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Objective This study aimed to investigate the dose calculation accuracy of individualized bulk electron density ( IBED) assignment approaches for simulated magnetic resonance imaging ( MRI)-only planning of thoracic tumors via the use of a 3DVH system. Methods 8 patients with thoracic cancer were included in this study. Based on standard planning CT,single-arc dynamic conformal therapy ( DCT) and double-arc volumetric modulated arc therapy ( VMAT) plans with a 6 MV photon beam were generated as a baseline plan ( Plan-CT) for each patient. The simulated MRI-only planning ( Plan-IBED) was implemented by copying the Plan-CT and forcing the electron density of each region of interest to its av-erage value and recalculating the dose distribution. A 3DVH system was used to visualize and compare the dosimetric differ-ences between Plan-CT and Plan-IBED,and the criteria of the 3D-Gamma pass rate were set to 1. 0％/1. 0 mm. Results The maximum percentage relative deviation ( MPRD) of the dosimetric parameters D2 ,D95 ,D98 ,and Dmean of planning tumor volumes (PTVs) between Plan-CT and Plan-IBED was less than 1. 3％. The MPRD of the average dose for organs at risk (OARs) was less than 1. 5％. The MPRDs of the lung V5,V20,and V30 were 1. 29％,3. 26％,and 2. 78％,respectively. Gamma analysis revealed an averaged pass rate of>95. 0％ for the body,as well as between 91. 9％ and 98. 2％ for OARs. Conclusion The proposed IBED assignment in simulated MRI-only treatment planning allows for dose calculation with com-parable accuracy to the baseline plan and is appropriate for thoracic tumors.