Objective: To determine the incidence of interval cancers which occurr
ed in the first 12 months after mammographic screening at a mammograph
ic screening service. Design: Retrospective analysis of data obtained
by crossmatching the screening Service and the New South Wales Central
Cancer Registry databases. Setting: The Central & Eastern Sydney Serv
ice of BreastScreen NSW. Participants: Women aged 40-69 years at first
screen, who attended for their first or second screen between 1 March
1988 and 31 December 1992. Main outcome measures: Interval-cancer rat
es per 10 000 screens and as a proportion of the underlying incidence
of breast cancer (as estimated by the underlying rate in the total NSW
population). Results: The 12-month interval-cancer incidence per 10 0
00 screens was 4.17 for the 40-49 years age group (95% confidence inte
rval [CI], 1.35-9.73) and 4.64 for the 50-69 years age group (95% CI,
2.47-7.94). Proportional incidence rates were 30.1% for the 40-49 year
s age group (95% CI, 9.8-70.3) and 22% for the 50-69 years age group (
95% CI, 11.7-37.7). There was no significant difference between the pr
oportional incidence rate for the 50-69 years age group for the Centra
l & Eastern Sydney Service and those of major successful overseas scre
ening trials. Conclusion: Screening quality was acceptable and should
result in a significant mortality reduction in the screened population
. Given the small number of cancers involved, comparison of interval-c
ancer statistics of mammographic screening programs with trials requir
es age-specific or age-adjusted data, and consideration of confidence
intervals of both program and trial data.