This is a randomized comparison of two different radiotherapy field set-ups for post-mastectomy treatment of locally advanced breast cancer. Patients who have undergone modified radical mastectomy including axillary lymph node dissection will be randomized to receive one of two radiotherapy treatment arms, A and B following the completion of adjuvant chemotherapy. The radiotherapy for treatment Arm A consists of irradiation of the chest wall only while Treatment Arm B includes irradiation of the chest wall and the ipsilateral supraclavicular field. Patients on both treatment arms will receive radiation with a shortened fractionation schedule. Patients will be evaluated for local control, regional control, survival and treatment toxicity.