The relative biological effectiveness of fractionated doses of fast neutrons (42 MeVd → Bc) for normal tissues in the pig

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Abstract
The late effects of irradiation with single and fractionated doses of X rays (250 kV) and fast neutrons (42 meVd.fwdarw.Be), on the cutaneous and subcutaneous tissues of the pig, have been evaluated from measurements of changes in relative field length. These were determined at intervals of 26-104 weeks after irradiation. For fractionated irradiation with X rays the average fractions exponent, N, obtained from a log-log plot of iso-effect dose (ED50) against fraction number was 0.41. This was independent of the period of assessment, with no significant indication of a time factor. However, the exponent N did vary with the level of effect and was in the range of 0.33-0.51. It was greatest for a .gtoreq. 10% reduction in relative field length. Assuming the validity of the linear quadratic model of cell survival, the .alpha./.beta. ratio was 1.95 Gy. However, this model fitted the data less well for the least severe levels of damage, and for these the .alpha./.beta. ratios were not significantly different from zero. Irradiation with fast neutrons showed a small effect of fractionation for doses given in .gtoreq. 6 fractions compared with a single dose. There was no significant increase in iso-effect dose when the dose was given in 30 fractions compared with 6 fractions. The relative biological effectiveness for late cutaneous and subcutaneous damage for the energy of fast neutrons used did not vary with the period of assessment, i.e. 26-52 weeks compared with 65-104 weeks, and was not significantly different from that previously obtained for ischaemic dermal necrosis, seen after higher doses, at 12-20 weeks after irradiation.