If a patient develops a local relapse after lumpectomy/radiation she is usually advised to have a mastectomy, though it may be an option to do another lumpectomy with or without more radiation (go here and here).

The next most common situation is to radiate the chest wall area after a mastectomy in patients with more advanced cancer
     see postoperative radiation
or radiation can be used if the cancer relapses on the chest wall area
    see radiation for chest wall relapses
Also review the sections on side effects of radiation
    see radiation side effects
Radiation can be used to stop ovarian hormone production which will benefit premenopausal women (go here)

Occasionally patients develop distant spread (metastases) and radiation can often be effective in these situations
     see bone metastases
     see brain metastases
     see liver metastases