Statistical analysis
The primary end point of the present study was ipsilateral breast tumor recurrence (IBTR), defined as recurrence in the ipsilateral breast. The time to IBTR was calculated from the date of breast conserving surgery to the occurrence of IBTR, and it was censored at the time of the last follow-up or the time of death among patients who did not suffer IBTR. Patients characteristics of the two groups were compared by means of the chi-square test and the Kruskal–Wallis test, where appropriate. Kaplan–Meier method was used to estimate IBTR-survival, distant-metastasis-free survival (DMFS), and overall survival (OS). Differences between groups were compared with the log-rank test. Multivariate analyses were carried out using the Cox proportional hazards model. All values were two sided, and statistical significance was defined as P <0.05. All calculations were performed with SPSS 19.0 software (SPSS Inc., IBM, Chicago, USA).