The contribution of radiotherapy, per se, in late cardiotoxicity remains controversial. An experimental model was developed, in which rat hearts were exposed to a range of ionizing radiation doses, to clarify its impact on the development of early cardiac dysfunction. Rat’s hearts were exposed to daily doses of 0.04, 0.3 and 1.2 Gy, for 23 days, achieving cumulative doses of 0.92, 6.9 and 27.6 Gy, respectively. Myocardial deformation, assessed by global longitudinal strain, was impaired (a relative percentage reduction of > 15% from baseline) in a dose-dependent manner. Moreover, by scanning electron microscopy, the microvascular density in the cardiac apex was significantly decreased exclusively at 27.6 Gy dosage. Prior to GLS impairment detection, several tools (qRT-PCR, mass spectrometry and western blot) were used to assess cardiac tissue remodeling. The number/expression of several genes, proteins and KEGG pathways, related to inflammation, fibrosis and cardiac muscle contraction, were found to be differently expressed in the cardiac tissue, according to the cumulative dose. Subclinical cardiac dysfunction occurred in a dose-dependent manner as detected by cardiac tissue remodeling, a predictor of the severity of global longitudinal strain impairment. Moreover, there is no dose threshold below which no myocardial deformation impairment was detected. These findings may contribute to i) develop new markers and explore non-invasive magnetic resonance imaging to assess cardiac tissue remodeling as an early predictor of cardiac dysfunction; ii) improve radiation-based diagnostic and therapeutic cardiac procedures; iii) highlight the need for personalized clinical approaches.