Objective: Clinical databases have been invented to monitor treatment outcomes, therapies or diseases, often in great detail. The traditional population-based cancer registry has been invented to collect a minimum of information about all incident cancers. Do clinical databases render population-based cancer registers obsolete as sources of cancer cases for epidemiological study?
Methods: We compared the study base of first incident breast cancer cases in Denmark in 1978–1994 known from the national cancer register and from the national clinical database on breast cancer patients. The clinical database is used for monitoring protocoled treatment.
Results: Combining the two data sources we found 48,522 first primary breast cancers in Denmark 1978–1994. Of these, 37,640 were included in both data sources, 2151 were included only in the clinical database, and 8731 were included only in the cancer register. A major part of the difference between the two data sources was due to treatment-focused data collection in the clinical database, and a minor part due to differences in the registration of second primaries, date of diagnosis and invasiveness.
Conclusions: Cancer incidence data are sensitive to registration procedures and definitions. Clinical cancer databases cannot generally replace the traditional cancer register as a reliable data source for incident cancer cases in a national population.