Solid Carcinoma of the Canine Mammary Gland: a Histological Type or Tumour Cell Arrangement?

Mammary neoplasms are the most frequently diagnosed tumours in female dogs and are classified into various histological types, including solid carcinomas. We proposed a subclassification of solid carcinomas based on morphological and immunohistochemical characteristics, and correlated the subtypes with prognostic factors. A total of 135 cases of solid mammary carcinoma were selected from 3,400 canine mammary neoplasms in the archives of the Laboratory of Comparative Pathology, Institute of Biological Sciences, Federal University of Minas Gerais, Brazil. Epidemiological and survival data were obtained, and immunolabelling for chromogranin A, pancytokeratin, cytokeratin 14, Ki67 and p63 was performed. Solid carcinomas were classified into six subgroups: malignant adenomyoepithelioma (68/135), carcinoma with solid pattern (22/135), malignant myoepithelioma (16/135), basaloid carcinoma (14/135), neuroendocrine carcinoma (10/135) and solid papillary carcinoma (5/135). Shorter survival time was associated with the presence of lymphatic invasion (P = 0.009) in the initial clinical staging (I-III). When considering all clinical stages (I-V), vascular invasion (P <0.001) and the presence of regional metastasis (P = 0.004) were important prognostic factors. Basaloid carcinoma and solid papillary carcinoma did not reach the median survival time for early-stage cases, and malignant myoepithelioma had the highest median survival in advanced stages. Carcinoma with a solid pattern was associated with a higher number of regional metastases. Distinguishing the various histological and immunophenotypic subtypes that exhibit a solid arrangement, using histological and immunohistochemical criteria, is essential for understanding the behaviour of these neoplasms and for the selection of more appropriate and specific therapies.