Poroid neoplasm is a skin appendage tumor that has both benign and malignant counterparts. It has traditionally been regarded as of eccrine origin and has four types: intraepidermal poroma (hidroacanthoma simplex), poroma, dermal duct tumor, and poroid hidradenoma. Here we describe the case of a 64-year-old woman who had a verrucous, erythematous to brownish tumor on her left buttock for many years. Histopathology revealed an intra-epidermal poroid tumor with both benign and malignant parts. The benign part had intra-epidermal nests of poroid cells, which were smaller, monomorphic and sharply marginated from adjacent keratinocytes. The malignant part showed similar cell types, but had a higher nuclear/cytoplasmic ratio, pleomorphism, and prominent mitoses. Ductal structures were noted in neoplastic cells and an epithelial membrane antigen stain was strongly positive. Interestingly, peripheral palisading and primitive follicular germ formation were also observed in the neoplasm, which suggests follicular differentiation. We made a final diagnosis of porocarcinoma in situ with follicular differentiation, which may support the folliculosebaceous-apocrine unit theory, but a tumor with such a combination has not been described before.
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