Radiotherapy to the remaining breast tissue after lumpectomy, or other breast conserving operations, greatly reduces the risk of cancer coming back in that breast (local recurrence).
Without radiotherapy, about 27 women in 100 will have a local recurrence of their cancer following breast conserving surgery. With radiotherapy, this goes down to less than 9 women in every 100. So, having radiotherapy reduces the risk of local recurrence by about two thirds or 66%.
Because of this UK national guidelines recommend that women have radiotherapy after breast conserving surgery for breast cancer.