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Purpose  The aim of this study is to evaluate the dose accuracy of bulk relative electron density (rED) approach for application in 1.5 T MR-Linac and assess the reliability of this approach in the case of online adaptive MR-guided radiotherapy for nasopharyngeal carcinoma (NPC) patients.    Methods  Ten NPC patients formerly treated on conventional linac were included in this study, with their original planning CT and MRI collected. For each patient, structures such as the targets, organs at risk, bone, and air regions were delineated on the original CT in the Monaco system (v5.40.02). To simulate the online adaptive workflow, firstly all contours were transferred to MRI from the original CT using rigid registration in the Monaco system. Based on the structures, three different types of synthetic CT (sCT) were generated from MRI using the bulk rED assignment approach: the sCT ICRU  uses the rED values recommended by ICRU46, the sCT tailor  uses the patient-specific mean rED values, and the sCT Homogeneity  uses homogeneous water equivalent values. The same treatment plan was calculated on the three sCTs and the original CT. Dose calculation accuracy was investigated in terms of gamma analysis, point dose comparison, and dose volume histogram (DVH) parameters.    Results  Good agreement of dose distribution was observed between sCT tailor  and the original CT, with a gamma passing rate (3%/3 mm) of 97.81% ± 1.06%, higher than that of sCT ICRU  (94.27% ± 1.48%,  p  = 0.005) and sCT Homogeneity  (96.50% ± 1.02%,  p  = 0.005). For stricter criteria 1%/1 mm, gamma passing rates for plans on sCT tailor , sCT ICRU , and sCT Homogeneity  were 86.79% ± 4.31%, 79.81% ± 3.63%, and 77.56% ± 4.64%, respectively. The mean point dose difference in PTV nx  between sCT tailor  and planning CT was −0.14% ± 1.44%, much lower than that calculated on sCT ICRU  (−8.77% ± 2.33%) and sCT Homogeneity  (1.65% ± 2.57%), all with  p  &amp;lt; 0.05. The DVH differences for the plan based on sCT tailor  were much smaller than sCT ICRU  and sCT Homogeneity .    Conclusions  The bulk rED-assigned sCT by adopting the patient-specific rED values can achieve a clinically acceptable level of dose calculation accuracy in the presence of a 1.5 T magnetic field, making it suitable for online adaptive MR-guided radiotherapy for NPC patients.

Dosimetric Accuracy of MR-Guided Online Adaptive Planning for Nasopharyngeal Carcinoma Radiotherapy on 1.5 T MR-Linac