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Case Reports

Abstract

A 69-year-old woman with a history of diffuse large B-cell lymphoma and melanoma in situ of the chin presented with a 6 mm scaling papule on the radial aspect of the right third finger. Shave biopsy revealed acanthosis, hypergranulosis, and multifocal epidermal invaginations with dyskeratotic upper epidermal keratinocytes and overlying parakeratosis. While the diagnosis of verruca plana was considered, the absence of papillomatosis and viral cytopathic changes and distinctive foci of abnormal dyskeratotic cells are most consistent with the diagnosis of porokeratotic acanthoma. Only 23 cases have been reported to date as solitary lesions most commonly located in the distal upper and lower extremities.  Numerous variants of porokeratosis have been described, including disseminated superficial actinic porokeratosis, porokeratosis of Mibelli, linear porokeratosis, disseminated superficial porokeratosis, and porokeratosis palmaris et plantaris disseminata. They occur most commonly in immune compromised patients and are thought to represent clonal expansions of keratinocytes resulting from a biallelic deficiency in genes encoding enzymes of the mevalonate pathway.  They harbor a risk of malignant transformation, mostly to squamous cell carcinoma, estimated in the range of 6.4% to 30%.  For this reason complete excision or ablative treatment is recommended. This case highlights the key histologic features that enable recognition of this rare entity.