 Radiotherapy has been shown to reduce breast-cancer specific mortality in patients at high risk for distant dissemination. It has also been shown to increase rates of non-breast cancer deaths and morbidity due to cardiovascular and pulmonary toxicity. Although treatment planning has improved significantly through the years, recent reports still demonstrate treatment-related morbidity even with 3-dimensional planned techniques. Thus, while 3D planning represents the state of the art treatment for loco-regional radiotherapy for breast cancer, further improvement is needed to continue to decrease heart and lung exposure. The ultimate goal of the proposed research is to determine whether treatment planning using intensity-modulated radiotherapy IMRT, the next generation of radiation treatment delivery systems, results in less radiation exposure to the heart and lungs than the best current RT technique in women with node positive breast cancer. This proposal will test the potential clinical value of IMRT compared to the best standard 3D plan partially wide tangent fields, PWTF in the treatment of breast cancer. These two treatment techniques will be studied in a Phase II randomized trial using quantitative indicators of potential cardiac and lung toxicity. The preliminary data generated from this trial will be used to ultimately justify a multi-institutional comparison of the two treatment techniques with long-term clinical cardiac and pulmonary toxicity as endpoints. 