A comparative study of canine and human breast cancer.

The incidence of mammary tumours in the bitch is probably three times as great as in women. While many of these tumours are mixed mammary tumours about one-third are carcinomas which resemble human breast carcinomas. Allowing for differences in life span, the age at onset is similar in both species. The World Health Organization classification of tumours and dysplasias of the canine mammary gland follows as far as possible the WHO classification for human breast tumours. Clinical staging of canine mammary tumours has now been completed. Some prognostic factors are similar in both species but regional lymph node metastasis does not seem to be of major importance in the bitch; mitotic activity may also not be as important as in women. Metastatic spread is broadly similar in both species except that involvement of the liver and skeleton is not as common in the bitch as in women. In older normal Beagles hyperplastic and neoplastic nodules commonly appear in the mammary gland, and they occur earlier in animals receiving large doses of progestogens. This has produced problems for the drug industry when conducting long-term carcinogenicity tests on progestogens present in the human contraceptive pill. Despite considerable endocrinological differences between the two species, oophorectomy is sparing for breast cancer in both. As in women, oestrogen and progesterone receptors have been detected in mammary carcinomas in bitches. Canine tumours can be grown in tissue culture but cloned cell lines have not yet been obtained. Transplantation can be made into nude mice and immunosuppressed neonatal dogs. The prognosis following mastectomy for invasive tubular adenocarcinoma and invasive solid carcinoma in the bitch is poor and these histological types make the best models for breast cancer in women. International trials are planned using chemotherapy and/or immunotherapy following mastectomy and, as results can be obtained within 3 years of commencement, it is expected that canine mammary tumours will play an increasingly important role in research which may lead to improved methods of treatment in human breast cancer.