Abstract

Background: 

Sebaceous carcinoma in situ is an exceptionally rare intraepidermal neoplasm with few cases reported in the literature. It primarily affects the head and neck region, while other areas of the body are rarely involved. 

Case Presentation: 

A 63-year-old male presented with a left inferior lateral posterior arm lesion. Microscopic examination revealed a proliferation of poorly differentiated atypical neoplastic sebocytes confined to the epidermis with pleomorphic nuclei and prominent nucleoli. Clear cell changes were noted. The neoplastic cells demonstrated immunoreactivity for Adipophilin, Androgen Receptor, EMA, P63, BerEP4, and CK7. Interestingly, mismatch repair immunostaining was preserved for MLH1, PMS2, MSH2, and MSH6. The findings were consistent with the diagnosis of poorly differentiated carcinoma in situ with clear cell changes. The differential diagnosis included sebaceous carcinoma in situ, squamous carcinoma in situ with sebaceous differentiation/clear cell squamous carcinoma in situ, and less likely clear cell superficial basal cell carcinoma. The tumor was completely excised with no recurrence after 1 month.

Discussion: 

Early diagnosis and removal are critical as sebaceous carcinomas have a high incidence of metastasis and generally poor prognosis. Dermatopathologists should have a high index of suspicion for a sebaceous carcinoma in situ diagnosis when assessing patients with skin lesions in unusual locations, such as those on the arm. Accurate diagnosis and complete excision are warranted to avoid recurrence and possible metastasis.