The relative effects of fast neutrons and X rays on tumour and normal tissue in the rat. Part II. Fractionation: recovery and reoxygenation

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Abstract
1. In rats an experimental sarcoma, known to contain hypoxic cells, and the normal tissue of the foot have been irradiated with fractionated doses of X rays and of fast neutrons, each dose separated by 24 hours. 2. For equal damage to normal tissue the therapeutic effect of X rays is improved by fractionation, but in this experimental system neutrons are always still more effective on the tumour. 3. This result is explained by the fact that the population of tumour cells surviving a dose of radiation is largely hypoxic, but that the proportion of hypoxic cells is reduced in time. However, at 24 hours after irradiation this proportion is still greater than in the untreated tumour. 4. Late damage and deformity of the normal tissue, relative relative to early damage, is the same for single doses, two or five fractions of X rays or of neutrons. However, for all treatments, once above a certain dose level, a small increase led to a small increase in early reaction, followed by a very large increase in late reaction and deformity. 5. The rate of increase of neutron RBE with increasing numbers of fractions is similar to that found in pig skin and in human skin. 6. By comparison with the single dose, “recovery” after neutron irradiation is less than after X rays by 0·4 for the early skin reaction and by 0·6 for the late reaction and for deformity.