A review of interval breast cancers diagnosed among participants of the Nova Scotia breast screening program

Purpose: To conduct a radiologic review of interval breast cancer cases to determine rates of true interval and missed cancers in Nova Scotia, Canada. Materials and Methods: This quality assurance project was exempt from institutional review board approval. Interval cancer cases were identified among women aged 40-69 years who were participants in the Nova Scotia Breast Screening Program from 1991 to 2004. For each case, the index negative screening mammogram was reviewed blindly by three radiologists from a pool of experienced radiologists. Cases were identified as those with normal or abnormal findings, the latter being a case that required further investigation. True interval cases were identified as cases in which a minimum of two radiologists reviewed the findings as normal. True interval and missed cancer rates were calculated separately for women according to age group and screening interval (for ages 40-49 years, a 1-year interval; for ages 50-69 years, a 1-year and a 2-year interval). Results: The rate of missed cancers per 1000 women screened was one-half of the true interval rate among women screened annually (for ages 40-49 years, 0.45 vs 0.93; for ages 50-69 years, 1.08 vs 2.22). Among women aged 50-69 years who were screened biennially, the rate of missed cancers per 1000 women screened was one-third of the true interval rate (0.90 vs 3.15). Similarly, the rate of missed cancers per 10 000 screening examinations was one-half of the true interval rate among those 40-49 years old (1.95 vs 3.99) and one-third of the true interval rate among those 50-69 years old (3.34 vs 10.44). Conclusion: In screening programs, true interval cancer rates should be differentiated from missed cancer rates as part of ongoing quality assurance.