Unusual presentation of ectopic extramammary Paget disease.

Extramammary Paget disease (EMPD) is a malignant tumor typically found in apocrine-rich areas of the skin, particularly in the anogenital region. Some germinative apocrine-differentiating cells might exist on the trunk, preferentially in Asian individuals. Ectopic EMPD arises in nonapocrine-bearing areas, specifically the nongerminative milk line. We present a case of a 67-year-old Thai man with a slowly progressive, pruritic, erythematous to brown plaque on the right lower back of 30 years' duration. Histopathologic examination of 2 scouting biopsies revealed a proliferation of large cells with pleomorphic nuclei, prominent nucleoli, and abundant pale to clear cytoplasm within the epidermis. In one of the biopsies, tumor cells were found in the dermis with an infiltrative growth pattern. Immunohistochemically, the tumor cells were positive for cytokeratin 7, carcinoembryonic antigen, and gross cystic disease fluid protein 15. Based on these findings, a diagnosis of ectopic EMPD was made.