Inclusion Criteria:  Female patients, aged 30-70yrs, diagnosed with stage 0-III breast cancer (NCCN-Breast Cancer V2.2019), undergoing the lumpectomy or mastectomy no more than 1 year. ALND or SLNB should be carried out and the lymph node status should be known. Patients with a deleterious germline mutation of BRCA 1/2 or PALB2. The patients with no contraindictation for irradiation. The patients consent for prophylactic irradiation to the contralateral breast. The patient may receive any regimen of adjuvant, neoadjuvant chemotherapy, according to the treating physician. The radiation therapy should start at least 2 weeks after the completion of all the chemotherapy cycles. The patient may receive endocrine therapy before, during or after study entry. The patient may receive target therapy (Trastuzumab) before, during or after study entry. The patient must be eligible for MRI examination of the contralateral breast. The patient refused prophylactic contralateral mastectomy and oophorectomy.  Exclusion Criteria:  Metastatic breast cancer. Past history of other cancer besides breast cancer Previous irradiation of the breast or chest wall, but not for breast cancer treatment Synchronous bilateral breast cancer Patients with active connective tissue diseases, pneumonia are excluded due to the potential risk of significant radiotherapeutic toxicity.