Fractionation with X rays and neutrons in mice: response of skin and C3H mammary tumours

The effectiveness of fast neutron therapy was tested where it was unlikely to show any advantage relative to conventional X rays, i.e. on a mouse mammary tumour which contains radioresistant hypoxic cells, but which reoxygenates extensively after a large X-ray dose. The therapeutic effects were estimated by comparing the degree of early skin reaction associated with a standard frequency of local tumour control assessed at 150 days. Five and nine fractions of X rays or fast neutrons were given at the expected optimum spacing for re-oxygenation after X rays. Single doses were also used. Five fractions of X rays given in nine days were indeed found to be as effective as either of the fractionated neutron treatments, which were closely similar to each other, but nine fractions of X rays in 18 days were considerably less effective. These results suggest that X rays can be made as effective as fast neutrons, by a sharply optimal choice of fractionation dose and interval. However, to make this choice requires detailed knowledge of the time course of re-oxygenation in the tumour to be irradiated. The fractionation schedule with fast neutrons appears to be much less critical.