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.