Estrogen Receptors in Canine Mammary Tumors1

The presence of estrogen receptor in 67 canine mammary lesions was correlated with pathological features of the dis ease. All tissue specimens were analyzed for estrogen receptor content by a sucrose gradient ultracentrifugation method pre viously used in analyzing human breast cancer cytosols. Patho logical features of the tissues were assessed by a veterinary pathologist without knowledge of results of estrogen receptor analysis. Sixty-two (92.5%) of the tissue samples analyzed were classified as epithelial neoplastic lesions, and 38 of these (61.3%), including 24 adenocarcinomas, were estrogen recep tor positive (i.e., estrogen receptor concentration equal to or greater than 10 fmol/mg cytosol protein). All five of the nonepithelial neoplastic lesions were estrogen receptor negative. Canine and human breast cancers share common histological types and have similar biological behavior. If a significant percentage of canine mammary cancer is also estrogen de pendent, the dog may be a useful model for hormonal studies and for the development of models of endocrine therapy for human breast cancer.