Mammary secretory carcinoma in a female domestic rabbit (Oryctolagus cuniculus): histopathological and immunohistochemical features

Abstract Secretory carcinoma of the breast is a rare histological subtype in women and is uncommon in female dogs and cats, with only two cases described in female dogs and one case described in a cat. This report describes the histopathological and immunohistochemical features of secretory carcinoma of the mammary gland in a domestic rabbit ( Oryctolagus cuniculus ). Microscopic evaluation revealed a non-encapsulated, well-demarcated neoplastic proliferation composed of epithelial cells arranged in solid and tubular patterns, supported by a delicate fibrous connective tissue stroma. The neoplastic cells had slightly eosinophilic cytoplasm, frequently containing large, well-demarcated vacuoles that displaced the nucleus to the periphery, giving the cells a signet-ring-like appearance. Periodic acid-Schiff (PAS) histochemical staining, performed with and without diastase digestion, showed positivity in the intracellular and extracellular secretory material in both approaches. Immunohistochemical evaluation was performed for the markers α-lactalbumin, Ki-67, estrogen receptor, progesterone receptor, and COX-2. Positive cytoplasmic immunolabeling for α-lactalbumin was observed in neoplastic cells and in intracellular and extracellular secretory material, confirming their secretory nature. The diagnosis of secretory carcinoma was established based on the association of histopathological, histochemical, and immunohistochemical features. Considering the secretory features of the mammary gland in rabbits, secretory carcinoma should be included in the differential diagnosis of mammary carcinomas with prominent secretory activity in this species.