Abstract

Extramammary Paget Disease (EMPD) is a rare intraepithelial adenocarcinoma, typically presenting in areas rich in apocrine and modified apocrine glands with the vulva and perianal region most affected. It usually manifests as a slowly expanding, sharply demarcated, erythematous plaque with scattered erosion and white hyperkeratosis. EMPD can be primary or, less frequently, secondary to an associated visceral malignancy. Ectopic EMPD (E-EMPD), found in areas without apocrine or modified apocrine glands, is exceedingly rare, with fewer than 50 cases documented. The majority of cases have been reported on the trunk of Japanese patients. The precise cell of origin and pathogenesis of E-EMPD continues to be elusive, with theories proposing an origin from an eccrine gland or a pluripotent germinative cell. Here we present a case of an 88-year-old Caucasian male with a history of numerous NMSCs who presented with an asymptomatic erythematous patch on the right forearm. Histopathological evaluation revealed a proliferation of isolated, scattered cells with ample pale cytoplasm and large, pleomorphic nuclei, including mitoses infiltrating the entire epidermal thickness. Immunohistochemical staining revealed positivity for CK AE1/AE3, CK7, CEA, CAM5.2, and GDDFP15. HMWtCK, p63, Mart-1, and Sox10 stains were negative, the profile confirming the diagnosis of primary E-EMPD. Given its rarity, E-EMPD poses a diagnostic challenge. This case adds to the scarce literature and is even more unusual in light of its presentation on the extremity of a non-Japanese patient.