The relative biological effectiveness of fractionated doses of fast neutrons (42 MeVd----Be) for normal tissues in the pig. II. Late effects on cutaneous and subcutaneous tissues.
 The late effects of irradiation with single and fractionated doses of X rays (250 kV) and fast neutrons (42 MeVd----Bc), 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 0.33-0.51.
 It was greatest for a greater than or equal to 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 greater than or equal to 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.
