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Case Reports

Abstract

Primary extramammary Paget disease (EMPD) is an uncommon intraepidermal adenocarcinoma typically occurring in apocrine-rich areas such as the anogenital or axillary regions. Occurrence outside these sites is exceedingly rare and poses diagnostic challenges. We present a case of a 59-year-old female who developed a solitary, irregularly shaped, erythematous plaque on the anterior shin. She had no known history of visceral malignancy. An initial biopsy was interpreted as squamous cell carcinoma in situ (SCCIS). The patient underwent multiple shave excisions with apparent resolution, but the lesion recurred after several years. Topical fluorouracil was then initiated with transient improvement followed by recurrence. Upon referral to our center, repeat biopsy revealed an intraepidermal proliferation of atypical cells with pagetoid scatter, displaying enlarged, hyperchromatic basophilic nuclei and palely eosinophilic, vacuolated cytoplasm. Immunohistochemical staining showed strong positivity for pancytokeratin, CK7, and CAM5.2, with patchy expression of CEA and focal positivity for mucicarmine. An immunopanel including CK5/6, p63, SOX10, neuroendocrine markers, CK20, AR, adipophilin, and GCDFP15 was negative, excluding SCCIS, melanoma in situ, Merkel cell carcinoma, and sebaceous carcinoma. These findings supported a diagnosis of primary EMPD arising on the lower extremity. This case highlights the importance of considering EMPD in the differential diagnosis of intraepidermal neoplasms, even in non-apocrine sites. Rare cases of EMPD in non-anogenital locations raise important questions about its enigmatic histogenesis, with proposed origins including eccrine structures, ectopic apocrine glands, or putative epidermal stem cells. Awareness of such presentations and thorough immunohistochemical work-up are essential to avoid misdiagnosis and ensure appropriate management.