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Merkel cell carcinoma (MCC) is a rare but aggressive skin cancer with limited evidence on the role of PET scanning. The primary aim of this study was to assess the impact of 18 F-FDG PET in the staging and management of MCC. Methods: A single-institution review using clinical outcome data collected until February 2012 was performed of patients with MCC who underwent staging PET scanning between January 1997 and October 2010. Management plans were recorded prospectively at the time of the PET request, and followup outcomes were recorded retrospectively. The clinical impact of PET was scored using our previously published criteria: "high" if the PET scan changed the primary treatment modality or intent; "medium" if the treatment modality was unchanged but the radiation therapy technique or dose was altered. The primary objective was to test the hypothesis that the true proportion of patients who have a high-or medium-impact scan would be greater than 25%. Results: The median follow-up of 102 consecutive patients was 4.8 y. The results of staging PET had an impact on patient management in 37% of patients (P , 0.003). High-and medium-impact scans were recorded for 22% and 15% of patients, respectively. PET staging results differed from conventional staging results in 22% of patients, with PET upstaging 17% and downstaging 5%. The 3-and 5-y overall survival was 60% (95% confidence interval, 50%-71%) and 51% (95% confidence interval, 41%-64%), respectively. In stratification by PET-defined stage, the 5-y overall survival was 67% for patients with stage I/II disease but only 31% for patients with stage III disease (log-rank P , 0.001). The 5-y cumulative incidence of locoregional failure, distant failure, and death was 16.6%, 22.3% and 14.3%, respectively. On multivariate analysis, only PET stage (P , 0.001) and primary treatment modality (P 5 0.050) were significantly associated with overall survival. The primary treatment modality was not associated with progression-free survival when stratification was by tumor stage. Conclusion: The use of 18 F-FDG PET scans had a great impact on patients and may play an important role in the prognostic stratification and treatment of this disease.

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