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Risk of mortality and ipsilateral breast cancer by age for the BCT treatment group. Age was grouped into 10 years intervals, and Cox proportional hazards model was used to estimate the relative risk of death and ipsilateral breast tumor recurrence (IBTR) in each age group for patients in the BCT treatment group. The reference age group was (30, 40). The smoothed version of these relative hazards was performed by cubine spline interpolation
