Single administration of avelumab induced a complete response in thyroid transcription factor 1‐positive combined Merkel cell carcinoma

Merkel cell carcinoma (MCC) is an aggressive neoplasm and patients with metastasis have poor survival outcomes. Recently, avelumab, an anti‐programmed death ligand 1 (PD‐L1) immune checkpoint inhibitor, was approved for first‐line treatment in patients with metastatic MCC. While the administration interval of avelumab is every 2 weeks, the durable effect of a single administration of avelumab is unknown. Additionally, the effect of avelumab in pure MCC or combined MCC concurrent with non‐MCC histology has not been fully elucidated. Herein, we report a case of combined MCC concurrent with squamous cell carcinoma; the patient had a complete response after a single administration of avelumab. Although the levels of avelumab were outside the detection limit within 12 weeks, a remarkable efficacy remained for more than 28 weeks after administration. Immunohistochemical analyses revealed that the expression of PD‐L1 and Merkel cell polyomavirus large T antigen was almost negative or only partial in the primary tumor lesion of this patient. Conversely, thyroid transcription factor 1 (TTF‐1) expression was positive in the primary MCC lesion, which is consistent with a previous report that combined MCC is positive for TTF‐1 expression. In conclusion, this case study presents a rare case of TTF‐1‐positive combined MCC showing complete response after a single administration of avelumab.