Second cancers following the diagnosis of Merkel cell carcinoma: a nationwide cohort study.

Merkel cell carcinoma (MCC) is a rare neuroendocrine carcinoma of the skin. MCCs and some other skin cancers, such as basal cell carcinomas, frequently harbour Merkel cell polyomavirus DNA. The purpose of the study was to investigate the frequency of second cancers following the diagnosis of MCC. We studied the incidence of second primary cancers after the diagnosis of MCC from the files of the Finnish Cancer Registry in 1979-2006. Among the 172 MCC patients identified a total of 34 second primary cancers were detected in 30 individuals after the diagnosis of MCC. Female MCC patients were diagnosed with 25 subsequent cancers (SIR, 2.35; 95% CI, 1.52-3.47; p<0.001) and male patients with 9 cancers (SIR, 2.32, 95% CI, 1.06-4.40; p<0.05). The MCC patients had an increased risk for a subsequent cancer (any site) compared to age-, gender- and calendar period-matched general population (standardized incidence ratio [SIR] 2.34; 95% confidence interval [CI], 1.62-3.27). The risks for basal cell carcinoma of the skin (O=11), SIR, 3.48; 95% CI, 1.74-6.22 and chronic lymphocytic leukemia (O=2), SIR, 17.9; 95% CI, 2.16-64.6 were significantly elevated. The SIRs for an overall second primary cancer risk did not change markedly with time since the diagnosis of MCC. We conclude that patients diagnosed with MCC have an increased risk for a second cancer. This risk may in part result from shared etiological factors between MCC and other tumour types, such as immunosuppression or possibly Merkel cell polyomavirus infection.