Objective <p>This study aimed to detect early changes in left ventricular systolic function in Beagle dogs after radiotherapy using two-dimensional speckle tracking echocardiography and to explore its potential value in evaluating radiation-induced heart disease.</p> Methods <p>Thirty-six Beagle dogs were randomized into a control group (<i>n</i> = 18) and an irradiation group (<i>n</i> = 18). The irradiation group received a single dose of 20&#xa0;Gy to the left ventricular anterior wall, while controls underwent sham irradiation. Conventional echocardiography and 2D speckle tracking echocardiography were performed at baseline and 3, 6, and 12&#xa0;months post-procedure. Additionally, six dogs were randomly selected from each group and euthanized at 3-, 6-, and 12-month post-irradiation, and their hearts were collected for histopathological testing.</p> Results <p>In the irradiation group, the global longitudinal strain of the left ventricle and regional strain in the irradiated area were significantly reduced versus baseline and controls by 3&#xa0;months, with progressive decline at 6 and 12&#xa0;months. Strain reduction correlated spatially with pathological injury. Conversely, there were no substantial differences in conventional echocardiographic parameters between the groups after 3&#xa0;months. Conventional parameters (e.g., LVEF) showed differences only at later timepoints. Histopathology revealed progressive cardiomyocyte damage, fibrosis, and microvascular injury in irradiated regions, extending to the posterior wall by 12&#xa0;months.</p> Conclusion <p>Two-dimensional speckle tracking echocardiography-derived strain parameters spatially correlate with radiation-induced pathological changes and detect subtle systolic dysfunction prior to irreversible remodeling. Speckle tracking may localize regions of peak radiation dose delivery.</p>