Dose Calculation Accuracy of Individualized Bulk Electron Density Assignment Approaches for Simulated MRI-only Planning of Thoracic Tumors

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 3 DVH 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 a6 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 average value and recalculating the dose distribution.A 3 DVH system was used to visualize and compare the dosimetric differences between Plan-CT and Plan-IBED,and the criteria of the 3 D-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 comparable accuracy to the baseline plan and is appropriate for thoracic tumors.