Merkel Cell Carcinoma — SCE Dermatology MCQ
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Correct answer: D — Approximately 80% in most series
The correct answer is D, approximately 80% in most series. Merkel cell polyomavirus (MCPyV) DNA integrates clonally into the tumour genome in the majority of Merkel cell carcinomas (MCC), driving oncogenesis through expression of viral large T and small T antigens that inactivate retinoblastoma protein function. Across the northern hemisphere, MCPyV DNA integrates into the host genome of approximately up to 80% of MCCs, although prevalence is lower in high UV-exposure regions such as Australia (around 30%). This bimodal aetiology (virus driven versus UV driven, high mutational burden) is now the accepted pathogenetic model in European and UK dermato-oncology guidance, and virus-positive tumours carry a comparatively better prognosis than virus-negative UV-mutated tumours. Why the other options are wrong: B. Only UV-associated MCCs are virus-positive: this reverses the biology; UV-associated MCC is the virus-negative subtype, driven instead by a very high UV-induced mutational burden rather than viral oncoproteins. C. 100%: a fifth of MCCs are virus-negative, arising through UV-mediated mutagenesis with a mutational rate up to 100-fold higher than virus-positive tumours, so universal positivity is incorrect. E. 50%: this understates the true proportion; published series consistently report a higher virus-positive fraction, particularly outside high-UV-exposure regions. A. Less than 10%: this figure describes neither subtype accurately and grossly underestimates the well-documented virus-positive majority seen in most Western case series. Key point: Roughly 80% of MCCs are MCPyV-positive (virus-driven, better prognosis), while the remaining 20% are virus-negative and driven by UV-induced mutations with a much higher mutational burden and worse prognosis.
Reference: European Dermatology Forum/EADO/EORTC consensus-based interdisciplinary guideline, Diagnosis and treatment of Merkel cell carcinoma, Update 2022, European Journal of Cancer