Spontaneous metastatic mammary carcinomas in 2 capuchin monkeys: pathology and immunohistochemical findings with a brief literature review

Spontaneous mammary neoplasms are uncommon in nonhuman primates (NHPs), particularly in neotropical species, and their pathologic aspects and biological behavior remain poorly characterized. Here, we describe the pathology and immunohistochemical features of spontaneous metastatic mammary carcinomas in 2 elderly captive capuchin monkeys ( Sapajus libidinosus ). A 26-y-old female and a 29-y-old male had unilateral mammary masses and died as a result of complications associated with advanced neoplastic disease. Grossly, both tumors had marked local invasion and systemic metastases involving lymph nodes, lungs, liver, and pericardium. Histologically, both carcinomas were poorly demarcated, infiltrative malignant epithelial proliferations arranged predominantly in solid nests and trabeculae, with central necrosis, marked anisocytosis and anisokaryosis, high mitotic activity, and vascular invasion. Immunohistochemically, both tumors expressed pancytokeratin, CK7, mammaglobin, HER2, and Ki67. The female had a triple-positive phenotype (ER+, PR+, HER2 2+), whereas the male had an HER2-enriched profile (ER low or negative, PR−, HER2 3+). Ki67 indices were high in both cases (31% and 35%, respectively), supporting elevated proliferative activity. The immunophenotypic and pathology findings resembled aggressive high-grade human breast carcinomas associated with poor prognosis and metastatic potential. A literature review confirmed the rarity of spontaneous mammary tumors in neotropical NHPs and highlighted the scarcity of cases with metastases. Collectively, our findings expand the current knowledge of mammary neoplasia in NHPs and reinforce the comparative relevance of these species for understanding similar human breast cancer.