Factors to be considered in deciding on fraction size for neutron therapy

The dose in radiotherapy is limited by the tolerance of the normal tissues. Dose fractionation (N) and the overall treatment time (T) are the two major factors affecting the response of normal tissues. The sparing effect of dose fractionation is much less with neutrons than with low-LET radiation. The same normal tissue response may be obtained if the neutron dose is given in 2, 4, 8 or 15 fractions providing the overall treatment time is the same. This means that the neutron treatment could be given in a few large fractions rather than the more conventional 15 to 30 small fractions without affecting normal tissue response. However, tumour sterilization is affected not only by N and T but may also be affected by the persistence of hypoxic foci throughout treatment. Any change in OER with dose fractionation must therefore be considered. There are theoretical reasons for expecting a reduction in OER with reduction in neutron fraction size. Experimental evidence on mouse tail necrosis presented in this paper shows that the OER does decrease with a decrease in fraction size. This means that small fractions will sterilize tumours with persistent hypoxic foci more effectively than large fractions. Thus a decrease in number of fractions and an increase in fraction size may have little effect on normal tissue response to neutrons but may decrease the chance of sterilizing the tumour. (author)