Inter-Fraction Cardiac Substructure Displacement Quantified by Magnetic Resonance (MR)-Guided Radiation Therapy

Abstract

Emerging evidence suggests that cardiac substructures are highly radiosensitive. However, the variability of substructure position after tumor localization has not been well characterized. This study quantifies the inter-fraction displacement of cardiac substructures by leveraging the excellent soft tissue contrast afforded by MR-guided radiation therapy. Sixteen patients who underwent radiotherapy for intrathoracic tumors with a 0.35 MR-guided Linac were retrospectively evaluated. Fourteen were treated at breath-hold (7 end-inhalation and 7 end-exhalation) using a 17-25 second TrueFISP scan (1.5 x 1.5 x 3 mm3) and 2 under free breathing condition (3-minute TrueFISP scan with 1.5 mm3 isotropic resolution). To mimic on-board IGRT, 3-4 daily MRIs (n = 63) were rigidly registered to the planning MR-simulation (MR-SIM) image based on tumor matching. Validated deep learning or atlas-based segmentation algorithms propagated 13 cardiac structures (e.g., whole heart, chambers, coronary arteries (CA), great vessels, etc.) to daily MRIs and contours were verified by 2 radiation oncologists. Daily centroid displacements from MR-SIM were quantified. Across the heart and substructures, inter-fraction displacements for 17.2% (left-right (L-R)), 11.4% (anterior-posterior (A-P)), and 22.5% (superior-inferior (S-I)) fractions were > 5 mm. Fewer than 2.5% of all structures were displaced > 10 mm in any direction over the studied fractions, often due to lack of compliance with breath-hold conditions. Table 1 summarizes key results for the left/right ventricles (LV/RV), left anterior descending artery (LADA), and ascending aorta (AA), which are significant in the pathogenesis of radiation-associated heart disease. For the chambers, the median absolute displacements were 2.4, 1.7, and 2.6 mm in the L-R, A-P, and S-I directions, respectively. Great vessels (superior vena cava, pulmonary artery, and AA) showed a tendency to have larger displacements in the S-I direction, with 46.6% of shifts > 3 mm whereas only 32.8 and 21.2% of displacements were observed in the L-R and A-P directions, respectively. Larger S-I displacements likely reflect the larger axial MRI slice thickness for most (14/16) patients. This exploratory work quantified the inter-fraction displacement of critical cardiac substructures and is a first step in deriving substructure-specific safety margins to ensure highly effective cardiac sparing. These findings require validation in a larger cohort for robust margin derivation and for applications in prospective clinical trials.Tabled 1Abstract 2729; TableAxisHeartLVRVLADAAAAbsolute Displacement Mean ± St Dev (mm)L-R2.5 ± 1.73.0 ± 3.13.3 ± 2.83.6 ± 3.62.5 ± 2.5A-P1.6 ± 1.62.8 ± 2.82.3 ± 2.03.5 ± 3.01.8 ± 1.9S-I3.2 ± 2.23.1 ± 2.43.1 ± 2.54.0 ± 3.83.2 ± 2.5Fractions < 3 mm (%)L-R7163605473A-P8063715279S-I5457605152 Open table in a new tab