Characteristics of spontaneous regression of Merkel cell carcinoma: a retrospective analysis of patient and tumour features.

Merkel cell carcinoma (MCC) is an aggressive cutaneous neuroendocrine neoplasia. Complete spontaneous regression (CSR) of MCC has been linked to good prognosis. We conducted a retrospective review of a single-centre cohort of patients with MCC to analyse the characteristics of patients and tumours that exhibited regression. The calculated incidence of CSR was 4.5% (n = 5/112). The median age at diagnosis was 69 years (interquartile range 31), tumours were primarily located on the head and neck, and the cohort exhibited a low rate of immunosuppression. After a median follow-up period of 4 years, none of the patients who underwent CSR showed recurrence or distant metastases. Merkel cell polyomavirus (MCPyV) positivity was associated with CSR (P = 0.045). There is limited understanding of the mechanisms underlying regression in MCC. However, biopsy-induced immune responses linked with host and tumour factors, such as age, immunosuppression and MCPyV positivity, may play an important role. More research in this field is crucial to improve our understanding of this rare phenomenon.