Age-specific interval breast cancers in New South Wales and meta-analysis of studies of women aged 40-49 years

Objective: To compare interval cancers in the 40-49 year age group with other age groups in New South Wales and with published trials and service studies. Setting: New South Wales data were derived from the population-based biennial mammography screening program, which achieved state-wide coverage in 1995. Women aged 40-49 years screened during 1995-1998 were included. Methods: Bilateral two-view mammography with reading by two radiologists was employed for biennial screening examinations. Interval cancers were detected by the screening program and by linkage with the state-wide cancer registry. Incidence of interval cancer based on the date of diagnosis was estimated as a proportion of the expected underlying breast cancer incidence for first- and second-year interval cancers. Sensitivity estimates were also calculated. Comparative data for the 40-49 year age group were derived from the published literature for meta-analyses of trial and service studies. Results: Interval cancer rates for New South Wales decreased with increasing age, with the highest proportional incidence in the 40-49 year age group for first year (56%, 95% confidence interval [CI] 50-62%) and second-year (86%, 95% CI 82-90%) interval cancers. Proportional incidence for women aged 50-69 years for first- and second-year interval cancers was 31% (95% CI 29-33%) and 50% (95% CI 47-52%) respectively. Sensitivity estimates for the program increased significantly with age, with lowest sensitivity estimates evident for women 40-49 years. In women aged 40-49 years the meta-analysed proportional incidence rate for randomised trials of screening for first- and second-year interval cancers was 42% (95% CI 21-62%) and 63% (95% CI 55-71%) respectively, while for service studies it was 44% (95% CI 31-58%) and 72% (95% CI 51-92%). Proportional incidence in the New South Wales program for women aged 40-49 years was not significantly different to the meta-analysed proportional incidences for trial and service studies in the first year, or for service studies in the second year. Conclusion: Proportional incidence of interval cancer was significantly higher in women aged 40-49 years in New South Wales relative to older age groups, but did not differ significantly from service studies of women in a similar age group. The effectiveness of mammography screening for this age group needs to be examined in view of the comparatively high rate of interval cancers.