Abstract

Extramammary Paget disease (EMPD) is a rare entity that can mimic melanoma dermoscopically and histopathologically. It is a challenging diagnosis, especially in the setting of melanocytic lesions such as melanocytic hyperplasia, melanoma in situ (MIS), or malignant melanoma. Synchronous EMPD and MIS are exceedingly rare with a high incidence of misdiagnosis. Herein, we present a 67-year-old female patient with a past medical history of prior multiple non-melanoma skin cancers who presents initially with a 1.5 cm brown macule on the left buttock. A skin shave biopsy of the lesion was performed and revealed malignant melanoma in situ with regression and positive lateral margins. Skin re-excision of the MIS was performed with a 0.5 cm margin of clinically normal-appearing skin. Histopathology revealed an acanthotic epidermis with large single cells which have abundant pale cytoplasm and vesicular nuclei with prominent nucleoli arranged in a pagetoid fashion. Atypical cells were positive for CK7 and negative for CK20, CK903, SOX10, and Melan-A. Additionally, a poorly demarcated pigmented lentiginous junctional growth of atypical melanocytes was noted. The atypical melanocytes are large, hyperchromatic with pleomorphic nuclei arranged in single cells and nests with scattered pagetoid cells. Atypical cells were positive for SOX10 and Melan-A while were negative for CK7. The overall findings were compatible with a collision tumor of EMPD and MIS. Subsequently, a vulvectomy was performed and revealed micro-invasive EMPD. In conclusion, cases of collision tumors of EMPD and MIS are extremely rare with overlapping morphologic features. A panel of immunohistochemical stains can be of assistance in such cases, to avoid diagnostic pitfalls. To our knowledge, this is the seventh confirmed case of synchronous EMPD and MIS tumor in the perianal area.

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