Mammary-type Metaplastic Carcinoma Arising From Vulvar Hidradenoma Papilliferum: A Case Report

Hidradenoma papilliferum is a benign neoplasm arising in the vulvovaginal region. These lesions are thought to originate from anogenital mammary-like glands and share morphologic and immunohistochemical features with breast tissue. Though rare, malignant transformation of hidradenoma papilliferum into mammary-type carcinoma, typically mammary-type ductal and mammary-type lobular carcinoma, has been documented. However, mammary-type metaplastic carcinoma arising from hidradenoma papilliferum is exceedingly rare. Herein, we describe a case of this entity in a patient presenting with a labial mass. Histologic examination revealed a papillary proliferation consistent with hidradenoma papilliferum, with focal ductal carcinoma in situ (DCIS). Adjacent to this was a biphasic malignant neoplasm composed of a high-grade carcinomatous component and an infiltrating sarcomatoid component with pleomorphic tumor cells. Immunohistochemically, the DCIS and carcinomatous components were positive for CAM5.2, CK7, claudin-4, and GATA-3, while the malignant sarcomatoid component was positive for p63 and GATA-3. Both invasive components demonstrated aberrant cytoplasmic p53 expression. Molecular analysis identified pathogenic mutations in TP53, PIK3CA, APC, and NF1. The aforementioned findings mirror those found in metaplastic carcinoma of the breast, and a diagnosis of biphasic tumor analogous to mammary-type metaplastic carcinoma, likely arising from hidradenoma papilliferum, was rendered. This case captures a continuum of disease progression from hidradenoma papilliferum to mammary-type metaplastic carcinoma and not only strengthens the association of mammary-type malignancies of the vulvovaginal region originating from breast-like structures native to this location, but expands on the spectrum of morphologic, immunohistochemical, and molecular features of these lesions.