Evaluating Synthetic CT Calculation Accuracy in a 1.5 T MR-Linac Clinical Workflow

Abstract

<h3>Purpose/Objective(s)</h3> The adapt to shape (ATS) workflow of 1.5 T MR-Linac 1.5 allows plan adaptation according to the new patient anatomy with a plan optimization on daily magnetic resonance imaging (MRI)-based synthetic computed tomography (syCT). This study evaluates the effectiveness of introducing into the clinical workflow a per/patient assessment of the minimum expected dosimetric errors associated with an MRI-based syCT adapted plan. <h3>Materials/Methods</h3> 55 patients treated at 1.5T MR-Linac in different tumors sites were consecutively included in this study. Each patient underwent CT and MR simulation scans for treatment plan preparation. The patient's target and organs at risk (OARs) were contoured in a treatment planning system, and a reference CT plan was generated using eight to twelve individual beam angles; the plan contains electronic density (ED) bulk assignment information i.e., the contours to use in the ATS workflow, and their corresponding average ED, the priority of each contour respect to density assignment in case of contour overlaps. To assess the dosimetric error made by using ATS procedure, for each patient the plan was recalculated on ideal syCT obtained from the reference CT by forcing the drawn contours to the average ED as indicated on the reference CT plan. Targets and OARs dose-volume histogram (DVH) of the CT and syCT plans and the dose distributions using gamma analysis 3%-3mm and 2%2mm criteria, were compared. <h3>Results</h3> For the dose distribution on syCT and CT, the average pass rate of gamma analysis using the 3%/3 mm and 2%/2 mm criteria were higher than 99% and 98%, for 47 patients out of 55; similarly targets and OARs DVH dose differences remained below 1%. For the remaining 8 patients, the source of discrepancy was investigated, and the CT reference plan was reworked to decrease the dosimetric differences on the syCT plan; the results obtained are shown in Table 1. <h3>Conclusion</h3> The accuracy of the plan adaptation on MR-based syCT may be affected by several factors. The inclusion of a per-patient assessment in the clinical workflow allows monitoring and possible optimization before the start of the radiotherapy course.