Background and aims <p>Radiotherapy, like chemotherapy, in breast cancer can cause cardiotoxicity. Research about the combined effect of radiotherapy and chemotherapy on left ventricular diastolic function (LVDD) is limited. Therefore, this study was conducted to see the early effect of radiotherapy in patients after completing chemotherapy.</p> Method <p>This research was a prospective analytical observational study with a pre-post design that was conducted from August 2023 to August 2024 in breast cancer patients who had completed their chemotherapy in Sardjito Hospital. The primary endpoint was cardiotoxicity in any level according to European Society of Cardiology (ESC) 2022 cardio-oncology guideline. Cardiotoxicity was diagnosed by considering patients’ history, physical examination, and echocardiography results.</p> Result <p>Twenty-nine female subjects were enrolled. Seven subjects (24.14%) had cardiotoxicity with one of them was symptomatic. McNemar test showed significant effect of radiotherapy (<i>p</i> = 0.016). Types of chemotherapy were not a confounding factor in this study.</p> Conclusion <p>Radiotherapy may cause mild cardiotoxicity in short term. A routine screening protocol is needed to prevent further complication.</p>