Extraocular Sebaceous Carcinoma Presenting as a Presumed Sebaceous Cyst in a Patient With Lynch Syndrome

Extraocular sebaceous carcinoma is a rare cutaneous malignancy that may clinically mimic benign lesions, potentially delaying diagnosis. It is associated with Muir-Torre syndrome, a phenotypic variant of Lynch syndrome characterized by sebaceous neoplasms and internal malignancies. We report a 60-year-old man with germline-confirmed Lynch syndrome and a history of resected urothelial carcinoma who presented with a painless, progressively enlarging right upper cheek mass that had been present for six months. Examination revealed a firm, freely mobile, 15 × 12 mm subcutaneous nodule without overlying skin changes or regional lymphadenopathy. Biopsy demonstrated well-to-moderately differentiated sebaceous carcinoma with invasion into skeletal muscle. Immunohistochemical evaluation demonstrated loss of MSH2 and MSH6 expression, consistent with mismatch repair deficiency and the patient's known germline Lynch syndrome. Given muscle invasion, the patient underwent re-excision with intraoperative frozen-section analysis, which showed no residual malignancy, and final pathology confirmed clear margins. Postoperative PET scan showed no evidence of metastatic disease. At four months, the patient remains recurrence-free and continues surveillance for local recurrence and other Lynch syndrome-associated malignancies. This case highlights the importance of maintaining a high index of suspicion for sebaceous carcinoma in patients with Lynch syndrome, even when lesions present as clinically benign-appearing subcutaneous nodules. Early biopsy, complete surgical excision, and appropriate systemic surveillance are essential for management and detection of associated malignancies.