The mammographic breast density distribution of Finnish women with breast cancer and comparison of breast density reporting using the 4th and 5th editions of the Breast Imaging-Reporting and Data System

Purpose: To examine the breast density distribution in patients diagnosed with breast cancer in an eastern Finnish population and to examine the changes in breast density reporting patterns between the 4th and 5th editions of the Breast Imaging-Reporting and Data System (BI-RADS). Method: 821 women (mean age 62.8 +/- 12.2 years, range 28-94 years) with breast cancer were included in this retrospective study and their digital mammographic examinations were assessed semi-automatically and then visually by two radiologists in accordance with the 4th and 5th editions of the BI-RADS. Intraclass correlation coefficients (ICCs) were used to evaluate interobserver reproducibility. Chi-square tests were used to examine the associations between the breast density distribution and age or body mass index (BMI). Results: Interobserver reproducibility of the visual assessment was excellent, with an ICCr = 0.93. The majority of breast cancers occurred in fatty breasts (93.8 %) when density was assessed according to the 4th edition of the BI-RADS. The distributions remained constant after correction for age and BMI. Using the 5th edition, there was an overall 50.2 % decrease in almost entirely fatty (p < 0.001), 19.4 % increase in scattered fibroglandular (p < 0.001), 28.7 % increase in heterogeneously dense (p < 0.001), and 2.1 % increase in extremely dense (p < 0.001) categories. Conclusions: Most breast cancers in eastern Finland occur in fatty breasts with an area density of < 50 %. Assessing breast density using the 5th edition of the BI-RADS greatly increased denser assessments.