Radiotherapy (RT) of the thorax can damage the pericardium, myocardium, heart valves, and coronary vessels, with the pericardium being the most frequently damaged. The objectives of this study were to evaluate RT-induced left ventricular dysfunction and myocardial injury by measuring cardiac biomarkers, troponin I (TnI) and brain natriuretic peptide (BNP), as well as determin cardiotoxicity and early signs of cardiovascular dysfunction using strain rate imaging (SRI) prior to and following RT. This study included 35 patients diagnosed with thoracic malignancy between January 2011 and October 2013. Of the 35 patients, 22 had left-sided lung cancer and 13 had left-sided breast cancer. SRI was performed and TnI and BNP levels were measured prior to and following RT. A total radiation dose of 40-60 Gy (mean, 54.9 +/- 8.9 Gy) was applied in lung cancer patients and 50-60 Gy (mean, 51.6 +/- 4.9 Gy) was applied in breast cancer patients. No significant difference was observed in TnI or BNP levels prior to and one month following RT. The left ventricular ejection fraction (LVEF) did not differ prior to or after the first month of RT. However, lower right ventricular strain (RVS) and left ventricular strain (LVS) SRI values were detected within the first month following RT. From the results of this study, we conclude that SRI is a sensitive method to detect RT-induced changes in cardiac function that are not typically detected by conventional echocardiographic methods and cardiac biomarkers.