Medial Canthal Collision Tumor with Seborrheic Keratosis Hiding Underlying Basal Cell Carcinoma

A 74-year-old man presented with a 3-year history of a tan papule on his right medial canthus (A). Excisional biopsy showed basket-weave hyperkeratosis (B, arrowhead), acanthosis (C, double-headed arrow), papillomatosis with squared fronds (C, ∗), and horn pseudocysts (D, ★) overlying nests of peripheral palisading atypical basaloid cells with surrounding retraction artifact (E, arrows), indicative of a collision tumor consisting of an overlying seborrheic keratosis (SK) and an underlying nodular-type basal cell carcinoma (BCC). The patient underwent Mohs micrographic excision and reconstruction for the residual BCC. Although SK are benign lesions, SK may rarely overlie and conceal cutaneous malignancies. (Magnified version of Figure A-E is available online at www.aaojournal.org).