Challenges of Treating Lung Cancer Patients at MR-Linac Using MR-Based Synthetic CT Calculation in the Adaptive Workflow

Abstract

Magnetic Resonance guided adaptive radiotherapy (MRgART) allows plan adaptation according to the new patient anatomy; the contours of the structures are adjusted based on the patient's daily MRI, and in the adapt to shape (ATS) workflow, the adapted plan is recalculated on the MRI-based synthetic CT (sCT) generated by bulk density assignment. For sites where there is a high electronic density (ED) gradient between the target and surrounding tissues, such as in lung cancer treatments, the assignment of an average ED may not be able to reproduce an accurate dose calculation. This study evaluates the accuracy of the sCT adapted plan calculation for lung cancer patients and assesses whether the assignment of an optimized ED can reduce dosimetric differences should they arise MATERIALS/METHODS: Nine lung cancer patients treated at Unity 1.5 MR-Linac were selected for this retrospective study. The patient's target and organs at risk (OARs) were contoured, and a CT reference plan containing the ED bulk assignment information i.e., the contours to use in the ATS workflow, and their corresponding average ED was generated. To assess the accuracy of the dosimetry of the adapted plan calculated on the sCT, the plan was recalculated on an ideal sCT (sCTref) obtained from the reference CT by forcing the drawn contours to the average ED as defined on the CT reference plan. Targets and OARs dose-volume histogram (DVH) of the CT and sCTref plans and the dose distributions using gamma (γ) analysis with 2%-2mm criteria were compared. In the case of a discrepancy between the DVHs, the average Eds used for the recalculation on the sCTref were adjusted by several attempts to obtain a sCT optimized (sCTopt) for which a superposition of DVHs on CT and sCTopt was achieved. For 7 of the 9 patients CT and sCTref target DVHs were not comparable, with a mean dosimetric difference of 5.55% (range 2.35%-7.46%) in the target volume receiving the prescription dose (VDpre), while OARs DVH dose differences remained below 1% for the nine patients. The adjustment of the ED of the homolateral lung in the sCTopt, reduced the mean target VDpre dosimetric difference between CT and sCTopt to 0.66% (range 0.17%-1.64%). In addition, the results of the gamma analysis increased from values ranging between 39.5%-70.3% to 88.5%-93.2%, as shown in the Table. Dosimetric errors in the use of the sCT calculation for targets in high ED gradient areas may arise; the use of optimized ED for sCT calculation may be a promising strand to investigate in order to proceed with MR-based sCT plan adaptation for lung cancer treatment.