Clinical characteristics and treatments of Merkel cell carcinoma in Japan: A multi-center retrospective study

Background: Merkel cell carcinoma (MCC) is a rare, aggressive cutaneous neuroendocrine carcinoma with limited data from Asian populations. Material and methods: We retrospectively analyzed data from 216 patients with stage I–III MCC who received surgery or radiotherapy, and 76 patients at any stage who received systemic therapy, across 36 Japanese institutions between April 2018 and March 2022. Results: The cohort demonstrated a higher incidence in females and a high rate of Merkel cell polyomavirus (MCPyV) positivity. Among stage I–III patients, 103 underwent surgery alone, 83 were treated with surgery and adjuvant radiotherapy, and 30 received radiotherapy alone. The 1-year recurrence-free survival (RFS) and overall survival (OS) rates were 73.7 % and 90.7 %, respectively. Although radiotherapy alone correlated with worse RFS and OS, no significant differences were observed among stage I–II patients treated with curative-intent surgery, surgery plus adjuvant radiotherapy, or radiotherapy alone. Among 76 patients with unresectable or metastatic MCC who received systemic therapy, 72 received first-line avelumab; with a 43.1 % response rate (RR), a median progression-free survival (PFS) of 9.9 months, and a median OS of 38.7 months. After avelumab failure, second-line chemotherapy yield a 46.2 % RR and a median PFS of 4.1 months. Conclusion: Japanese patients with MCC exhibited distinct clinical characteristics compared to Western populations, with comparable avelumab effectiveness. Outcomes did not differ significantly across modalities in patients with localized disease treated with curative intent. This largest real-world Asian MCC dataset provides key insights for optimizing treatment strategies for Asian patients.