Heart ultrasound and biomechanical evaluation of radiation-induced heart toxicity using transthoracic echocardiogram (TTE) and dynamic mechanical analysis (DMA).

Radiation-induced heart disease (RIHD) is a serious complication but difficult to assess in patients undergoing thoracic radiotherapy (RT). We aim to analyze RIHD using heart ultrasound and elastic modulus, exploring relationships between functional, anatomical or biomechanical changes of the heart and radiation dose. Twenty BALB/c mice were divided into four groups (control, 10 Gy, 20 Gy and 25 Gy) with a single fraction of image-guided volumetric modulated arc radiotherapy (VMAT) to murine heart on a linear accelerator. Transthoracic echocardiography (TTE) was performed on a small-animal ultrasound imaging system with a handheld microscan transducer. E-wave/A-wave ratio (E/A) and myocardial performance index (MPI) for diastolic performance were noninvasively evaluated weekly, as well as ejection fraction (EF%), fractional shortening (FS%), left ventricle (LV) mass and heart wall thickening for systolic performance. At the end of the fifth week, all mice were sacrificed for elastic modulus measurement on a dynamic mechanical analyzer (DMA) and for histopathological staining. All experiments were conducted in accordance with the local institution's animal research committee guideline. Significant difference was observed in E/A ratio between the control and 25 Gy irradiated groups (1.8±0.5 and 0.7±0.9, respectively; <0.05), indicating reduced diastolic performance and increased stiffness in left ventricle after high-dose heart radiation. Diastolic dysfunction in irradiated groups was also observed with significantly increased MPI. In contrast, posterior wall thickness, aortic peak velocity, heart rate, EF and FS were not significantly different after RT. Heart elasticity was reduced substantially with the increased radiation dose. HE and Masson Trichrome staining confirmed more fibrosis deposition in irradiated hearts. RIHD evaluation with ultrasound imaging noninvasively and biomechanical modulus measurement invasively in the image guided, precision dose-escalated murine heart irradiation is feasible. Increased myocardial stiffness, abnormal diastolic relaxation, more collagen deposition, and reduced tissue elasticity are observed in irradiated heart tissue. This study may facilitate our understanding of RIHD and facilitate improving patients' quality of life in the future.