Multiple keratin granulomata: A potential histologic clue to cutaneous squamous cell carcinoma responding to programmed cell death protein 1 inhibitor therapy.

We present histologic features of a locally advanced cutaneous squamous cell carcinoma (cSCC) treated with the programmed cell death protein-1 (PD-1) antagonist, pembrolizumab, with a partial response. This contributes to a growing body of literature supporting the efficacy of pembrolizumab in treatment of surgically unresectable cSCC. We also provide a detailed description of the histologic features, particularly keratin granulomata with adjacent lymphocytic aggregates and fibrosis, observed in cSCC under treatment with a PD-1 antagonist.