The Effect of Bulk Electron Density on the Accuracy of Treatment Planning for Postoperative Cervical Cancer

Abstract

MRI (Magnetic Resonance Images) only treatment planning is desirable due to its high soft tissue contrast and nature of non-radiation. This study investigates the impact of bulk electron density assignment on the dose accuracy in MRI-only treatment planning for postoperative cervical cancer treatment.20 cases of postoperative cervical cancer treatment plans were selected to generate new plans using bulk electron density assigning to OARs (Organs at Risk) and tissues on CT images in treatment planning systems. Contours include OARs such as bladder, rectum, small intestine, kidney, spinal cord, femoral head and Planning Target Volumes (PTV). The plans employed Volumetric Modulated Arc Therapy (VMAT) with double arcs (360°) and 0.3 cm × 0.3 cm × 0.3 cm calculation grid with 0.6 cm minimum subfield. New plans (PlanN) were generated from original plans with bulk electron density assigning to organs. Fluence optimization and IMRT dose constraints are the same as that with original plans (Plano). Dose Volume parameters (DVPs) of OARs and PTV were extracted from the two groups plans including: V30, V40-relative volume received 30 Gy, 40 Gy; mean dose-Dmean; D2, D98-dose received by 2%,98% of the target volume. The DVPs were compared between original and new plans using scatter plots. Paired-t test for DVPs was used to test the significance of the statistics.Relative deviation of most of the dosimetry parameters between original and new plans are less than 1%. PCTV1 Dose of the PlanN are higher 0.1% ∼ 0.2% (P > 0.05) than Plano. The dose difference of PCTV2 and PCTVln for the two group of plans is within 0.6(P < 0.05), 0.4% (P > 0.05), respectively. The OARs dosimetry differences including V20, V30, D1cc, Dmean for bladder, rectum, small intestine is within 0.2%, 0.6%, 0.2%. There is no noticeable distinction observed from dose distribution between Plano and PlanN. Monitor Units for PlanN are higher 0.84% than Plano. The statistical results of all parameters showed that there was no significant difference between the two groups.The dose deviation between the MRI-only plan and the routine CT-based plan was less than 2%, which met the clinical requirements. It provided a feasible method for MRI to replace CT in radiotherapy plan design and dose calculation.