Contrast-Enhanced 4D MRI for Internal Target Volume Generation in Treatment Planning for Liver Tumors

Purpose/Objective(s)

Treatment planning for liver tumors is challenging, as they have variable respiratory motion and are often invisible on four-dimensional commuted tomography (4DCT), even with use of intravenous contrast. Thus, imperfect imaging surrogates are used to estimate the motion of tumors. 4D magnetic resonance (MR) with hepatobiliary contrast has the potential to allow for direct visualization of tumors through the respiratory cycle. Thus, we assessed multiple reconstruction algorithms for directly visualizing liver tumor motion for radiotherapy planning and compared treatment plans with those generated using 4DCT.

Materials/Methods

Patients were simulated using a 3T MR and CT scanner. Three prototype binning algorithms (phase, amplitude, and two-directional) were applied to the 4DMRs. Phase and amplitude binning averaged signal from both the inhale and exhale trajectories for theoretically improved signal to noise ratio, while two-directional binning separated trajectories for theoretically improved spatial resolution. Image quality for tumor delineation was assessed using a qualitative clarity score and quantitative sharpness score. Radiation plans were generated for internal target volumes (ITVs) derived using 4DMR and 4DCT, and the dosimetry compared. Paired t-tests were used to compare sharpness scores and dosimetric data.

Results

Twelve patients with 17 liver tumors were scanned between May and November 2021. Compared to phase binning, two-directional binning demonstrated equal or better clarity and sharpness scores (end-expiration: 0.33 vs. 0.38, p=0.018, end-inspiration: 0.28 vs. 0.31, p=0.010). Compared to amplitude binning, two-directional binning captured hysteresis (anterior displacement of the tumor during respiration) of ≥3 mm in 35% of patients. Dosimetric evaluation of 4DCT-derived plans revealed resulting underdosage of the PTV where the 4DCT did not capture the full extent of 4DMRI-defined PTV, (PTV receiving 90% of prescription: 75.56% vs. 89.38%, p=0.002).

Conclusion

Using contrast-enhanced 4D MRI is feasible for directly delineating liver tumors throughout the respiratory cycle. Two-directional binning capture hysteresis in 1/3 of patients. The current standard of creating radiation plans optimized for 4DCT-derived targets achieved lower coverage of MRI-derived targets, suggesting that adopting MRI for motion management may improve radiation treatment of liver lesions and reduce the risk of marginal misses.