at risk will be predominated by the protons lost in the patient collimator. Furthermore, the analytical estimates suggest that the neutron dose scales linearly with the number of protons lost in the patient aperture, which means that the neutron dose is very sensitive to the beam usage efficiency.

Conclusion: The results of the study suggest that dominant part of the secondary neutron dose to the patient results from the patient specific aperture. Improving conformity of the radiation field to the patient specific aperture can significantly reduce secondary neutron dose 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.