This 90-year old man presented with a 0.5-centimeter papule on his right outer chest that was thought clinically to be an irritated seborrheic keratosis. Sebaceous carcinoma occurs most commonly in middle-aged or elderly patients. The head and neck, particularly the ocular area, are most prone to developing this neoplasm. It usually presents as an asymptomatic, nondescript nodule. Poorly differentiated cases, such as this one, are particularly treacherous and challenging because majority of the neoplastic cells have lost their sebocytic appearance. The histogenesis of the tumor, however, can be uncovered with careful and close inspection whereby one identifies scattered, albeit very few, cells with the unmistakable appearance of sebocytes. Basaloid (prominent peripheral palisading of neoplastic cells), squamoid (squamous metaplasia with formation of keratin pearls), and sarcomatoid (prominent spindle cells) variants of sebaceous carcinoma have been described. The differential diagnosis includes cutaneous clear cell neoplasms and basal cell carcinoma with sebaceous differentiation. The presence of sebocytes, sometimes aided by their dimorphic staining pattern for cytokeratin and epithelial membrane antigen, clinches the diagnosis.
Closer inspection reveals the tumor to be composed predominantly of variably atypical basaloid cells exhibiting large and pleomorphic nuclei, discernable nucleoli, and conspicuous mitotic activity. Admixed with these basaloid cells are a few well-differentiated sebocytes. A focus of squamous metaplasia is also present.