Whole breast radiation therapy (RT) after breast-conserving surgery is sometimes omitted in Japan; however, its impact on the outcome has not been properly evaluated. A multi-institutional retrospective study was conducted to clarify the impact of RT on local control after breast-conserving therapy (BCT). Data were collected from 3576 patients from 37 participating hospitals, of whom 1763 were eligible for analyses. Five hundred and five patients had ipsilateral breast tumor recurrence (IBTR) and 1258 patients did not. Details of IBTR were available for 245 of 505 patients who had IBTR, the location of IBTR was within or adjacent to the original tumor bed in 168 patients (68.6%). IBTR was salvaged with partial mastectomy in 119 patients (48.6%). Second recurrence in the ipsilateral breast was observed in 27 patients (11.0%). Univariate analyses demonstrated that administration of RT, the resection margin status, hormone responsiveness, T stage, N stage and stage were significantly related to IBTR. Multivariate analysis demonstrated that administration of RT, T stage and N stage were significantly correlated to IBTR. Among them, administration of RT had the largest impact on RT and it decreased the risk of IBTR by 77.3%. Omission of RT had the most significant impact on IBTR. RT should be given as a standard component of BCT.

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