Effects of X-Irradiation on Cell Proliferation in Normal Epithelium and in Tumours of the Hamster Cheek Pouch

The stratified squamous epithelium of the hamster cheek pouch is a steady-state renewal system which is particularly suitable for kinetic analysis, owing to the absence of hair follicles and sweat glands which complicate the study of skin. Tumours were induced in the cheek pouches of inbred Syrian-derived hamsters with 9,10 dimethyl-1,2 benzanthracene (DMBA). These morphologically typical, exophitic squamous carcinomata display varying degrees of differentiation. The kinetics of proliferation in both tumours and normal tissue were studied autoradiographically after administration of 3H-thymidine in single pulses and by continuous labelling techniques, before and after local irradiation of the cheek pouches with single doses of 500 and 1000 rad of 250 kVp X -rays. For the unirradiated hamster, the cell cycle in the normal epithelium of the cheek pouch was 12-13 times longer than in the tumours induced in the pouch. The fraction of proliferating cells (growth fraction) was 100% in the normal epithelium, and was considerably smaller, although variable, among the carcinogen- induced tumours. There was a diurnal rhythm in the progression of cheek pouch cells through the division cycle which was not seen in the tumours induced in the pouch. After a dose-dependent delay in their first post-irradiation division, cells of the cheek pouch epithelium showed a small (circa 20%) reduction In their cell cycle; in contrast, the tumour cell cycle was marginally elongated. These results are discussed in the context of changes which might occur in the proliferation kinetics of human tumours and normal tissues included in the field of irradiation during a fractionated course of radiotherapy. (author)