Standardized incidence ratios (SIR) comparing the incidence of ER-positive and ER-negative CBC to the incidence of unilateral breast cancer, overall and according to ER-status of the first breast cancer, for all patients and stratified on age.

26% of the CBC-cases had unknown ER-status at their first cancer, 34% of the CBC-cases had unknown ER-status of their second cancer. SIRs standardized for age at diagnosis in 5-year-categories and for period diagnosis in 5-year-categories. SIR = Standardized incidence ratio, CI = Confidence Interval, CBC = contralateral breast cancer, ER = Estrogen receptor.