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Background: The treatment of stage I Merkel cell carcinoma (MCC) usually includes wide local excision (WLE) combined with irradiation of the tumor bed (ITB). No randomized study has ever been conducted in MCC. The purpose of this study was to assess the efficacy and safety of prophylactic adjuvant radiotherapy on the regional nodes., Patients and Methods: In this randomized open controlled study, patients for a stage I MCC treated by WLE and ITB were randomly assigned to regional adjuvant radiotherapy versus observation. Overall survival (OS) and probability of regional recurrence (PRR) were primary end points. Progression-free survival (PFS) and tolerance of irradiation were secondary end points., Results: Eighty-three patients were included before premature interruption of the trial, due to a drop in the recruitment mainly due to the introduction of the sentinel node dissection in the management of MCC. No significant improvement in OS (P = 0.989) or PFS (P = 0.4) could be demonstrated after regional irradiation, which, however, significantly reduced the PRR (P = 0.007) with 16.7% regional recurrence rate in the observation arm versus 0% in the treatment arm. The treatment was well tolerated., Conclusion: The adjuvant regional irradiation significantly decreased the PRR in MCC, but benefit in survival could not be demonstrated.