Prospective evaluation of liver cancer volumes on non-contrast 1.5 T MR Linac imaging

Background: The 1.5 T MR-Linac (MRL) provides a platform for online adaptive stereotactic body radiation therapy (SBRT). This study aims to compare gross tumor volume (GTV) on multiphasic (MP) CT and MR simulation imaging versus non-contrast MRL images and to measure changes in GTV during SBRT. Methods: Primary or metastatic liver cancer patients treated with SBRT using MRL or cone beam CT (CBCT) were eligible for this prospective imaging study. At simulation, patients underwent MP CT and MP 3 T MR liver T1-weighted (T1w) imaging in exhale breath hold and T2-weighted (T2w) images on 1.5 T MRL (MRL0). Repeat MRL T2w images were acquired at each SBRT fraction. Liver tumors were contoured on simulation CT/MR and MRL0 images and on each subsequent MRL image. MRL0 GTV were compared to MP CT/MR GTV and changes during SBRT were measured. Results: From August 2019 to January 2023, 29 liver tumors (23 patients) treated with SBRT, were contoured on 23 MP CT/3T MR and 103 MRL imaging datasets. Simulation MRL0 interquartile median GTV was 43.3 cc (IQR 4.3-47.7 cc), GTV on MRL0 images were smaller than MP CT/3T MR volumes by an average of 7.1 cc (95 % CI 1.9-12.2, p = 0.008) or 21.9 % (95 % CI 13.5-30.2, p = <0.001). Over the course of SBRT there were no statistically significant predictable trends in tumor volume. Conclusion: GTV based on MRL T2w images were significantly smaller than GTV based on MP CT/3T MR images. This systematic difference should be considered during adaptive MRL liver SBRT. There were no predictable changes in tumor volumes during SBRT.