INTERVAL BREAST CANCERS IN THE SCREENING MAMMOGRAPHY PROGRAM OF BRITISH-COLUMBIA - ANALYSIS AND CLASSIFICATION

OBJECTIVE. Interval cancers in an annual mammography screening program are defined as cancers detected within 12 months after a mammographic screening in which findings are considered normal. Our objective was to analyze interval cancers for histologic type, tumor size, staging, and histologic grade. A classification for interval cancers is present ed. Detection of interval cancers is an integral part of quality contr ol and is required for the establishment of sensitivity rates for scre ening mammography, a necessary part of implementing the recommendation s for breast cancer screening. MATERIALS AND METHODS. A total of 47,58 3 mammographic screening examinations of 38,219 women were available f or review for the period of July 1988 to March 1991. Linkage with the provincial population-based cancer registry provided identification of all interval cancers of the breast. Histologic analysis of breast can cers was provided by an internationally recognized breast cancer patho logist who interpreted the specimens twice. RESULTS. A total of 207 br east cancers were detected at mammographic screening (true-positive ca ncers) during the first 33 months of the program, and 37 interval canc ers were identified during the 33 months of screening plus a 12-month interval. Blinded review by three radiologists experienced in screenin g mammography resulted in classification of 21 of 37 cases as true int erval cancers and 16 cases as misses. No interval cancers were missed because of poor mammographic technique. The sensitivity of screening m ammography was 85%, 63% for patients less than 50 years old and 89% fo r patients more than 50 years old. The specificity was 93% for all pat ients (both younger and older than 50 years). CONCLUSION. The availabi lity of a population-based cancer registry in the province of British Columbia makes an accurate determination of interval cancers and sensi tivity rates possible. These data are essential for quality analysis, program planning, and education of radiologists in screening mammograp hy. There were more true interval cancers than misses, and histologic analysis demonstrated that the majority of interval cancers were in st ages I and II.