Do incident and new subsequent cases of non-melanoma skin cancer registered in a Danish prospective cohort study have different 10-year mortality?

Background: The Danish Gerda Frentz Cohort (GFC) was created for registering all incident and new subsequent cases of non-melanoma skin cancer (NMSC) among patients seen by Danish dermatologists in 1995. We have recently found, in this cohort, a lower 10-year mortality than in the general population in patients with basal cell carcinoma (BCC). Differences in mortality between incident and new subsequent cases, incomplete registration or selection bias may be responsible for this finding. Methods: We aimed to quantify differences in mortality between incident and new subsequent cases of NMSC in the GFC and to compare mortality among incident cases recorded in the GFC and those recorded in the Danish Cancer Registry (DCR). We followed 10,830 skin cancer patients and 106,696 age-, gender- and residence-matched population controls through 2006 and computed their cumulative mortality and mortality rate ratio (MRR). Results: One-, 5-, and 10-year cumulative mortality of incident and new subsequent cases of BCC and SCC in the GFC were similar. Likewise, MRR for incident BCC (MRR = 0.91; 95% CI 0.84–0.98) and incident SCC (MRR = 1.29; 95% CI 1.05–1.56) among patients registered in the GFC were similar to their counterparts in the DCR (MRR = 0.96; 95% CI 0.91–1.00 and MRR = 1.36; 95% CI 1.22–1.52). Conclusion: Mortality of incident and new subsequent cases of NMSC was similar and thus did not explain the reduced mortality of BCC patients.