BACKGROUND: Liver tumors have low contrast on 4DCT. A novel Multitasking (MT)MR imaging technique has been implemented on the MR simulator, providing both T1 and T2-weighted 4DMR images in a single 8-min free-breathing scan for better tumor delineation and motion evaluation. This study reports our early clinical experience of MTMR regarding tumor visibility, motion characteristics, and resultant dosimetry compared to post-contrast 4DCT for liver SBRT. METHODS: Phantom motion validation was performed. Tumor contrast-to-noise ratio (CNR) and motion were analyzed in 54 patients. Replanning for 17 patients (21 tumor volumes) was performed, and planning target volume receiving greater than 90% of the prescription (PTV_V90) was compared based on optimized dose distributions for each 4D dataset-derived PTV. RESULTS: Phantom motions in both 4DCT and MTMR datasets were within <
1.8 mm of the programmed ground truth. The absolute CNR of MTMR-T1w and MTMR-T2w were significantly greater than post-contrast 4DCT. Tumor superior-inferior motions were significantly greater in MTMR than in 4DCT, while PTV volumes were not significantly different between the two 4D datasets. The PTV_V90 calculated from individual MTMR-T1w and 4DCT optimized plans were similar. However, a statistically significant 5 % reduction of PTV_V90 was observed when the optimized PTV_MTMR dose was superimposed on the respective PTV_4DCT, or vice versa for the re-planning patient cohort. CONCLUSION: This study demonstrates that the MTMR sequence offers superior tumor visualization and detects greater superior-inferior motion compared to 4DCT, enhancing the precision of radiotherapy planning for liver SBRT. While both imaging methods achieve comparable target volume coverage with individually optimized plans, discrepancies in tumor positioning lead to reduced coverage when plans are cross-applied, highlighting the importance of motion assessment accuracy. MTMR's ability to provide multiple contrast-weighted images in a single scan addresses limitations of traditional 4DCT and multi-sequence MR protocols, particularly for patients unable to receive contrast.