On the Selection of the Neutron to Photon Dose Ratio in Neutron-Photon Therapy for Cancer

Abstract

Purpose: To evaluate the methodological approaches to the prevention of radiation-induced complications after neutron-photon therapy considering the neutron-photon dose ratio in the tumor. Material and methods: The linear-quadratic model (LQM) and principles of neutron and photon dose distributions in a tissue-equivalent medium were used. Cases with the highest risk of radiation-induced complications (treatment by a single or two opposite fields) were discussed. The number of neutron-photon therapy sessions to ensure a combined total neutron and photon dose was determined where the RBE concept was used. When calculating the total effect (TE) and TDF factor characterizing the damage to the irradiated tissue, the effect of the radiation field size and subcutaneous fat layer on their values was taken into account. Results: Methods for selecting the ratio of the neutron and photon dose contribution to the total dose, providing the maximum permissible radiation dose, were developed. It was established that the dependences of TDF and TE factors and the differences in the values of the allowable number of photon therapy sessions on the depth of the tumor were less pronounced in cases with two opposite radiation fields compared to those with a single field. It can be explained by the fact that with increasing depth, an increase in the entrance dose is compensated by a decrease in the dose contribution formed during irradiation from the opposite field. Conclusion: For neutron-photon therapy using a linear-quadratic model, methodical approaches that might be used to provide an acceptable level of radiation-induced skin reactions for any ratio of neutron-photon doses in a tumor were proposed. The use of these techniques for planning neutron-photon therapy will minimize the risk of radiation-induced complications.