Evaluating Omitting of Internal Mammary Irradiation Among Early Stage Intermediate Risk (N1) Breast Cancer

Inclusion Criteria: * • Histologically confirmed invasive breast cancer * Underwent radical surgery with either mastectomy or breast conserving surgery and axillary lymph node dissection (ALND) * The number of positive lymph node should be 1-3 (N1). * Clinical high risk breast cancer (≥2 clinical risk factors) * Aged 18-80 years old * ECOG performance status ≤2 (Karnofsky ≥70%) Anticipative overall survival \>5 years Pathologically surgical margin \>2mm ER (estrogen-receptor), PR (progesterone-receptor), HER2 (human epidermal growth factor receptor 2) and Ki67 testing can be performed on the primary breast tumor Women of child-bearing potential must agree to use adequate contraception for up to 1 month before study treatment and the duration of study participation Ability to understand and willingness to participate the research and sign the consent form Exclusion Criteria: * • Axillary dissection of less than 10 lymph nodes * Pathologically positive ipsilateral supraclavicular lymph node * Pathologically or radiologically confirmed involvement of ipsilateral internal mammary lymph nodes * Pregnant or lactating women * Treated with breast reconstruction surgery * Severe non-neoplastic medical comorbidities * History of non-breast malignancy within 5 years with the exception of lobular carcinoma in situ, basal cell carcinoma of the skin, carcinoma in situ of skin and carcinoma in situ of the cervix * simultaneous contralateral breast cancer * Previous radiotherapy to the neck, chest and/or ipsilateral axillary region * Active collagen vascular disease * Definitive pathological or radiologic evidence of distant metastatic disease * Primary T4 tumor * Interval between radical surgery (mastectomy or breast conserving surgery) and radiotherapy was more than 12 weeks or interval between last dose of adjuvant chemotherapy and radiotherapy was more than 8 weeks