To conduct a radiologic review of interval breast cancer cases to determine rates of true interval and missed cancers in Nova Scotia, Canada.
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).
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).
In screening programs, true interval cancer rates should be differentiated from missed cancer rates as part of ongoing quality assurance.