Development of a MRI-based Motion Management Strategy for Liver SBRT Treatment Planning

Purpose: MR images have better soft tissue contrast and introduce no ionizing radiation dose to patient compared with CT images. MRI-based treatment planning for liver SBRT has the potential to contour a more precise target volume and reduce radiation dose to normal tissues. Considering the tumor motion, in this study we investigated the motion management strategy workflow. We also evaluated the dose error in MRI-based treatment planning for liver SBRT and the feasibility of slice-stacking method to generate internal target volume (ITV) in the workflow.

Methods and Materials: Five liver SBRT treatment plans were used to evaluate the dose errors caused by CT number assignment and MR geometric distortion. A bulk CT number was assigned to different structures manually contoured on CT images and MR geometric distortion was simulated on CT images to evaluate the dose errors. In addition, we used 4D XCAT digital phantom with regular and patient breathing motions to test the slice-stacking method to generate maximum intensity projection (MIP). The slice-stacking method is to generate MIP by determining the maximum pixel intensity throughout all scanning images instead of throughout 10 phases from Four-dimensional magnetic resonance imaging (4D-MRI). A simulation patient study was designed to test the workflow, and the dose error was calculated between MRI-based treatment plan and CT-based treatment plan.

Results: The dose error caused by CT number assignment uncertainties and MR geometric distortion was generally small (< 1Gy), except for large geometric distortion (> 3mm). Using slice-stacking method, the DSC of ITV was > 0.99 and the relative ITV volume difference was < 1.5% when using 5 repeated scanning images with regular breathing motion to reconstruct MIP. For patient’s breathing motion, when using 5 repeated scanning images to reconstruct MIP, the mean DSC of ITV is larger than 0.97 and the relative ITV volume difference is nearly 4%. In the patient study, for the patient with larger tumor motion, the PTV maximal dose error, minimal dose error and mean dose error was -0.75Gy, 4.90Gy and 0.00Gy. For the patient with smaller tumor motion, the PTV maximal dose error, minimal dose error and mean dose error was 0.36Gy, -2.25Gy and -0.09Gy, respectively.

Conclusions: The workflow of motion management strategy for liver SBRT has been developed based on a simulation patient study. In this workflow, the dose error is generally small and it is feasible to generate a fast clinically acceptable ITV using slice-stacking method.