• Division of Cancer Prevention, National Cancer Institute, United States. Electronic address: pp4f@nih.gov.
• Division of Cancer Prevention, National Cancer Institute, United States.
• Division of Cancer Epidemiology and Genetics, National Cancer Institute, United States.

BACKGROUND: Ascertaining incident cancers is a critical component of cancer-focused epidemiologic cohorts and of cancer prevention trials. Potential methods: for cancer case ascertainment include active follow-up and passive linkage with state cancer registries. Here we compare the two approaches in a large cancer screening trial.

METHODS: The Prostate, Lung, Colorectal and Ovarian (PLCO) cancer screening trial enrolled 154,955 subjects at ten U.S. centers and followed them for all-cancer incidence. Cancers were ascertained by an active follow-up process involving annual questionnaires, retrieval of records and medical record abstracting to ascertain and confirm cancers. For a subset of centers, linkage with state cancer registries was also performed. We assessed the agreement of the two methods in ascertaining incident cancers from 1993 to 2009 in 80,083 subjects from six PLCO centers where cancers were ascertained both by active follow-up and through linkages with 14 state registries.

RESULTS: The ratio (times 100) of confirmed cases ascertained by registry linkage compared to active follow-up was 96.4 (95% CI: 95.1-98.2). Of cancers ascertained by either method, 86.6% and 83.5% were identified by active follow-up and by registry linkage, respectively. Of cancers missed by active follow-up, 30% were after subjects were lost to follow-up and 16% were reported but could not be confirmed. Of cancers missed by the registries, 27% were not sent to the state registry of the subject's current address at the time of linkage.

CONCLUSION: Linkage with state registries identified a similar number of cancers as active follow-up and can be a cost-effective method to ascertain incident cancers in a large cohort.