Identification of "safe" dose volume constraints (DVC) for the heart from breast cancer radiotherapy would permit maximal oncologic benefit from radiation, identifying which patients require more complicated and resource consumptive radiation methods such as respiratory-gating or intensity modulated radiotherapy (IMRT) and establish patients who need targeted follow-up for their long term cardiac risk. Two recent technical developments now make it conceivable to identify the dose-volume relationship between heart dose and subsequent cardiac event risk: 1. CT based three dimensional conformal radiation delivery methods (3DCRT) where the precise radiation dose to any given heart volume is known, and 2. Cardiac MRI (CMR) that is more sensitive to prior methods of cardiac evaluation (e.g. SPECT) for detecting and measuring cardiac injury.