Exercise Testing for Early Stage Breast Cancer Patients Receiving Radiation Treatment

Inclusion Criteria: * Female * ECOG performance status 0 or 1 * Medical clearance from attending radiation oncologist or medical oncologist to undergo a symptom-limited CPET * Currently undergoing intact breast radiotherapy for early stage breast cancer (including TisN0, T1N0, T2N0) or has completed intact breast RT in the last 3 months. * Receiving or received a prescribed dose of 4240 cGy in 16 fractions to the whole breast without treatment directed at nodal basins. Patients may also be receiving or have received a boost to the lumpectomy bed at the discretion of the treating physician. * Subjects may be treated in the supine or prone position at the discretion of the treating physician. * Subjects may be treated with or without a deep inspiratory breath hold technique at the discretion of the treating physician. * Identified by the treating radiation oncologist as having significant treatment-related fatigue or minimal treatment-related fatigue and not simply baseline fatigue. A score of "0" on the RTOG fatigue scale will be considered minimal fatigue, while a score of "2" or greater will be considered significant fatigue (moderate fatigue causing difficulty performing some ADLs). Exclusion Criteria: * Currently undergoing post-mastectomy radiation * Has received chemotherapy previously or has a plan to receive chemotherapy during the timeframe of study assessment. * Any of the following absolute contraindications to CPET, as per American Thoracic Society (ATS) recommendations * Acute myocardial infarction (within 3-5 days of any planned study procedures) * Unstable angina * Uncontrolled arrhythmia causing symptoms or hemodynamic compromise * Recurrent syncope * Active endocarditis * Acute myocarditis or pericarditis * Symptomatic severe aortic stenosis * Uncontrolled heart failure * Acute pulmonary embolus or pulmonary infarction within 3 months of any planned study procedures * Thrombosis of lower extremities * Suspected dissecting aneurysm * Uncontrolled asthma * Pulmonary edema * Room air desaturation at rest ≤ 85% * Respiratory failure * Acute non-cardiopulmonary disorders that may affect exercise performance or be aggravated by exercise (i.e. infection, renal failure, thyrotoxicosis) * Mental impairment leading to inability to cooperate