A 30-Year Single-Centre Series of Unknown Primary Merkel Cell Carcinoma: Management and Prognosis

Background: Merkel cell carcinoma (MCC) is a rare, aggressive cutaneous neuroendocrine malignancy. Like cutaneous malignant melanomas, a subset of MCCs can clinically present without an identifiable primary tumour, termed Merkel cell carcinoma of unknown primary (UPMCC), with incompletely understood behaviour and prognosis. Our objectives are to evaluate the clinical presentation, management and survival outcomes in UPMCC and compare overall survival with metastatic MCC of known-primary origin. Methods: A retrospective review of 252 consecutive MCC patients (1992–2023) identified 20 cases of histologically confirmed nodal or metastatic UPMCC. Demographics, anatomical distribution, treatment and oncological outcomes were analysed. Overall survival was compared with patients presenting with metastatic MCC of known primary. Results: The cohort included 15 males and 5 females (mean age 76 years). Presentation most commonly involved inguinal (n = 7) and axillary (n = 6) nodes, followed by parotid and cervical basins (n = 4). Management was multimodal, including lymphadenectomy, radiotherapy and systemic therapy. Six patients remained disease-free at a mean follow-up of 63.3 months. The mean overall survival was 43.65 months for UPMCC versus 39.29 months for known-primary metastatic MCC. Conclusions: UPMCC most commonly presents as inguinal or axillary nodal disease. Survival outcomes suggest a trend toward improved prognosis compared to known-primary metastatic MCC.