Unity MR-Linac allows daily plan adaptation according to the new patient anatomy; the contours of the structures are adjusted based on the patient's daily MRI, and in the adapt to shape (ATS) workflow, the adapted plan is recalculated on the MRI-based synthetic CT (sCT) generated by bulk density assignment[1]. The accuracy of this calculation is strongly dependent on how the sCT has been generated, and for sites where there is a high electronic density gradient between the target and surrounding tissues, such as in lung cancer treatments, the assignment of an average electronic density (ED) may not be able to reproduce an exact dose calculation[2, 3]. This study aims to evaluate the accuracy of sCT calculation in this type of situation mainly encountered in lung cancer treatments assessing separately tumor sites in different electronic density gradient areas.