Fractionation and therapeutic ratio with neutron therapy.

Dose-response relationships of most tumours and normal tissues after neutron irradiation are characterized by relatively high alpha/beta ratios. Late reacting tissues are not as protected by low doses per fraction as they are after low linear energy transfer (LET) irradiation. Using the linear-quadratic (LQ) model with the time factor (when needed), we have calculated the therapeutic ratio (TR) as the ratio of the effect on a tumour (or an acutely responding tissue) over that on late reacting tissue. Hypothetical cases are given as well as a case derived from LQ parameters obtained experimentally in animals. It is shown that X-ray fractionation schemes that lead to a high TR will lead to a low neutron TR. For epithelial tumours, a neutron gain factor, GF (ratio of TR) is possible if neutron fractionation is compared to hypofractionated X-irradiation. Neutron treatment over conventional overall times of rapidly proliferating tumours carries a low TR and offers no GF. Neutron therapy for tumours with low alpha/beta ratios offer a potential advantage compared to tumours with high alpha/beta ratios.