The study is a single-center, prospective, non-randomized controlled study. The primary objective is to examine the clinical outcomes of utilizing Deep Inspiration Breath Hold (DIBH) in conjunction with hypo-fractionated radiotherapy for patients diagnosed with left-sided breast cancer with a particular focus on its potential to reduce the incidence of cardiac-related clinical and subclinical events.Participants will revieve large segmentation sadiotherapy ± DIBH for left breast cancer and be followed up to gather their clinical cardiac imformation.

Inclusion Criteria:

1. 18-70 years old, female, life expectancy > 5 years
2. ECOG 0, 1, 2 points
3. Pathologically diagnosed with left breast cancer and underwent breast-conserving surgery or modified radical mastectomy
4. The patient will receive radiation therapy to the entire breast or chest wall ± the lymphatic drainage area above and below the clavicle ± the lymphatic drainage area in the axilla.
5. No prior neoadjuvant chemotherapy or breast reconstruction.
6. No active cardiac disease, myocardial infarction, or congestive cardiac failure at baseline.
7. Patients can hold their breath for at least 30 seconds after training
8. Patients who can be followed up and agree to follow the plan.
9. Sign the consent form.

Exclusion Criteria:

1. Lesions were observed on both sides of the breast.
2. Diagnosis could not be confirmed by pathology.
3. Distant metastasis was identified.
4. The patient had undergone neoadjuvant chemotherapy or breast reconstruction surgery.
5. Severe cardiac insufficiency; myocardial infarction or uncorrected unstable cardiac arrhythmia or uncorrected unstable angina within the last 3 months; or pericardial disorders
6. The patient's New York Heart Association (NYHA) cardiac classification is within categories 2-4.
7. A history of chronic lung disease, including conditions that may contribute to ductal dilatation, such as chronic obstructive pulmonary disease (COPD) and interstitial pneumonia.
8. Previous mediastinal radiotherapy.
9. Previous or concurrent second primary malignant tumor (except skin cancer that is not a malignant black pigmented tumor, papillary/follicular carcinoma of the thyroid, carcinoma in situ of the cervix, and contralateral non-invasive breast cancer).
10. Irradiation of the lymphatic drainage area of the internal breast is required.