Sebaceous Carcinoma in Situ Masquerading Clinically and Histologically as Paget Disease of the Breast.

Sebaceous carcinoma in situ is a poorly understood and ill-defined entity. In situ sebaceous carcinoma exhibits a similar location pattern to its invasive counterpart in that most commonly has a periorbital distribution. Review of the literature found only seven cases of extraocular sebaceous carcinoma in situ. We present a unique and challenging case of sebaceous carcinoma in situ masquerading both clinically and histologically as Paget's disease of the breast. A 61-year-old female presented to her dermatologist complaining of a 6 mm erythematous waxy papule on her medial right breast. The patient's past medical history was significant for Muir-Torre syndrome. Clinically, the differential diagnosis included Paget disease of the breast, squamous cell carcinoma, and sebaceous carcinoma. A shave biopsy revealed an atypical proliferation of large single cells limited to the epidermis infiltrating in a pagetoid pattern, as well as cohesive nests of round neoplastic cells with mild nuclear atypia, prominent nucleoli, and vacuolated cytoplasm. Histologically, the differential diagnosis included Paget's disease of the breast, squamous cell carcinoma in situ, melanoma in situ, and sebaceous carcinoma in situ. A battery of immunohistochemical stains was performed including CK7, EMA, CAM5.2, CK20, and MART-1. The lesional cells were positive for adipophilin, factor XIIIa, CK7, and EMA and were negative for CAM5.2, CK20, and MART-1 supporting a diagnosis of sebaceous carcinoma in situ. Multiple deeper sections were examined and invasion beyond the epidermis was not identified. This case adds to the paucity of information available regarding extraocular sebaceous carcinoma in situ and warns clinicians of this potential diagnostic pitfall especially in patients with Muir-Torre syndrome.