Clinical utility of a circulating tumor cell assay in Merkel cell carcinoma

Background: Quantitation of circulating tumor cells (CTCs) has utility in managing breast, colon, and prostate carcinomas. Objective: We sought to determine whether a commercially available CTC assay provides prognostic information in Merkel cell carcinoma (MCC), insight into treatment responses, or both. Methods: We analyzed CTCs in 52 specimens from 34 patients with MCC. Results: The presence of CTCs correlated with extent of disease at blood draw (P = .004). Among 15 patients with regional nodal disease, CTC-negative patients had 80% disease-specific survival at 2 years after the test, versus 29% for CTC-positive patients (P = .015). Among the entire cohort, those without CTCs had 72% MCC-specific survival whereas CTC-positive patients had 25% survival (n = 34, median follow-up 19 months, P = .0003). Fifty seven percent of patients with MCC had a cytokeratin dot visible in 20% or more of CTCs, a feature that was absent among CTCs from other carcinomas (0 of 13 cases). Limitations: CTC assay was performed at variable times after diagnosis and heterogeneity in extent of disease affects interpretability of the data. Conclusion: CTC detection in MCC is feasible and appears to add prognostic information, particularly in patients with regional nodal disease. It may also assist clinical management in certain situations, including differentiating metastatic MCC cells from those of other carcinomas.