Inter-Fraction Cardiac Substructure Displacement Quantified by Magnetic Resonance (MR)-Guided Radiation Therapy
Zhu S, Ghanem AI, Morris ED, and Glide-Hurst C. Inter-Fraction Cardiac Substructure Displacement Quantified by Magnetic Resonance (MR)-Guided Radiation Therapy. International Journal of Radiation Oncology Biology Physics 2020; 108(3):e324.
International Journal of Radiation Oncology Biology Physics
Purpose/Objective(s): 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.
Materials/Methods: 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.
Results: 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.
Conclusion: 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.