Determination of colony numbers in pig epidermis as an estimate for radiosensitivity. A rapid assay based on in vitro BrdU-labelling

A rapid assay has been developed for the quantitation of colonies arising f rom surviving clonogenic cells in pig epidermis after irradiation. The numb er of surviving clonogenic cells per unit area was related to the epidermal in vivo response of moist desquamation. After irradiation with single dose s, ranging from 20 to 36 Gy, skin biopsies were taken and incubated in disp ase for enzymatic separation of the epidermis and dermis. Full thickness ep idermal sheets were labelled with bromodeoxyuridine (BrdU) in vitro. Prolif erating cells were visualized using standard immunohistochemical procedures . Cell groups containing greater than or equal to 16 cells were counted as colonies. These colonies were first seen on day 14/15 after irradiation. Th e number of colonies per cm(2), as a function of skin surface dose, yielded a cell survival curve with a D-0 (+/- SE) of 3.87 +/- 0.57 Gy. The ED50 fo r the epidermal in vivo reaction of moist desquamation corresponded with a colony density of 2.7 colonies per cm(2). After higher doses, abundant smal ler colonies of 4-8 BrdU-positive cells were seen and these were more radio resistant, as represented by higher D-0 values.