Interval cancers in the Norwegian Breast Cancer Screening Program: Frequency, characteristics and use of HRT

Breast cancers diagnosed between screening examinations among women who att end a breast cancer screening program are defined as interval cancers. The Norwegian Breast Cancer Screening Program started as a pilot project in 199 6, and data from the first 2-year interval are available. Our study quantif ies interval cancers in the pilot project and explores characteristics and factors that may be associated with interval cancer. Interval cancers in th e screening population were identified through the Cancer Registry of Norwa y. The frequency of invasive interval cancer was calculated as cases per 10 ,000 screened and as observed/expected ratio. Characteristics of the interv al cancers were compared to screening-detected and clinical cancers. Breast density was assessed in a blinded review of 3 categories of screening mamm ograms. Information on hormone replacement therapy (HRT) use was collected from a questionnaire. The frequency of invasive interval cancers was 18.2 ( 15.9-20.7) per 10,000 screened and the observed/expected ratio was 0.49 (0. 43-0.56). The frequency in the second year of the interval was higher than reported from other programs. The median tumor size of the interval cancers was 19.5 mm and 44.0% of the patients had affected axillary lymph nodes. T he interval cancer cases had higher proportions of dense breasts and report ed use of HRT compared to screen normal and screening-detected cases. The r eported frequency of interval cancers is similar to comparable programs. Th e interval cancers differed significantly from the cancers detected in the first screening round and were more similar to clinical cancers. Interval c ancer was associated with dense breasts and use of HRT. Screening programs must keep these associations in focus. (C) 2001 Wiley-Liss, Inc.