Assessment of PTV Margins for Unity MR-Linac Treatments Using Cine-MRI Target Motion
Abstract
Purpose
In comprehensive motion management (CMM), Elekta Unity MR-Linac uses cine-MRI to monitor intra-fraction target motion. Due to the lack of efficient tools to determine patient-specific treatment margin, an isotropic GTV-to-PTV margin of 5 mm is used for all liver, pancreas, and kidney patients. The purpose of this study was to assess whether the margin is adequate using cine-MRI.
Methods
Cine-MRI recordings of 15 Unity treatments were retrospectively analyzed, including 5 cases each of liver, pancreas, and kidney. An in-house software tool was developed to extract target coverage within the PTV envelope and motion trajectories in the superior–inferior (SI), anterior–posterior (AP), and left–right (LR) directions. The classical van Herk margin recipe was used to estimate site and direction specific PTV margins based on intra-fraction motion characteristics.
Results
Across all three anatomical sites, the largest motion was observed in the SI direction, followed by AP and LR motion. Liver exhibited the largest average motion range of 3.24 cm. The target remained within the PTV envelope for an average of 94.96% (s.d. 3.19%) of recorded treatment time for pancreas cases, 87.30% (s.d. 9.07%) for kidney cases, and 64.13% (s.d. 11.74%) for liver cases. The estimated PTV margin for liver in the LR, SI, and AP directions were 2.0 mm, 4.1 mm, and 3.3 mm, respectively. For pancreas, the estimated margins were 1.6 mm (LR), 3.1 mm (SI), and 1.4 mm (AP), while for kidney the margins were 3.0 mm (LR), 2.3 mm (SI), and 1.6 mm (AP).
Conclusion
A uniform 5 mm isotropic PTV margin exceeds the estimated site and direction specific margins for all three anatomical sites, suggesting that margin reduction may be feasible for certain sites and motion directions. These findings support the potential for motion-characteristic-based PTV margin selection in Unity MR-Linac treatments.