15-Year Experience of 18F-FDG PET Imaging in Response Assessment and Restaging After Definitive Treatment of Merkel Cell Carcinoma

The objective of this study was to evaluate the utility of F-18-FDG PET in restaging and response assessment of patients who underwent definitive treatment for Merkel cell carcinoma (MCC). Methods: A retrospective review of patients undergoing F-18-FDG PET imaging for MCC between January 1997 and October 2010 at the Peter MacCallum Cancer Centre with follow-up until February 2015 was performed. Data analysis was performed on patients who were treated definitively and underwent post-treatment PET imaging performed either as a restaging scan for ongoing monitoring, suspicion of recurrence, or assessment for suitability of salvage treatment or as response assessment within 1-6 mo of treatment. Management plans were recorded prospectively before F-18-FDG PET imaging and compared with post-imaging management to assess the impact of the study as per our previously defined categories: high if the primary treatment modality or intent was changed and medium if the radiotherapy technique or dose was altered. In total, 62 patients were included in the analysis. Thirty-six patients underwent 53 restaging scans, and 37 patients underwent a response-assessment scan. The median follow-up of patients in the restaging group was 5.3 y (95% confidence interval [CI], 4.6-9.4), and it was 5.7 y (95% CI, 4.3-10.8) in the response-assessment group. Results: Restaging F-18-FDG PET scans had a high impact in 24 of 53 cases (45%) and a medium impact in 6 of 53 cases (11%). In the response-assessment group, 24 of 37 patients had a complete metabolic response (CMR). Patients without a CMR had a 15% 1-y overall survival (95% CI, 0.04-0.55). Those with a CMR had an 88% 2-y overall survival (95% CI, 0.75-1.00) and a 68% 5-y overall survival (95% CI, 0.49-0.95). The presence of a CMR (P < 0.001) and nodal involvement (P = 0.016) were statistically significant prognostic factors for overall survival. Conclusion: F-18-FDG PET imaging had a high impact on restaging after definitive treatment in patients with MCC. Metabolic response was significantly associated with overall survival. (1)8(F)-FDG PET may play an important role in ongoing post-treatment management of MCC.