RATIONALE: Radiation therapy uses high-energy x-rays to kill tumor cells. Giving radiation therapy in different ways may kill any tumor cells that remain after surgery. It is not yet known whether whole breast radiation therapy is more effective than partial breast radiation therapy in treating breast cancer.

PURPOSE: This randomized phase III trial is studying whole breast radiation therapy to see how well it works compared to partial breast radiation therapy in treating women who have undergone surgery for ductal carcinoma in situ or stage I or stage II breast cancer.