Tumor–Vessel Motion Correlation In MR-Guided Lung Radiotherapy
Abstract
Purpose
To evaluate the correlation between lung tumor and pulmonary vessel motion under MR-guided radiotherapy and to inform surrogate tracking feature selection when direct tumor tracking is challenging.
Methods
Correlation between Tumor-vessel motion was decomposed into temporal phase and displacement amplitude components. Cine MR images from 11 treatment fractions (3 patients) were analyzed, motion trajectories were extracted, and displacements relative to a reference position were calculated in both AP and SI directions. Tracking points were manually placed in various pulmonary vessels to track vessel motion. Phase lag was defined as the temporal shift that maximized the cross-correlation between tumor and vessel motion, and the maximum normalized cross-correlation coefficient (ccMax) quantified temporal synchrony. The displacement amplitude relationship was quantified using linear regression between tumor and tracking point displacements.
Results
Pulmonary vessels showed strong temporal synchrony with tumor motion (median ccMax > 0.74), with phase lag centered near 0 s (median 0 s, range 0–0.83 s in AP and -0.71–0.39 s in SI). Phase correlation strength depended on tumor-vessel spatial relationship, with closer vessels showing stronger correlation. In contrast, displacement amplitude correlation was variable across datasets, with mean regression slopes of 0.60 and 1.07, and mean R2 values of 0.53 and 0.44 in the AP and SI directions, respectively. Phase and amplitude correlations were more consistent in the AP than in the SI direction.
Conclusion
This study shows that pulmonary vessels track the phase well but show variable amplitude coupling with tumor motion. These preliminary findings suggest that vessel-based surrogates may be suitable for phase-based gating, while displacement-based surrogate tracking requires further evaluation. The observed variability indicates that surrogate selection may need to be evaluated on a patient- and feature-specific basis in MR-guided lung radiotherapy.