Differential response of rapidly- and slowly-proliferating hair follicles of mice to fractionated irradiation.

Single and fractionated dose response curves for hair follicles in two different growth activities were compared to analyze the difference in sparing associated with fractionation. Doses per fraction ranged from 8.0 Gy to 2.5 Gy from a 137Cs source. The maximum score for epilation, either at 4 weeks (growing follicles) or at 8.5 weeks (resting follicles) from treatment, was evaluated from a 6-point, graded scale of epilation. The dose to produce a given hair response in 50% of animals (HRD50) was calculated using logit analysis. For both growing and resting follicles, the HRD50 values increased with decrease in fraction size. However, when the dose per fraction was decreased below 4.0 Gy, the increase of isoeffect dose was less for growing follicles but continued at the same rate for resting follicles. This difference in the slope of isoeffect curves was analyzed in terms of alpha/beta ratio in the linear quadratic model. The estimates with 95% confidence intervals were 6.0 Gy (5.2-6.8 Gy) and 3.6 Gy (3.1-4.2 Gy) for growing and resting follicles, respectively, for doses per fraction less than 7 Gy. For resting follicles, the plot of isoeffect inverse total dose versus dose per fraction was nonlinear. There was no correlation between the response within the same animal of early occurring epilation in growing follicles to the late occurring epilation in resting follicles though the site and origin of the tissue was the same and it differed only in turnover kinetics. This suggests that the response of organs is related to random cell killing rather than to animal specific variations in radiosensitivity, at least in an inbred strain of mice.