Radiation therapy is a simple, painless and generally well-tolerated tool for treating and even curing breast cancer. One of the most common side effects of radiation therapy to the breast (after a lumpectomy) or to the chest wall (after a mastectomy) is fatigue. This reaction and its extent is different for every woman. Because radiation therapy is often such an important part of breast cancer treatment, it is important to know how to mitigate its side effects in order to gain the greatest benefit from the therapy.