Background: Patients' chances of surviving breast cancer have increased thanks to multimodal therapy.
After breast-conserving surgery, hypofractionated radiation (RT) is quickly becoming one of the alternatives for breast cancer patients (BCS).
 Objectives: The present study aimed to assess the thoracic radiotherapy's effects on patients with breast cancer's ability to breathe normally and engage in physical activity.
Patients and methods: This was a prospective study   which was carried out at Oncology Department & Chest Department of Qena University Hospitals. Spirometry was performed to all included patients to assess their lung function.
 Results: The mean and range of spirometer values 3 months after radiotherapy. The mean FEV1 was 65 % predicted, the mean FVC was 62.7 % predicted, the mean FEV1/FVC was 83.7% and the mean FEF_25%-75% was 70.27 % predicted.
Conclusion: In radiotherapy, the lung is a major organ at risk because of the risk of radiation-induced lung injury. Silent radiation pneumonitis is a common side effect of radiotherapy. FEV1 and FVC parameters are decreased after radiotherapy due to acute radiation-induced lung injury. Spirometry can be used in assessing lung functions after radiotherapy. N3 stage showed significant decrease in FEV1 and FVC compared to other lower N stages. Max. lung dose was found to be the best predictor of the occurrence of radiation pneumonitis.