Abstract: Radiotherapy (RT) is an important treatment for breast cancer. As the left breast is adjacent to heart, heart irradiation cannot be completely avoided. Radiotherapy on breast cancer increases the risk of ischemic heart disease, pericarditis and valvular disease. Tumor location and treatment choices influence cardiac dose with complex interactions. Radiation technology plays an important role in cardiac toxicity of radiotherapy. Multi-field IMRT may be the optimal one which can balance PTV coverage and organ at risk sparing for left-sided breast cancer after mastectomy; as to the patients after breast-conserving surgery irradiation, volumetric-modulated arc therapy (VMAT) offer certain dosimetric advantages over fixed-field IMRT plans. Compared with whole breast irradiation, partial breast irradiation shows a significant reduction in radiation dose for the heart. As to lymph node-positive left-sided breast cancer patients, VMAT or tomotherapy retains target homogeneity and coverage and allows maximum doses to organs at risk to be reduced; deep inspiration breath hold results in a significant reduction in radiation dose to the heart and left anterior descending coronary artery compared with an free breathing, especially for the patients who need to irradiate chest wall and regional lymph node; and for some patients with large breast after surgery, prone setup can significantly reduce the amount of heart volume. Moreover, left-sided radiotherapy concurrent with trastuzumab or AI increases the cardiac toxicity risk. In conclusion, left-sided breast radiotherapy plan needs to consider patients' age, BMI, primary tumor location, size, postoperative breast size, whether need regional lymph node radiation; giving the optimal radiotherapy plan according to the existing radiation therapy equipment, at the same time, reducing the synchronous cardiac toxicity increasing treatment, so as to minimize cardiac adverse event.