Cardiotoxicity is a devastating complication of thoracic radiotherapy. However, current practice ignores radiosensitivities and complex motion trajectories of individual substructures. Current imaging protocols in radiotherapy are insufficient to decouple and quantify cardiac motion, limiting substructure-specific motion considerations in treatment planning. We propose a 5D-MRI workflow for comprehensive substructure-specific motion analysis. Approach: Our 5D-MRI workflow was implemented in 10 healthy subjects (23-65 years) and two patients with lung cancer (67-69 years), with local low-rank iterative reconstruction at end-exhale/inhale and active-exhale/inhale for end-systole/diastole. For motion assessment, proximal coronary arteries, chambers, great vessels, and cardiac valves/nodes were contoured across all images and verified. Centroid/bounding box excursion was calculated for cardiac, respiratory, and hysteresis motion. Distance metrics were tested for statistical independence across substructure pairings. Three thoracic motion-perturbed radiotherapy plans were retrospectively analyzed using volunteer-derived IRVs. Main Results: 5D-MRI images were successfully acquired. Cardiac motion exceeded 1 cm for right-heart substructures and was greatest for the right coronary artery. Respiratory motion was largest for the inferior vena cava/left ventricle. Respiratory hysteresis was generally 5 mm for some subjects. For cardiac motion, statistically significant differences were observed between coronary arteries/chambers/great vessels and between right/left-sided substructures. Respiratory motion differed significantly between the heart base/apex for the volunteer cohort. 5DMRIs for 2 thoracic cancer patients demonstrated tumor-induced changes in motion patterns relative to the volunteer cohort. Motion-perturbed treatment plans in 3 thoracic cancer patients demonstrated an increase in D0.03cc up to 21.5 Gy across volunteer-derived cardiorespiratory IRVs. Significance: Our 5D-MRI workflow successfully decouples cardiorespiratory motion in a ~5-minute free-breathing acquisition. In volunteers, cardiac motion was >5mm for coronary arteries/chambers, while respiratory motion was >5 mm for all substructures. Statistically significant differences were observed between cardiac substructures for cardiac and respiratory motion. Interplay between tumor location, motion source, and motion magnitude affected substructure doses.
Ruff et al. (Wed,) studied this question.