Cardiac Substructure Segmentation with Deep Learning for Improved Cardiac Sparing

Purpose: Radiation dose to cardiac substructures is related to radiationinduced heart disease. However, substructures are not considered in radiation therapy planning (RTP) due to poor visualization on CT. To address this, we developed a novel deep learning pipeline leveraging the soft tissue contrast of MRI coupled with CT for state-of-the-art substructure segmentation requiring only non-contrast CT inputs. Methods: Thirty-two left-sided wholebreast cancer patients underwent cardiac T2 MRI and CT-simulation. A rigid cardiac-confined MR/CT registration enabled ground-truth delineations of 12 substructures (chambers, great vessels, coronary arteries, etc.). Paired MRI/ CT data for 25 patients were placed into separate image channels to train a 3- dimensional Neural Network (3D U-Net) using the entire 3D image and all substructures simultaneously. A Dice-weighted multiclass loss function was utilized along with deep supervision. Segmentation results were assessed pre/post augmentation (scaling, rotation, flipping, and translation). Results for 11 test patient CTs (7 unique patients) were compared to ground-truth and our previously developed multi-atlas method. Prediction versus ground-truth evaluation was performed via Dice similarity coefficient (DSC) and mean distance to agreement (MDA). Results: The model stabilized in ∼17 hr after 200 epochs (training erro