Skin carcinogenesis in rats by 3-methylcholanthrene and 7,12-dimethylbenz(alpha)anthracene. Influence of dose and frequency on tumour response and its histological type.

Skin tumours were induced in male inbred rats of the Lister and Wistar strains and in (WixBN) F1 hybrids by topical application of 3-methylcholanthrene (MCA) and 7,12-dimethylbenz(alpha)anthracene (DMBA). Several schedules of application were compared. At a weight basis DMBA was more potent than MCA and the tumour response to DMBA was most clearly related to total dose applied, while the frequency of application was very important for the tumour crop on MCA-treated rats. DMBA led to a marked predominance of squamous celled tumours (SQCT) when treatment was potent, while basal celled and adnexal tumours (BCAT) prevailed after any dosage of MCA. The carcinogenic effect of DMBA in a low total dose approached that of the most potent dosage of MCA. At this level of activity DMBA induced more BCAT than SQCT in both strains and in the hybrids, while MCA induced an increased number of SQCT. Strain differences did not interfere with these main results.