Data quality at the Bulgarian National Cancer Registry: An overview of comparability, completeness, validity and timeliness.

Reporting of neoplasms in Bulgaria has been compulsory since a directive from the Ministry of Health in 1951. The quality of cancer registry data has been estimated rather infrequently in past years. We aimed to provide a comprehensive evaluation of the quality of the data at the Bulgarian National Cancer Registry (BNCR). Quantitative and semi-quantitative methods were applied for cancers diagnosed during the whole period 1993-2010, and also for cases diagnosed in 2006-2010. The methods used include historic data methods, mortality-to-incidence ratios (M:I), capture-recapture and death-certificate methods, proportions of morphologically verified cases (MV%), death-certificate-only cases (DCO%), and cases with missing information (primary site unknown, PSU%; stage unknown, SU%). The BNCR coding and classification systems follow international standards. The overall completeness was estimated at 92.6-94.7% for the period 2006-2010, with variations between cancer sites (86.7-98.5%). During the period 1993-2010, M:I decreased to 0.5 for males and 0.4 for females, MV increased to 87.4%, DCO and SU decreased to 4.8% and 18.8%, respectively, and PSU remained at the same level of about 4% for both sexes together. Sub-analysis revealed differences by site, sex and age groups. The comparison with other registries from the region showed similar incidence rates and directions of trends: M:I, MV% and DCO% that were not significantly different. The underreporting in 2008 and 2009 due to timely publication was estimated at an overall 0.8% and 0.5%, respectively. The present review showed that the BNCR yields internationally comparable data that are reasonably accurate, timely, and close to complete, especially in recent years. This is a prerequisite for the BNCR to expand its role to more areas of cancer control.