Optimized cardiac functional MRI of small-animal models of cancer radiation therapy

Dose-dependent changes in cardiac function, strain and remodelling in a preclinical model of heart base irradiation

BACKGROUND AND PURPOSE

Radiation induced cardiotoxicity (RICT) is as an important sequela of radiotherapy to the thorax for patients. In this study, we aim to investigate the dose and fractionation response of RICT. We propose global longitudinal strain (GLS) as an early indicator of RICT and investigate myocardial deformation following irradiation.

METHODS

RICT was investigated in female C57BL/6J mice in which the base of the heart was irradiated under image-guidance using a small animal radiation research platform (SARRP). Mice were randomly assigned to a treatment group: single-fraction dose of 16 Gy or 20 Gy, 3 consecutive fractions of 8.66 Gy, or sham irradiation; biological effective doses (BED) used were 101.3 Gy, 153.3 Gy and 101.3 Gy respectively. Longitudinal transthoracic echocardiography (TTE) was performed from baseline up to 50 weeks post-irradiation to detect structural and functional effects.

RESULTS

Irradiation of the heart base leads to BED-dependent changes in systolic and diastolic function 50 weeks post-irradiation. GLS showed significant decreases in a BED-dependent manner for all irradiated animals, as early as 10 weeks after irradiation. Early changes in GLS indicate late changes in cardiac function. BED-independent increases were observed in the left ventricle (LV) mass and volume and myocardial fibrosis.

CONCLUSIONS

Functional features of RICT displayed a BED dependence in this study. GLS showed an early change at 10 weeks post-irradiation. Cardiac remodelling was observed as increases in mass and volume of the LV, further supporting our hypothesis that dose to the base of the heart drives the global heart toxicity.

Methods to assess radiation-induced cardiotoxicity in rodent models

Cancer survivors who have received thoracic radiation as part of their primary treatment are at risk for developing radiation-induced cardiotoxicity (RICT) due to incidental radiation delivered to the heart. In recent decades, advancements in radiation delivery have dramatically improved the therapeutic ratio of radiation therapy (RT)-efficiently targeting malignancies while sparing the heart; yet, in many patients, incidental radiation to the heart cannot be fully avoided. Cardiac radiation exposure can cause long-term morbidity and contribute to poorer survival in cancer patients. Severe cardiac effects can occur within 2years of treatment. Currently, there is no way to predict who is at higher or lower risk of developing cardiotoxicity from radiation, and the critical factors that alter RICT have not yet been clearly identified. Thus, pre-clinical investigations are an important step towards better prevention, detection, and management of RICT in cancer survivors. The overarching aim of this chapter is to provide researchers with foundational and technical knowledge in the use of mice and rats for RICT investigations. After a brief overview of RICT pathophysiology and clinical manifestations, we discuss important considerations of RICT study design, including animal selection and radiation planning. We then provide example protocols for murine tissue harvesting and processing that can support use in downstream applications of the reader's choosing.

Myocardial Contractility Pattern Characterization in Radiation-Induced Cardiotoxicity Using Magnetic Resonance Imaging: A Pilot Study with ContractiX

Radiation therapy (RT) plays an integral role in treating thoracic cancers, despite the risk of radiation-induced cardiotoxicity. We hypothesize that our newly developed magnetic resonance imaging (MRI)-based contractility index (ContractiX) is a sensitive marker for early detection of RT-induced cardiotoxicity in a preclinical rat model of thoracic cancer RT. Adult salt-sensitive rats received image-guided heart RT and were imaged with MRI at 8 weeks and 10 weeks post-RT or sham. The MRI exam included cine and tagging sequences to measure left-ventricular ejection fraction (LVEF), mass, myocardial strain, and ContractiX. Furthermore, ventricular torsion, diastolic strain rate, and mechanical dyssynchrony were measured. Statistical analyses were performed between the sham, 8 weeks post-RT, and 10 weeks post-RT MRI parameters. The results showed that both LVEF and myocardial mass increased post-RT. Peak systolic strain and ContractiX significantly decreased post-RT, with a more relative reduction in ContractiX compared to strain. ContractiX showed an inverse nonlinear relationship with LVEF and continuously decreased with time post-RT. While early diastolic strain rate and mechanical dyssynchrony significantly changed post-RT, ventricular torsion changes were not significant post-RT. In conclusion, ContractiX measured via non-contrast MRI is a sensitive early marker for the detection of subclinical cardiac dysfunction post-RT, and it is superior to other MRI cardiac measures.

Cardiac Magnetic Resonance for Early Detection of Radiation Therapy-Induced Cardiotoxicity in a Small Animal Model

Background

Over half of all cancer patients receive radiation therapy (RT). However, radiation exposure to the heart can cause cardiotoxicity. Nevertheless, there is a paucity of data on RT-induced cardiac damage, with limited understanding of safe regional RT doses, early detection, prevention and management. A common initial feature of cardiotoxicity is asymptomatic dysfunction, which if left untreated may progress to heart failure. The current paradigm for cardiotoxicity detection and management relies primarily upon assessment of ejection fraction (EF). However, cardiac injury can occur without a clear change in EF.

Objectives

To identify magnetic resonance imaging (MRI) markers of early RT-induced cardiac dysfunction.

Methods

We investigated the effect of RT on global and regional cardiac function and myocardial T1/T2 values at two timepoints post-RT using cardiac MRI in a rat model of localized cardiac RT. Rats who received image-guided whole-heart radiation of 24Gy were compared to sham-treated rats.

Results

The rats maintained normal global cardiac function post-RT. However, a deterioration in strain was particularly notable at 10-weeks post RT, and changes in circumferential strain were larger than changes in radial or longitudinal strain. Compared to sham, circumferential strain changes occurred at the basal, mid-ventricular and apical levels (p<0.05 for all at both 8-weeks and 10-weeks post-RT), most of the radial strain changes occurred at the mid-ventricular (p=0.044 at 8-weeks post-RT) and basal (p=0.018 at 10-weeks post-RT) levels, and most of the longitudinal strain changes occurred at the apical (p=0.002 at 8-weeks post-RT) and basal (p=0.035 at 10-weeks post-RT) levels. Regionally, lateral myocardial segments showed the greatest worsening in strain measurements, and histologic changes supported these findings. Despite worsened myocardial strain post-RT, myocardial tissue displacement measures were maintained, or even increased. T1/T2 measurements showed small non-significant changes post-RT compared to values in non-irradiated rats.

Conclusions

Our findings suggest MRI regional myocardial strain is a sensitive imaging biomarker for detecting RT-induced subclinical cardiac dysfunction prior to compromise of global cardiac function.