Clinicopathological Characteristics of Merkel Cell Carcinoma in Younger Patients: A Retrospective Study.

Merkel cell carcinoma (MCC) is a rare and aggressive cutaneous neuroendocrine carcinoma that predominantly affects older adults. MCC arising in younger patients is exceedingly rare, and its clinicopathologic features remain poorly described. We retrospectively reviewed clinical records, histopathology, and outcomes of patients aged 18 to 50 years diagnosed with MCC at a single tertiary care center between 2009 and 2025. A total of 13 patients were identified with a median age of 40 years (range: 18 to 50 y); 61.5% were female. Of 11 patients with available data, 36.4% had pre-existing immune suppression. Tumors arose most commonly on the extremities (46.2%), followed by the head and neck (23.1%), trunk (15.4%), and unknown primary (15.4%). The most common growth pattern was nodular (63.6%), followed by infiltrative (27.3%). Median Breslow depth was 10.5 mm and median mitotic rate was 46/mm². Immunohistochemical staining was consistent with MCC in all cases. Merkel cell polyomavirus was detected by large T antigen chromogenic in situ hybridization and/or immunohistochemistry in 62.5% of assessable cases (5/8). Lymph node metastases were present at presentation in 60.0% of patients, and distant metastases developed in 54.5%. Kaplan-Meier estimated overall survival was 74.1% at 2 years and 59.3% at 5 years. In this single-institution cohort, early-onset MCC was associated with elevated rates of immune suppression and a substantial burden of regional and distant metastatic disease, highlighting clinicopathologic distinctions from predominantly older historical MCC populations.