The Impact of the 1.5 T Transverse Magnetic Field in Radiotherapy for Esophageal Cancer Patients
Authors: Li Yukun , Li Baosheng , Chen Jinlong , Yin Yong , Li Zhenjiang 
Published: June 2022

Abstract
Purpose: Daily imaging in magnetic resonance (MR)-guided radiotherapy (MRgRT) allows one to visualize static anatomy, to capture internal tumor motion and to extract quantitative image features for treatment verification and monitoring outcomes. Due to the presence of magnetic fields, the use of online MR imaging (MRI) requires dedicated efforts to perform accurate dose measurements and calculations. The purpose of the study was to investigate the suitability of treating esophageal cancer patients with MR-linac and to characterize the dosimetric impact at tissue-air interfaces resulting primarily from the electron return effect. These capabilities enable treatment adaptation, with potential benefits in terms of personalized medicine.Methods: We prospectively enrolled 22 patients with esophageal squamous cell carcinoma (ESCC) in a cohort study who underwent four- and three-dimensional computed tomography (4DCT and 3DCT) scans before radiotherapy (RT). The heart was contoured on 3DCT images, 4DCT end expiration (EE) images, and 4DCT end inhalation (EI) images by the same radiation oncologist. The reference RT plans were designed on 3DCT images, and the dose distributions of the plans were imported into the Monaco treatment planning system (TPS) and adjusted to each phase to generate distributions for different phases. For each patient, we generated five types of plans. Variations in dose-volume parameters for the organs at risks (OARs) (heart, lung, skin and chest wall, etc.) and target area were compared among different types. All the optimizations and calculations in this work were performed using Monaco 5.40.02.Results: Most prominently, slight dose distortions at air-tissue interfaces were observed in the presence of the magnetic field. Compared to that in nonmagnetic field conditions, the dose of the magnetic field at air-tissue (chest wall and heart wall) interfaces was slightly higher in some patients (2.4% of the tissue increased by 2.3 Gy on average). Compared to planEI (the reference plan for EI phase) and planEE (the reference plan for EE phase), the average clinical target volume (CTV) coverage V100 dropped from 99% to 95%. The average dose-volume histogram (DVH) variation between the original plan and the (planEI and planEE) was within 2.5%. The displacement in the SI direction was significantly larger than that in the LR and AP directions (p<0.05).Conclusion: This study showed that the SI direction shift of lower esophageal cancerous regions observed during RT is larger than that observed in other directions. The dosimetric impact of the magnetic field, including the electron return effect (ERE) at tissue-air boundaries, can be observed during RT. The changes in OAR dose may be valuable indicators of organ impairment and target dose changes when using the 1.5 T linac method, especially for cardiac tissue. However, reoptimizing the plan in the presence of a 1.5 T magnetic field increases the feasibility of achieving a clinically acceptable treatment plan for esophageal cancer patients.