Impact of contrast-enhanced CT in the dosimetry of SBRT for liver metastases treated with MR-Linac

Background: To investigate the impact of using contrast-enhanced computed tomography (CHCT) in the dosimetry of stereotactic body radiation therapy (SBRT) for liver metastases treated with MR-Linac. Methods: A retrospective study was conducted on 21 liver cancer patients treated with SBRT (50Gy in 5 fractions) using a 1.5 Tesla Unity MR-Linac. The clinical treatment plans optimised on plain computed tomography (pCT) were used as reference. The electronic density (ED) of regions of interest (ROIs) including the liver, duodenum, esophagus, spinal cord, heart, ribs, and lungs, from pCT and CHCT, was analysed. The average ED of each ROI from CHCT was used to generate synthetic CT (sCT) images by assigning the average ED value from the CHCT to the pCT. Clinical plans were recalculated on sCT images. Dosimetric comparisons between the original treatment plan (TPpCT) and the sCT plan (TPsCT) were performed using dose-volume histogram (DVH) parameters, and gamma analysis. Results: Significant ED differences (p<0.05) were observed in the liver, great vessels, heart, lungs, and spinal cord between CHCT and pCT, with the lungs showing the largest differences (average deviation of 11.73% and 12.15 % for the left and right lung, respectively). The target volume covered by the prescribed dose (VDpre), and the dose received by 2% and 98% of the volume (D2%, and D98%,respectively) showed statistical differences (p < 0.05), while the gradient index (GI) and the conformity index (CI) did not. Average deviations in target volume dosimetric parameters were below 1.02%, with a maximum deviation of 5.57% for Dmin. For the organs at risk (OARs), significant differences (p < 0.05) were observed for D_0.35cc and D1.2cc of the spinal cord, D_10cc for the stomach, D0.5cc for the heart, and D30% for the liver-GTV, with mean deviations lower than 1.83% for all the above OARs. Gamma analysis using 2%-2mm criteria yielded a median value of 95.64 % (range 82.22% to 99.65%) for the target volume and 99.40 % (range 58% to 100%) for the OARs. Conclusion: The findings indicate that using CHCT in the SBRT workflow for liver metastases may result in minor target volume overdosage. This potential discrepancy suggests that CHCT should not be adopted as a standard practice in clinical settings.