Abstract: Objective To explore the short-term efficacy of electron beam irradiation of internal mammary chain (IMC) and the risk factors of radiation pneumonia in patients with axillary lymph node metastasis ≥ 4 after modified radical mastectomy for right breast cancer. 
Methods In total, 100 patients with right breast cancer and axillary lymph node metastasis ≥ 4 were selected for modified radical mastectomy, including 66 patients with N2 stage and 34 patients with N3 stage. All patients receive standardized neoadjuvant or postoperative adjuvant chemotherapy, targeted and endocrine therapy. The radiation therapy plan involved intensity modulated radiation therapy (IMRT) in the upper and lower areas of the right clavicle, and electronic wires were used in the right chest wall field and right inner breast area. The radiation dose was 50 Gy/25 times. The short-term efficacy and incidence of radiation pneumonia in these patients were observed, and the high-risk factors for the occurrence of radiation pneumonia were further explored. 
Results The median number of axillary lymph node dissection in the patients was 22, while the median number of lymph node positivity was 7. During a median follow-up of 39.5 months, there was 1 patient with regional lymph node recurrence, 2 patients with chest wall recurrence, 4 patients with distant metastasis, and 1 patient with death. There were 38 patients (38%) with grade 1 radiation pneumonia and 2 patients (2%) with grade 2 radiation pneumonia. Logistic analysis showed that grade 2 lymphocyte depletion after radiotherapy was an independent factor leading to radiation pneumonia. 
Conclusion It is safe and feasible to use electron beam irradiation of IMC after modified radical mastectomy for right breast cancer without additional adverse reactions. The occurrence of grade 2 or above lymphocyte depletion after radiotherapy is a high-risk factor for the development of radiation pneumonia. 

Key words: unilateral breast neoplasms, internal mammary chain, radiotherapy