Hidroacanthoma simplex (HAS) is a rare benign
tumor that is also known as
intraepidermal poroma. While there have been a few reports of HAS with malignant transformation (porocarcinoma), we report an unusual case of porocarcinoma, arising in a pigmented HAS, the latter also showing secondary
amyloid deposits. An 80-year-old Japanese man presented with a cutaneous
tumor on his left buttock, which had first been noticed in his childhood. The
tumor consisted of flat pigmented plaque and a depigmented papule with erosion. Histologic analysis revealed many pigmented and well-defined nests within the epidermis of the flat pigmented portion. The nests were composed of cuboidal to oval and occasionally elongated, bland, basaloid cells with numerous
melanin granules. In addition, there were infrequently ductal structures and small clusters of sebocytes, and abundant
amyloid deposits in the upper dermis. These findings were consistent with pigmented HAS with
amyloid deposition. In the depigmented portion, markedly atypical cells with occasional ductal structures and intracytoplasmic lumina extended throughout the entire thickness of the epidermis, with minimal invasion of the dermis. We considered this portion of the
tumor to be a porocarcinoma. Since the two portions of the
tumor were continuous, we made a final diagnosis of porocarcinoma arising in pre-existing pigmented HAS with
amyloid deposition.