Extramammary Paget disease with underlying hidradenoma papilliferum: guilt by association?

Extramammary Paget disease (EMPD) is a heterogenous entity representing either an intraepidermal adenocarcinoma in situ with apocrine differentiation or an expression of underlying malignancy of the skin or of the intestinal or genitourinary tract. The coexistence of EMPD with a benign underlying hidradenoma papilliferumn (HP) is, however, exceptional. We present the case of a 79-year-old woman with diffuse and patchy gray-white lesions involving her left vulva as well as an underlying 0.7-cm asymptomatic firm nodule. Histologically, the epidermis and dermis showed features characteristic of EMPD and HP, respectively. Malignant transformation in HP giving rise to EMPD in the overlying epithelium has been reported. In our case, however, failure to demonstrate continuity between the two lesions together with the lack of cytologic atypia, mitoses, and necrosis in the HP lends additional support to the possibility that the HP is "innocent" and that its association with EMPD is thus coincidental. A common histogenetic derivation of these two lesions from the mammary-like glands or from related germinative cells in the epidermis is suggested to explain this rare presentation.