Intrafractional Displacement of Cardiac Substructures Among Patients With Mediastinal Lymphoma or Lung Cancer

Abstract

Purpose: The radiation dose to specific substructures of the heart may be more critical than the dose to the whole heart. Yet, these substructures are sensitive to intrafractional motion from breathing and cardiac motion, which can affect their dose-volume histograms. We sought to investigate intrafractional motion of the heart and its substructures among free-breathing patients undergoing radiation for mediastinal lymphoma or lung cancer. Methods and materials: After institutional review board approval, the medical records of 20 patients (12 with mediastinal lymphoma; 8 with lung cancer) were retrospectively reviewed. Patients underwent 4-dimensional computed tomography simulation and a contrasted scan for treatment planning. Using MIMVista software, the heart, coronary arteries, chambers, and valves were contoured on the 50% phase, and these contours were propagated to the other phases and edited. Each substructure was graded on the basis of its ease of contouring across all phases (1 = no difficulty; 2 = minor difficulty; 3 = moderate difficulty; and 4 = very difficult). The centroid position and volume of each substructure for all phases were exported to Excel to calculate basic statistics and the independent t test. Results: The heart, 4 chambers, and atrioventricular valves were easily identified with a mean score of 1 to 1.2, and the pulmonic valve, left anterior descending artery, aortic valve, circumflex, and right coronary artery were minor-to-moderately difficult with a mean score of 2.1 to 3.2. The smallest centroid displacement was seen in the 4 chambers and mitral and pulmonic valves (0.7-1.1 cm). Greater displacement was seen in the coronary vessels and tricuspid and aortic valves (1.2-1.5 cm). The greatest displacement was in the Z direction (craniocaudal) for all substructures; however, the displacement was significantly greater among patients with lymphoma for the right ventricle, aortic valve, and left anterior descending artery (P < .05). However, patients with lung cancer had more displacement in the X and Y directions, which was statistically significant for the right atrium, tricuspid valve, right ventricle, and heart. When calculating overall displacement, no statistically significant difference was observed between patients with lymphoma and patients with lung cancer. Conclusions: Intrafractional motion of the cardiac substructures ranged from 0.7 to 1.5 cm, mostly in the Z direction. Further investigation of the respiratory motion effect on the dose-volume histogram of the substructures is needed for patients treated with contemporary radiation techniques.