The incidence of interval cancers in the German mammography screening program: results from the population-based cancer registry in North Rhine-Westphalia.

BACKGROUND: Cancer screening programs must meet high standards for quality and effectiveness, because many healthy persons need to be screened to detect relatively few cases of cancer. In this study, we use the rate of interval cancers as an important surrogate indicator for evaluating the German mammography screening program (MSP).
METHODS: All first-time participants in the MSP in the German federal state of North Rhine-Westphalia over the period 2005-2008 whose screening mammogram had been read as negative were followed over the next 24 months for the potential development of breast cancer (an "interval cancer" or IC). The screening data in the MSP database were compared in anonymized fashion with reports of cancer that were recorded at the Epidemiological Cancer Registry North Rhine-Westphalia.
RESULTS: Among the 878,764 women with negative screening mammograms in the first screening round, 2036 (23.2 per 10,000) developed an IC in the ensuing 24 months. These ICs accounted for 40% of all T2-T4 breast cancers occurring in first-time participants in the screening program in the 2 years after screening. The relative rate of IC compared to the background incidence of breast cancer before introduction of the MSP was 27% in the first year and 58% in the second. Screening detected 78% of all breast cancers that occurred during a maximum of 2 years after screening.
CONCLUSION: The IC rates in the implementation phase of the MSP agree with those found in other, established European programs. The present study is the first one to assess this important surrogate parameter to characterize the effectiveness of the German MSP among women in North Rhine-Westphalia, Germany's largest state by population.