A one-year comparative study of post-irradiation collagen content and microscopic fibrosis in mouse skin

Mice were irradiated with 5 Gy, 10 Gy, 15 Gy or 25 Gy, to the thorax, with a dual-head 137Cs source. Skin specimens were taken 1-48 weeks later. In 5 μm tissue sections the ratio of collagen to total protein was determined. Adjacent sections were also evaluated microscopically for fibrosis. A dose-dependent increase in collagen content was seen at 1 week, 4 weeks and 12 weeks after irradiation; it declined from 12 weeks to 24 weeks, and increased again at 36 weeks and 48 weeks. Such increases ranged to 50% higher than controls. In contrast to the biochemical detection of increased collagen content within 1 week at all dose levels, microscopic fibrosis was first detected 4 weeks after irradiation and then only in the animals given 25 Gy. At all other dose levels, microscopic changes were not found until 12 weeks or more after irradiation. The authors concluded that: (1) cutaneous collagen content increased within 1 week after irradiation and remained elevated, even after 5 Gy, for almost a year; (2) microscopic detection of fibrosis was an inadequate end-point; and (3) the post-irradiation increase in cutaneous collagen was an early, not a late event. (author)