Assessment of Dose Accuracy for MR-Guided Online Adaptive Radiotherapy for Nasopharyngeal Carcinoma

Purpose/Objective(s) MR-guided radiotherapy relies on the daily assignment of a relative electron density (rED) map to acquire the fraction specific dose calculation. The approach to acquire the rED map is used bulk rED assignment based on the rED values of the delineated region of interests (ROIs) to generate a synthetic CT (sCT). This study aims to evaluate the dose accuracy of this approach for application in 1.5 T MR-Linac, to assess the reliability of this approach in the case of online adaptive MR-guided radiotherapy for Nasopharyngeal Carcinoma (NPC) patients. Materials/Methods The original CT and MRI T2 were acquired for 10 NPC patients and targets, organs at risk (OARs), bone and air regions were delineated on original CT in a treatment planning software. Each patient's MRI/CT image pair was aligned using a rigid registration algorithm, and the contours were copied to MRI from original CT. The sCT was then generated in MRI using bulk rED assignment approach, based on the rED values of the delineated region of interests (ROIs). Three different types of sCT images were generated: The sCTICRU uses the rED values recommended by ICRU46, the sCTtailor uses the patient-specific mean rED values, and the sCTHomogeneity was created using water equivalent value of 1 g/cm3 for entire patient data set. The same treatment plan was calculated on three sCTs and original CT. The dose calculation accuracy was investigated in terms of gamma analysis, point dose comparison and dose volume histogram (DVH) parameters. Results For both criteria, 1%/1mm and 2%/2mm gamma passing rate results of plans on sCTtailor were always higher than plans on sCTICRU and sCTHomogeneity when compared with original CT based plans (P = 0.005). The difference on point dose in high dose planning target volume for each case calculated on sCTtailor (-0.14% ± 1.44%) was smaller if compared to those calculated on sCTICRU (-8.77% ± 2.33%) and sCTHomogeneity (1.65% ± 2.57%). The DVH differences for plan based on sCTtailor were much smaller than sCTICRU and sCTHomogeneity. Conclusion The sCT generated with bulk rED assignment approach guarantees an acceptable level of dose accuracy for NPC patients with 1.5 T MR-Linac, making this approach suitable to online adaptive MR-guided radiotherapy. The accuracy can be improved by using patient-specific rED values.