LAUSR

PURPOSE Radiation dose to cardiac substructures is related to radiation-induced heart disease. However, substructures are not considered in radiation therapy planning (RTP) due to poor visualization on CT. Therefore, we developed a novel deep learning (DL) pipeline leveraging MRI's soft tissue contrast coupled with CT for state-of-the-art cardiac substructure segmentation requiring a single, non-contrast CT input. MATERIALS/METHODS Thirty-two left-sided whole-breast 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 (GVs), coronary arteries (CAs), etc.). Paired MRI/CT data (25 patients) were placed into separate image channels to train a 3-dimensional (3D) neural network using the entire 3D image. Deep supervision and a Dice-weighted multiclass loss function were applied. Results were assessed pre/post augmentation and post-processing (3D conditional random field (CRF)). Results for 11 test CTs (7 unique patients) were compared to ground-truth and a multi-atlas method (MA) via Dice similarity coefficient (DSC), mean distance to agreement (MDA), and Wilcoxon signed-ranks tests. Three physicians evaluated clinical acceptance via consensus scoring (5-point scale). RESULTS The model stabilized in ~19 hours (200 epochs, training error <0.001). Augmentation and CRF increased DSC 5.0±7.9% and 1.2±2.5%, across substructures, respectively. DL provided accurate segmentations for chambers (DSC=0.88±0.03), GVs (DSC=0.85±0.03), and pulmonary veins (DSC=0.77±0.04). Combined DSC for CAs was 0.50±0.14. MDA across substructures was <2.0mm (GV MDA=1.24±0.31mm). No substructures had statistical volume differences (P<0.05) to ground-truth. In four cases, DL yielded left main CA contours, whereas MA segmentation failed, and provided improved consensus scores in 44/60 comparisons to MA. DL provided clinically acceptable segmentations for all graded patients for 3/4 chambers. DL contour generation took ~14s per patient. CONCLUSIONS These promising results suggest DL poses major efficiency and accuracy gains for cardiac substructure segmentation offering high potential for rapid implementation into RTP for improved cardiac sparing.