Objective: To measure the accuracy of position differences in anatomical landmarks in gated MRI and four-dimensional CT (4D-CT) fusion planning for radiation therapy in patients with hepatocellular carcinoma (HCC). Methods: From April to December 2009, gated MR and planning 4D-CT images were obtained from 53 inoperable HCC patients accrued to this study. Gated MRI and planning 4D-CT were conducted on the same day. Manual image fusions were performed by matching the vertebral bodies. Liver volumes and three specific anatomical landmarks (portal vein conjunction, superior mesenteric artery bifurcation, and other noticeable points) were contoured from each modality. The points chosen nearest the centre of the four landmark points were compared to measure the accuracy of fusion. Results: The average distance differences (¡standard deviation) of four validation points were 5.1 mm (¡4.6 mm), 5.6 mm (¡6.2 mm), 5.4 mm (¡4.5 mm) and 5.1 mm (¡4.8 mm). Patients who had ascites or pulmonary disease showed larger discrepancies. MRI-CT fusion discrepancy was significantly correlated with positive radiation response (p,0.05).Conclusions: Approximately 5-mm anatomical landmark positional differences in all directions were found between gated MRI and 4D-CT fusion planning for HCC patients; the gap was larger in patients with ascites or pulmonary disease. Advances in knowledge: There were discrepancies of approximately 5 mm in gated MRI-CT fusion planning for HCC patients. Many studies have reported that the treatment response and survival of hepatocellular carcinoma (HCC) patients are related to the delivered radiation dose [1][2][3]. Recently, detailed information on HCC and liver motion gained from the use of advanced techniques, such as a fiducial marker combined with fourdimensional (4D) planning CT, has enabled the delivery of higher doses of radiation therapy (RT) with reduced normal liver toxicity [3][4][5].Although triphasic CT can provide much information about HCC, the lesion/liver contrast is higher in MRI than in CT [6,7]. To take advantage of these benefits, there have been many efforts to incorporate liver MRI in the RT planning process [8,9]. Moreover, several groups have demonstrated the feasibility of using cine-MRI and 4D-MRI to measure liver tumour motion for RT planning [10][11][12]. However, a consensus has not yet been reached on the best strategies to compensate for liver motion and adapt RT planning and delivery using planning 4D-CT combined with liver MRI.The primary goal of this prospective study was to evaluate the accuracy of gated MRI and 4D-CT fusion planning by measuring the discrepancies in the specific anatomical landmark points of the liver between exhalephase images of gated liver MRI and 4D-CT. We also evaluated possible factors affecting gated liver MRI and 4D-CT fusion discrepancy and RT response. Materials and methods EligibilityThis study investigated the accuracy of gated liver MRI and 4D-CT fusion planning for RT in patients with HCC. The eligibility criteria were a diagnosis of HCC base...