Ascertainment of cancer cases in the German National Cohort (NAKO): methods and initial results

BACKGROUND: The German National Cohort (NAKO Gesundheitsstudie) is a prospective cohort study with 205 053 participants. Its goal is to investigate risk factors for chronic diseases, including cancer. METHODS: We describe the methods of ascertaining cancer cases in NAKO (linkage with cancer registries and self-reporting), the incidence and prevalence figures obtained so far, and the ratio of observed to expected (O/E) case numbers. RESULTS: Altogether, 2774 prevalent (diagnosed within the 5 years prior to study enrollment) and 4295 incident (diagnosed up to 5 years after enrollment) cancer cases were identified. Breast, prostate, lung, and colorectal cancers made up 55% of the incident cases. Fewer incident cases were found than would have been expected on the basis of incidence rates in the general population (O/E ratio 0.80, 95% confidence interval [0.76;0.83] for the first 2 years of prospective follow-up). The O/E ratio varied, however, across tumor sites (breast 1.02, prostate 1.12, lung 0.38, colorectal 0.62). Possible explanations include bias caused by the fact that most study participants were healthy, delayed tumor reporting, incomplete data linkage, and incomplete self-reporting. CONCLUSION: Rising case numbers for the most commonly occurring types of cancer are expected in the next few years. This will enable comprehensive epidemiological analysis of the risk factors for cancer.