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 identify risk factors for chronic diseases, including cancer. METHODS:We describe the methods of ascertaining cancer cases in NAKO (i.e., linkage with cancer registries and self-reporting), the incidence and prevalence figures obtained so far, and the ratios of observed to expected (O/E) case numbers. RESULTS:Case ascertainment identified 2774 existing cancer cases diagnosed within the five years prior to study enrollment and 4295 new cancer cases diagnosed up to five years after enrollment. Cancers of the breast, prostate, lung, and colorectum made up 55% of the incident cases. Fewer incident cases were found than would have been expected on the basis of incidences in the general population (O/E ratio 0.80, 95% confidence interval [0.76;0.83] for the first two-years of prospective follow-up); the O/E ratio varied across tumor sites (breast 1.02, prostate 1.12, lung 0.38, colorectum 0.62). Possible explanations include healthy volunteer bias, delayed reporting, as well as incomplete data collection from registries and self-reporting. CONCLUSION:Rising case numbers for the most common types of cancer are expected in the next few years and will enable comprehensive epidemiological analysis of the risk factors of cancer.