Vulvar Poroid Hidradenoma

To the Editor: We report a case of poroid hidradenoma, a recently described variant of the eccrine poroma group of tumors, and the first to present in the vulva. A 46-year old female presented with dyspareunia for 2 weeks. She noticed a dark spot on the external surface of the right labia majora, which had gradually increased in size over the past 12 months. The lesion was excised. Gross examination revealed a skin covered, well-circumscribed nodule measuring 1.4 × 1.1 × 1.0 cm. The cut surface showed a partially cystic lesion filled with cloudy opaque fluid. On H&E sections, a well-circumscribed solid-cystic epithelial lesion confined to the dermis with no connection to the overlying epidermis was seen (Fig 1). The cyst wall was composed of tumor cells of two types that formed the solid component. Predominantly the tumor comprised of small, dark staining, monomorphous, poroid cells surrounding few tubular lumina and cystic spaces of various sizes. The nuclei were round-to-oval with small but prominent nucleoli. The other cell type comprised of larger, paler, more pleomorphic cuticular cells with abundant pink cytoplasm. Some of these cells were multinucleate with distinct nucleoli. Vacuolar spaces were seen within the cytoplasm of some cuticular cells probably representing early ductular differentiation. Well-defined ductal structures surrounded by the cuticular cells, few of them lined by eosinophilic cuticle were also noted. Peripheral palisading was occasionally observed at the edges of the tumor nests. There was no evidence of cellular atypia, increased mitosis, or necrosis (Fig 2). Periodic Acid Schiff (PAS) stain demonstrated glycogen granules in the cytoplasm of both types of tumor cells and highlighted the cuticular luminal border of ductal structures. The majority of the smaller poroid cells and small groups of the larger cuticular cells showed positive cytoplasmic staining for epithelial membrane antigen (EMA) (Insert, Fig 2). Fewer cells were positive for carcinoembryonic antigen (CEA).FIG. 1.: Well-circumscribed solid-cystic neoplasm confined to the dermis. Cystic spaces and luminal structures can be identified.FIG. 2.: Predominantly the tumor is composed of smaller, darker poroid cells with round-to-oval nuclei. Admixed are larger, paler cuticular cells showing conspicuous ductular differentiation (arrow). Few of these cells are multinucleate. Note the absence of atypia, mitosis, and necrosis. Insert: Strong cytoplasmic positivity for EMA in the tumor cells.Overall, the features are consistent with poroid hidradenoma, a rare benign appendageal tumor of purported eccrine origin first described in 1990 (1). This tumor shows architectural characteristics of hidradenoma (a tumor confined to the dermis) and cytologic features of a poroid neoplasm (poroid and cuticular cells, the latter showing ductal differentiation), hence the name poroid hidradenoma (2). Four histopathologic types of poromas are defined according to the architectural features (1): hidroacanthoma simplex or intraepidermal poroma; eccrine poroma, when the cells involve basal layer of the epidermis and extend to the superficial part of the dermis; dermal duct tumor, when the tumor is composed of solid aggregates of cuboidal and cuticular cells devoid of cysts and restricted to the dermis; poroid hidradenoma, which is solid-cystic dermal poroma. The term hidradenoma has been classically applied to benign neoplasma with eccrine differentiation. However, some of these show apocrine differentiation. Ackerman and Abenoza (1) characterize hidradenomas into two types: apocrine hidradenoma, the larger set which is comprised of clear, squamoid, and mucinous cells; and eccrine hidradenoma with poroid and cuticular cells also known as poroid hidradenoma. Eccrine tumors are believed to arise by proliferation and expansion of the basal cells of intradermal eccrine ducts. Some tumor cells may differentiate towards luminal cells of the eccrine ducts, forming ductal lumina, whereas others differentiate towards the abluminal eccrine secretory regions, forming cystic spaces (3). Eckert et al ascribe the origin of eccrine poroma to a pluripotent stem cell of the transitional zone between the dermal and intraepidermal segments of the eccrine duct (4). Similar to previous studies, both the poroid and cuticular cells in our case were positive for EMA suggesting a close relationship between eccrine poroma and poroid hidradenoma and confirming that poroid hidradenoma is a type of poroma (5,2). Carcinoembryonic antigen positive cells adjacent to cystic spaces have been implicated to cause cyst formation in cases of poroid hidradenoma (6,7). To date, about a dozen cases of poroid hidradenoma have been reported in literature, mostly in the extremities and the head and neck (1,5,6,7,8,9). Occurrence in the vulva probably suggests that there is no site of predilection for this rare tumor. Salem O. Alowami, M.D., F.R.C.P.C. Abha Malik, M.D. Wedad Hanna, M.D., F.R.C.P.C.

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