Hidradenoma in Axillary Lymph Nodes: A Diagnostic Pitfall Mimicking Metastatic Carcinoma.

Axillary lymph nodes are a common site of metastatic carcinoma, particularly from the breast. Epithelial inclusions within axillary lymph nodes are uncommon but well recognized, most frequently encountered as small, incidental clusters of mammary-type epithelium; squamous, Müllerian, and mixed types are also sporadically identified. Hidradenoma is a benign adnexal neoplasm classically composed of epidermoid/squamoid, intermediate/basaloid, and mucinous cells that most commonly originates in the skin but can arise in the breast parenchyma, with morphologic and genetic features that overlap with mucoepidermoid carcinoma. Rare instances of hidradenomas within pelvic and axillary lymph nodes have been reported, raising the specter of metastasis. We report 3 cases of axillary lymph node hidradenoma presenting without a concurrent breast or cutaneous lesion. Morphology, immunohistochemistry, and molecular studies were evaluated. Two cases were found to harbor CRTC1::MAML2 rearrangements, while a rarer CRTC3::MAML2 fusion was identified in a third case. Following excision, the 3 patients were without evidence of residual disease, with clinical follow-up of ∼2 years each. Recognition of this entity is essential to avoid misclassification as metastatic carcinoma and prevent overtreatment.