Primary Nodal Poroid Hidradenocarcinoma With YAP1::NUTM1 Fusion: Report of a Rare Case

Poroid hidradenocarcinoma is a rare malignant sweat gland neoplasm. Although primary nodal hidradenoma has been reported, primary nodal poroid hidradenocarcinoma has not been described. We report a case of a 56-year-old man presenting with a nontender 5.5 cm mass within the axillary lymph node, and no identifiable primary cutaneous lesion on clinical examination or imaging. Histopathologic examination of the axillary lymph node dissection revealed involvement of 1 of 42 lymph nodes by a multinodular, solid and cystic neoplasm composed of poroid and cuticular cells with ductal differentiation, consistent with a background poroid hidradenoma. There was an abrupt transition to malignant areas with marked cytologic atypia, nuclear pleomorphism, increased mitotic activity, elevated Ki-67 proliferation index and necrosis, supporting the diagnosis of poroid hidradenocarcinoma. Tumor cells were positive for CK5 and CK7. Molecular analysis identified a YAP1::NUTM1 fusion and oncogenic mutations in EGFR exon 20 and FBXW7. At 30 months of follow-up after axillary lymph node dissection and chemoradiation, there was no evidence of disease progression. This case represents the first report of primary nodal poroid hidradenocarcinoma, expanding its clinicopathologic spectrum. Awareness of this entity is important to avoid misdiagnosis as metastatic carcinoma and to prevent potential overtreatment.