Validation of the International Breast Cancer Intervention Study (IBIS) Model in the High Risk Ontario Breast Screening Program: A Retrospective Cohort Study.

PURPOSE: Women with a remaining lifetime risk of breast cancer ≥25%, estimated using the International Breast Cancer Intervention Study (IBIS) model, are eligible for the High Risk Ontario Breast Screening Program (OBSP). This study examined the performance of IBIS 10-year risk estimates in the Program.

METHODS: This retrospective study included 7487 women aged 30-69 years referred to the High Risk OBSP between July 1, 2011, and December 31, 2016, with follow-up until December 31, 2018. Model calibration and discrimination were assessed. Analyses were conducted overall and stratified by age (< or ≥50 years). Different 10-year risk thresholds were compared to the current eligibility criteria.

RESULTS: Overall, IBIS over-estimated the risk of breast cancer with an E/O ratio of 1.17, 95% CI 1.04, 1.35. Over-estimation was highest in women ages 50-69 years (E/O=1.29, 95% CI 1.03, 1.69), and for those in the top quartile of risk. Overall discrimination was fair with a c-statistic of 0.66 (95% CI 0.63, 0.70). Further, when using different 10-year risk eligibility thresholds most cases would have been missed in the 30-49 age group using the 8% 10-year risk threshold, while relatively few women aged 50-69 would have been ineligible at any of the thresholds examined.

CONCLUSION: We found IBIS over-estimates the risk of breast cancer in this screening cohort but has adequate discrimination. Age-specific risk thresholds should be considered to optimize Program eligibility criteria.