Existing data sources for clinical epidemiology: Danish Cancer in Primary Care cohort

BACKGROUND: In this paper, we describe the settings, content, and possibilities of the Danish Cancer in Primary Care (CaP) cohort as well as some of the key findings so far. Further, we describe the future potential of the cohort as an international resource for epidemiological and health services research studies. METHODS: The CaP cohort comprises information from three Danish subcohorts set up in 2004–2005, 2007–2008, and 2010 on newly diagnosed cancer patients aged 18 years or older. General practitioner (GP)-reported and patient-reported data from six questionnaires generated information on causes and consequences of delayed diagnosis of cancer, and these data were supplemented with complete information on, eg, death, migration, health care utilization, medication use, and socioeconomic data from Denmark’s comprehensive health and administrative registers. The cohort is followed up in terms of emigration, death, hospitalization, medication, and socioeconomics, and data are updated regularly. RESULTS: In total, we identified 22,169 verified incident cancer cases. Completed GP questionnaires were returned for 17,566 (79%) of the verified cases, and patient questionnaires were completed by 8,937 (40%) respondents. Patients with participating GPs did not differ from patients with nonparticipating GPs in regard to one-year survival, comorbidity, or educational level. However, compared with nonparticipating GPs, patients listed with participating GPs were more likely to be women, younger, to have a higher disposable income, to have more regional or distant spread of tumors, were also more likely to have breast cancer, and were less likely to have prostate cancer. Responding patients were more likely to be women, aged 45–74 years, and diagnosed with breast cancer or malignant melanoma, and have higher one-year survival rates, more localized tumors, higher educational background, and higher disposable income. CONCLUSION: The cohort is an international resource for epidemiological and health service research, and data are accessible for well defined and approved collaborative studies.