Abstract Body (Do not enter title and authors here): Background: Proton beam irradiation to the ventricular myocardium may enable non-invasive targeting of ventricular tachycardia. Previous work has demonstrated the feasibility of imaging ablation lesion with late gadolinium enhanced (LGE) cardiac magnetic resonance imaging (MRI).

Hypothesis: We hypothesize that proton beam irradiation will lead to progressively increasing density of LGE corresponding to lesion formation during follow-up after treatment delivery.

Aim: The aim of the current study was to determine the time course of LGE intensity in ventricular myocardium following proton beam therapy.

Methods: Eleven swine were irradiated with 40 Gy proton beam in the anterior or lateral left ventricular (LV) followed by LGE cardiac MRI every 4 weeks for up to 24 weeks. After manual contouring of the endocardial and epicardial myocardium, a region of interest (ROI) in the area irradiated with 0-5 Gy was contoured (remote ROI). The mean and standard deviation (SD) of the signal intensity (SI) of the remote ROI were calculated. Light LGE was defined as voxels with SI > mean+2SD but < mean+3SD of the remote ROI, while voxels with SI > mean+3SD of the remote ROI were classified as having dense LGE. Voxels identified as bright due to image artifacts or noise were manually removed. The analysis was conducted in a fashion blinded to swine ID and time point after irradiation.

Results: A total of 33 time points were analyzed (3.1 ± 1.4 MRI studies per swine). Figure 1A shows a representative time course of LGE in the same swine, progressing from diffuse light LGE at 8 weeks to dense LGE at 16-24 weeks. Figure 1B demonstrates the proportion of light versus dense LGE across timepoints in irradiated myocardial regions. At 8 weeks, light and dense LGE areas were evenly distributed. At 12 to 24 weeks, dense LGE increased to approximately 75%. The proportion of dense versus light LGE was significantly higher at 12, 16, and 24 weeks, as compared to 8 weeks.

Conclusion: In ventricular myocardium, diffuse light LGE evident at 8 weeks progresses to denser LGE at 12-24 weeks following proton beam irradiation corresponding with lesion formation.