Breed- and age-related differences in canine mammary tumors.

Triple-negative breast cancer is a type of breast cancer that does not express the genes for estrogen receptor (ER), progesterone receptor (PR), and human epidermal growth factor receptor-2 (HER-2). It is an important and clinically relevant condition as it has a poor prognosis and is difficult to treat. Basal-like triple-negative cancer is highly prevalent in both African-Americans and adolescents. We therefore examined whether such a cancer likewise occurs in specific breeds and age groups in dogs, focusing on basal-like triple-negative cancer in particular. In this study, 181 samples from dogs with malignant mammary carcinoma from the 5 most common breeds and 2 age groups in Korea were analyzed. Histological classification and molecular subtyping, including assessment of immunohistochemical findings, were carried out. Twenty-five of 28 (89.3%) triple-negative carcinomas were identified as basal-like triple-negative carcinomas. Analysis of associations of classified factors revealed that the shih tzu breed (9/25, 36.0%) and advanced-age (19/25, 76.0%) groups were characterized by higher prevalence of basal-like triple-negative tumors with diverse histological types and of a higher grade. These results suggest that breed- and age-related differences can be identified in canine mammary carcinoma and, notably, in the shih tzu breed and at older ages. Further investigation of these distinguishing characteristics of the shih tzu breed is warranted.