Invasive carcinoma of no special type with medullary pattern in the canine mammary gland: histopathological and immunohistochemical characterization.

Abstract

Medullary carcinoma (MC) is an extremely rare neoplasm of the human mammary gland that is essentially not reported in veterinary literature. The present report describes a case of a 14-year-old female mixed-breed dog presenting with a mammary mass. Histological examination revealed a poorly demarkated, multinodular neoplasm composed predominantly of solid sheets of epithelial cells with high nuclear-to-cytoplasmic ratio, indistinct cytoplasmic borders and marked pleomorphism. A prominent lymphoplasmacytic infiltrate was also observed. Immunohistochemical analysis showed tumour cells were positive for cytokeratin 14, 34βE12 and epidermal growth factor receptor. The tumour cells were negative for HER2 and oestrogen receptor but had diffuse labelling of progesterone receptor. The proliferation index, assessed by Ki-67 labelling, was 22%, and cyclooxygenase-2 labelling was observed in less than 10% of cells. CD3 and CD20 immunolabelling indicated a predominance of T lymphocytes within the inflammatory infiltrate. These findings led to a diagnosis of invasive carcinoma of no special type with medullary pattern and T lymphocyte enriched inflammatory infiltrate. This case reinforces the importance of recognizing MC in dogs as it shares several histopathological and immunohistochemical features with its human counterpart. Further studies with larger cohorts are needed to elucidate the prognostic significance of this rare tumour subtype in dogs.