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Hidroacanthoma simplex (HS) is a rare variant of eccrine poroma. Clinically, it manifests as sharply demarcated, brownish, flat or verrucous plaque, and often resembles seborrheic keratosis or Bowen's disease. Histologically, it mimics clonal seborrheic keratosis (CSK), but can be differentiated by the density of CD1a cells inside the tumor nest.1 Pigmented HS is even more rare. We report a new case manifesting as a large, pigmented seborrheic keratosislike plaque on the right temporal area. Differentiation of HS from CSK was facilitated by the finding of a low density of CD1apositive dendritic cells within the tumor nests. A 73-year-old Taiwanese male with a history of squamous cell carcinoma on his left arm presented with a large, asymptomatic plaque on his right temporal scalp for 5 years. Examination revealed a 4.5 cm 5 cm, well-circumscribed hyperkeratotic plaque consisting of coalescing, brownish seborrheic keratosis-like nodules (Figure 1A). No regional lymph node was palpable. The tentative diagnosis was seborrheic keratosis or Bowen's disease. Histological examination of a biopsy specimen revealed epidermal hyperplasiawith bulbous elongation of rete ridges containing large, discrete nests of uniform basosquamous cells (Figure 1B). Numerous pigmented dendrites were visible in the tumor nests (Figure 1C). The tumor cells displayedminimal nuclear atypia, however, some mitotic figures were visible. Immunohistochemical study showed that the tumor cells did not express carcinoembryonic antigen (CEA) or epithelial membrane antigen (EMA) except for occasional tumor cells showing membranous staining. The nests showed a marked reduction of CD1aþ cells, compared with the surrounding epidermis. The number of CD1aþ cells within the nests was approximately 5.42 cells/mm2 (Figure 1D). For comparison, we counted CD1a cells in the nests of a typical case of CSK, which showed a much higher density of 14.50 cells/mm2. The density of CD1aþ cells was lower in the nests than outside the nests. With the presence of pigmented dendrites in the tumor nests, the final diagnosis was pigmented HS. Our patient preferred less aggressive treatment, therefore, the lesion was treated with cryotherapy with satisfactory results. HS, first described by Smith and Coburn in 1956, is an intraepidermal variant of eccrine poroma derived from the acrosyrigium.2 HS commonly affects the elderly without sex predilection, and usually manifests as hyperkeratotic erythematous or brown plaques,2

Pigmented hidroacanthoma simplex of the scalp mimicking clonal seborrheic keratosis