OBJECTIVE: 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. METHODS: Thirty-six Beagle dogs were randomized into a control group (n = 18) and an irradiation group (n = 18). The irradiation group received a single dose of 20 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 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. RESULTS: 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 months, with progressive decline at 6 and 12 months. Strain reduction correlated spatially with pathological injury. Conversely, there were no substantial differences in conventional echocardiographic parameters between the groups after 3 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 months. CONCLUSION: 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.