Promotion pattern and tumour types in two-stage rat skin carcinogenesis

Carcinogenic studies were performed on the skin of inbred male Lister rats using 7,12-dimethylbenz[alpha]anthracene (DMBA) in low doses (2.5 mg and 5.0 mg), followed by promotion with croton oil (CO) in one half of the animals. The clinically observed tumour rate did not differ after the two carcinogen doses and was practically unaffected by promotion, only a marginal increase in effect being noted after DMBA 2.5 mg. Also the latency periods were alike. The total tumour crop was increased by promotion, but histologically the predominant clinical skin tumour was a sebaceous proliferation with or without associated papillomatous squamous cell hyperplasia. This lesion did not in microscopic structure or cellular characteristics fulfil the criteria of a neoplastic tumour. Basal cell carcinomas, adnexal tumours and pure squamous cell tumours were rare and were not promoted by CO. CO had a marked promotion effect, but only on the mixed sebaceous-papillomatous hyperplasias, which were more than doubled in number. In conclusion, carcinogenesis was weak after the low DMBA-doses, and promotion by CO of neoplastic tumours was not seen.