Approximately one‐third of Merkel cell carcinoma (MCC) patients eventually develop distant metastatic disease. Little is known about whether the location of the primary lesion is predictive of initial distant metastatic site, or if survival likelihood differs depending on the metastatic site. Such data could inform imaging/surveillance practices and improve prognostic accuracy. Multivariate and competing‐risk analyses were performed on a cohort of 215 MCC patients with distant metastases, 31% of whom had two or more initial sites of distant metastasis. At time of initial distant metastasis in the 215 patients, metastatic sites (n = 305) included non‐regional lymph nodes (present in 41% of patients), skin/body wall (25%), liver (23%), bone (21%), pancreas (8%), lung (7%), and brain (5%). Among the 194 patients who presented with MCC limited to local or regional sites (stage I‐III) but who ultimately developed distant metastases, distant progression occurred in 49% by 1 year and in 80% by 2 years following initial diagnosis. Primary MCC locations differed in how likely they were to metastasize to specific organs/sites (P < .001). For example, liver metastases were far more likely from a head/neck primary (43% of 58 patients) versus a lower limb primary (5% of 39 patients; P < .0001). Skin‐only distant metastasis was associated with lower MCC‐specific mortality as compared to metastases in multiple organs/sites (HR 2.7; P = .003), in the liver (HR 2.1; P = .05), or in distant lymph nodes (HR 2.0; P = .045). These data reflect outcomes before PD1‐pathway inhibitor availability, which may positively impact survival. In conclusion, primary MCC location is associated with a pattern of distant spread, which may assist in optimizing surveillance. Because it is linked to survival, the site of initial distant metastasis should be considered when assessing prognosis.

IMPORTANCE Merkel cell carcinoma (MCC) often behaves aggressively; however, disease-recurrence data are not captured in national databases, and it is unclear what proportion of patients with MCC experience a recurrence (estimates vary from 27%-77%). Stage-specific recurrence data that includes time from diagnosis would provide more precise prognostic information and contribute to risk-appropriate clinical surveillance. OBJECTIVE To estimate risk of stage-specific MCC recurrence and mortality over time since diagnosis. DESIGN, SETTING, AND PARTICIPANTS This prospective cohort study included 618 patients with MCC who were prospectively enrolled in a Seattle-based data repository between 2003 and 2019. Of these patients, 223 experienced a recurrence of MCC. Data analysis was performed July 2019 to November 2021. MAIN OUTCOMES AND MEASURESStage-specific recurrence and survival, as well as cumulative incidence and Kaplan-Meier analyses. RESULTS Among the 618 patients included in the analysis (median [range] age, 69 years; 227 [37%] female), the 5-year recurrence rate for MCC was 40%. Risk of recurrence in the first year was high (11% for patients with pathologic stage I, 33% for pathologic stage IIA/IIB, 30% for pathologic stage IIIA, 45% for pathologic stage IIIB, and 58% for pathologic stage IV), with 95% of recurrences occurring within the first 3 years. Median follow-up among living patients was 4.3 years. Beyond stage, 4 factors were associated with increased recurrence risk in univariable analyses: immunosuppression (hazard ratio [HR], 2.4; 95% CI, 1.7-3.3; P < .001), male sex (HR, 1.9; 95% CI, 1.4-2.5; P < .001), known primary lesion among patients with clinically detectable nodal disease (HR, 2.3; 95% CI, 1.4-4.0; P = .001), and older age (HR, 1.1; 95% CI, 1.0-1.3; P = .06 for each 10-year increase). Among 187 deaths in the cohort, 121 (65%) were due to MCC. The MCC-specific survival rate was strongly stage dependent (95% at 5 years for patients with pathologic stage I vs 41% for pathologic stage IV). Among patients presenting with stage I to II MCC, a local recurrence (17 arising within/adjacent to the primary tumor scar) did not appreciably diminish survival compared with patients who had no recurrence (85% vs 88% MCC-specific survival at 5 years). CONCLUSIONS AND RELEVANCEIn this cohort study, the MCC recurrence rate (approximately 40%) was notably different than that reported for invasive melanoma (approximately 19%), squamous cell carcinoma (approximately 5%-9%), or basal cell carcinoma (approximately 1%-2%) following definitive therapy. Because more than 90% of MCC recurrences arise within 3 years, it is appropriate to adjust surveillance intensity accordingly. Stage-and time-specific recurrence data can assist in appropriately focusing surveillance resources on patients and time intervals in which recurrence risk is highest.

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