Ten years of breast screening in the Screening Mammography Program of British Columbia, 1988-97.

Objectives: To evaluate 10 years outcomes of the Screening Mammography Program of British Columbia (SMPBC) and determine if breast screening targets were being achieved among women aged 40-80+ years.

Setting: Organised breast screening programme in British Columbia, Canada.

Methods: Rates of participation, abnormal referral, cancer detection, and interval cancer were calculated for asymptomatic women receiving an SMPBC mammography from 1988-97.

Results: 895,849 screening mammographies were provided to 335,433 women. 51.3% of women were age 50-69 years. Abnormalities were identified on 57,454 screens (6.4%) from which 3304 cancers were detected. Abnormal call rates were higher on first (9.8%), compared with subsequent screens (4.4%) and declined with age: 7.7% at age < 40 to 5.4% for age 70-79 years. Cancer detection rates were higher on first (5.0 per 1000) compared with subsequent screens (2.8 per 1000) and increased smoothly with age from 1.4 to 8.2 per 1000 from age < 40 to age 80 years and older. Twenty per cent of cancers were non-invasive. The median size of invasive cancers was 14 mm and 81% had no axillary lymph node metastases. The 12 month interval cancer rate was 0.6 per 1000 and did not vary significantly with age or screening history. The prevalence to expected incidence ratio was 3.1 for women age 50-79 years.

Conclusion: Across a broad range of ages, surrogate indices of screening mammography success have been achieved in a population based, North American, organised breast cancer screening programme.