Merkel cell carcinoma (MCC) is a rare and aggressive skin cancer that occurs in elderly patients. The risk of MCC is increased by immunosuppression due to conditions such as human immunodeficiency virus infection, hematological malignancies, and organ transplantation. These risk factors can also be associated with a poor prognosis in MCC. Chronic active Epstein-Barr virus (CAEBV) infection is a rare Epstein-Barr virus (EBV) infectious disease characterized by persistent infectious mononucleosis-like symptoms. Recently, programmed cell death ligand 1 (PD-L1) blockade using avelumab was approved as the first-line therapy for patients with metastatic MCC. Here, we report a case of extremely hyperprogressive MCC occurring in a patient with CAEBV.
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