This study is to prospectively investigate the cardiac dose-sparing effect and clinical benefit of deep inspiration breath-hold (DIBH) technique. Patients with left-sided breast cancer treated with breast conserving surgery followed by radiotherapy is enrolled. Radiotherapy is delivered with either free-breathing or deep inspiration breath-hold (DIBH) technique. The cardiac dose parameters and cardiac toxicity are prospective evaluated, and the dose-effect relationship is analyzed.

Patients with left-sided breast cancer treated with breast conserving surgery are prospectively enrolled. Two types of radiation treatment are delivered based on attending physician's preferences: radiation to whole breast +- regional nodal areas with free-breathing technique, or with deep inspiration breath-hold (DIBH) technique. The cardiac dose parameters including heart dose and the dose to left anterior coronary artery are assessed, and the cardiac toxicities regularly assessed with cardiac enzymes,electrocardiogram(ECG) and normal gated single-photon emission computed tomography-myocardial perfusion imaging before, during and after radiation.The dose-effect relationship of heart injuries is analyzed.

Inclusion Criteria:

1. Left-sided breast cancer
2. Underwent breast conserving surgery
3. Planned for postoperative whole breast +- regional nodal radiotherapy
4. No basic heart disease

Exclusion Criteria:

1. Abnormal cardiac baseline examination
2. History of chest radiation
3. Can not tolerate deep inspiratory breath-holding

female only