Age as a modifying factor of 7,12-dimethylbenz(a)anthracene-induced mammary carcinogenesis in the lewis rat.

The influence of age at DMBA administration on (1) susceptibility to mammary tumor development, (2) the proportion of tumors which are ovary-dependent, and (3) the frequency of spontaneous tumor regression was examined in female Lewis rats between 25 and 200 days of age. Tumor development was monitored for a 12-month period after carcinogen administration. Age at carcinogen administration affected (1) the number of tumors induced, (2) the latent period of tumor development, and (3) the number of tumors which regressed spontaneously. More tumors developed in young rats; however, a large fraction of these tumors regressed spontaneously. The frequency of spontaneous tumor regression was strongly age-related, being highest in rats treated up to 70 or after 150 days of age. Mean latent period of development of tumors which regressed spontaneously (SRT) was significantly shorter than that of tumors which grew progressively (PGT). Furthermore, mean latent period of SRT development was significantly shorter in rats treated at 25 or 50 days of age. In contrast, all age-groups were equally susceptible to the development of progressively growing tumors (PGT). PGT incidence, mean latent period of PGT development, mean numbers of PGT per PGT-bearing rat and the relative proportion of PGT which were ovary-dependent were similar for all age-groups at the end of 12 months. These results indicate that, in the Lewis rat strain, age does not confer significant or permanent refractoriness. Furthermore, spontaneous tumor regression is a frequent and strongly age-related phenomenon.