Eruptive squamous cell carcinoma in a patient treated with concomitant pembrolizumab and imiquimod

Pembrolizumab, a humanized monoclonal antibody against programmed cell death 1, is used for various malignant neoplasms. Toll-like receptor (TLR) agonists, specifically targeting the TLR9 subfamily (TLR7-9), are treatment options for solid tumors and hematological malignancies. We experienced a case of eruptive squamous cell carcinoma (SCC) in a patient treated concomitantly with pembrolizumab and imiquimod, a TLR7 agonist. A 75-year-old woman who was given a diagnosis of bladder cancer with lung metastasis received pembrolizumab for 3 months when she was referred to our department for the evaluation of skin rashes on her hands. Her skin lesions were diagnosed as well-differentiated SCC and treated with topical imiquimod. Two months after the start of imiquimod, more than 10 reddish papules appeared on her hands. The histological diagnosis of a new plaque was the same as an earlier biopsy. We herein describe this case in detail and provide a published work review.