The Dosimetric Feasibility of the 1.5T MR-Linac System for Esophageal Cancer: The Impact of the Magnetic Field

Abstract

The MR-Linac (MRL) provides image guidance with high soft-tissue contrast. Esophageal cancer patients are a potential group to benefit from better guidance in the MRL. However, due to the presence of a transverse magnetic field (1.5T), electron return effect (ERE) may affect the target dose distribution, particularly at air-tissue interfaces. The purpose of the study was to investigate the suitability of treating esophageal cancer patients with the MRL and to characterize the dosimetric impact at air-tissue interfaces resulting primarily from the ERE. We prospectively enrolled 20 cases with ESCC into a cohort study which underwent re-planned using the research version of Monaco (V 5). All patients received 4DCT and three dimensional (3D) CT scans before Radiotherapy. Intensity modulated radiotherapy (IMRT) plans were designed on 3DCT images and 0% phase of the 4DCT images. In each case, we generated two types of plans: the original plan generated without a magnetic field; the optimized plan generated by a full optimization with a 1.5 T magnetic field. The prescribed dose was 60 Gy to the primary tumor (PTV60) and the esophageal wall contralateral (CE) to the tumor was contoured as an avoidance structure. All generated treatment plans were optimized for fulfilling their respective planning constraints. The dose distributions of the plans were imported into MIM Maestro and deformed to each phase to generate distributions for MIP and AIP phases. Most prominently, slight dose distortions at air-tissue-interfaces were observed in the presence of the magnetic field. Compared to non-magnetic field conditions, the dose of the magnetic field at air-tissue interfaces are slightly higher in some cases (1.8% of the tissue increased by 1.3 Gy on average). When considering breathe motion, 4DCT is more affected by magnetic field than 3DCT. The dose recalculation showed the presence of the 1.5 T magnetic field can slightly reduce PTV dose (D95). The low dose areas (V5, V10) of the lung sites are higher than the conventional Linac. The MRL results in considerable increases in Dmax on the esophageal wall in all cases. No significant differences between both plans were seen in terms of V20Gy of the lungs and V40Gy of the heart. The treatment of patients in the MRL using a conventional IMRT technique, at 1.5 T, implies an increase in esophageal wall and V5, V10 of the lung sites dose. The dosimetric impact of magnetic field at air-tissue boundaries is minor and does not negatively impact target conformity index. This opens possibilities for developing MR-guided esophageal cancer treatments in the MRL.