Background: Radiotherapy is an integral component in the management of breast cancer, significantly improving local control and survival. However, radiation pneumonitis (RP) remains a notable adverse effect, potentially affecting treatment outcomes and patients’ quality of life.

Aim: To evaluate the incidence and clinical impact of radiation pneumonitis in breast cancer patients undergoing radiotherapy.

Methods: This prospective observational study was conducted in the Department of Radiotherapy at Sree Mookambika Institute of Medical Sciences, Kulasekharam, Tamil Nadu, over 18 months. A total of 200 histopathologically confirmed breast cancer patients receiving adjuvant radiotherapy were included. Baseline clinical and treatment-related parameters were recorded. Dosimetric variables such as mean lung dose (MLD) and V20 were analyzed. Patients were followed up at 1, 3, and 6 months post-radiotherapy for clinical and radiological evidence of RP, which was graded using CTCAE version 5.0.

Results: The incidence of radiation pneumonitis was 13%. Most cases were mild to moderate (Grade 1–2), with only 2.5% experiencing severe toxicity. A significant association was observed between RP and higher dosimetric parameters (MLD and V20) (p < 0.001). The majority of cases presented within 3 months of treatment, with dry cough and dyspnea being the most common symptoms. Approximately 50% of affected patients required corticosteroid therapy, while hospitalization was rare.

Conclusion: Radiation pneumonitis is a relatively common but manageable complication of breast radiotherapy. Strict adherence to lung dose constraints and early detection are essential to minimize its impact and improve patient outcomes.

KEYWORDS: Breast Cancer, Radiation Pneumonitis, Radiotherapy, Mean Lung Dose, V20, Pulmonary Toxicity, CTCAE.