Tumor primary site as a prognostic factor for Merkel cell carcinoma disease-specific death

Background: Merkel cell carcinoma (MCC) primary site has not been fully investigated as a potential prognostic factor. Objective: To determine the incidence by tumor primary site of death due to MCC. Methods: We undertook a retrospective analysis of the Survival, Epidemiology, and End Results database. MCC patients treated between 1973 and 2016 were grouped by tumor primary site and a competing risks analysis was performed to test the impact of primary site on disease-specific death. Cumulative incidence of Merkel cell carcinoma-specific mortality (CMMI) at 5 years was estimated for each primary site. Results: Of 9407 MCC patients identified, 6305 (67.0%) had localized disease, 2397 (25.5%) had regional metastasis, and 705 (7.5%) had distant metastasis. Tumor primary site was predictive of CMMI and varied by stage at diagnosis. Tumors involving the scalp/neck carried the highest CMMI among localized MCC (26.0%). Tumors involving the lip had the highest CMMI among MCC with regional metastasis (56.7%) and distant metastasis (82.1%). Limitations: Tumor size data were missing for a large proportion of patients, precluding stratification by stage according to current American Joint Committee on Cancer guidelines. Conclusions: Probability of MCC disease-specific death varies by primary site. The primary site of the tumor may be useful as a prognostic indicator for MCC.