Assessment of the Air-Electron Streaming Effect for MRI Guided Lung SBRT: A Treatment Planning Study

Abstract

Electron streams outside the body are inevitably generated in the presence of magnetic field during MRI guided radiotherapy. This study aims to investigate the air-electron streaming effects for lung Stereotactic Body Radiotherapy (SBRT) treated on the 1.5T MRI Linac. Three lung SBRT cases with prescription dose ranging from 30Gy to 60Gy in four to five fractions were investigated in this study. The original curative treatment plan was designed by using step-and-shoot intensity modulated radiotherapy (IMRT) technique and then calculated with Monaco (v5.40.01) Monte Carlo dose engine under the 1.5 T magnetic field. To investigate the effect of air-electron streaming, another comparable treatment plan without 1.5T magnetic field was generated using the same planning parameters. In order to display the in-air dose distribution outside the body in Monaco system, an auxiliary structure was created by extending the external with margin of 8cm in anterior. The two plans with or without magnetic field were then compared in terms of in air dose distribution outside the body and dose to the chin. Electron stream effects outside the body were obviously observed for all 3 lung SBRT cases when in the presence of magnetic field, while which were not observed in the absence of magnetic field. From sagittal view, the electron stream can travel a long distance in air along the magnetic field direction, even can reach to the patient’s chin. But only two of the three cases have CT scan including the chin region. For that two cases, the chin surface skin doses were 1.47Gy (2.45% of 60Gy) and 2.24Gy (7.47% of 30Gy) respectively. While for the plans without magnetic field, the corresponding chin surface skin dose were 0.18Gy (0.3% of 60Gy) and 0.32Gy (1.07% of 30Gy) respectively. For the last case, the streaming electrons were expected to reach at the chin area, but due to which was not scanned in CT image, it’s hard to evaluate quantitatively. Electron streaming effects outside the body were also observed in MRI guided lung SBRT plans and which could lead to a higher chin dose compared to conventional scenario without magnetic field. It is therefore recommended for the MRI guided lung SBRT patients that the chin region should be scanned and included in planning CT image. Other suggestions for example shielding with bolus or keeping the patient’s head tilted to the opposite side should also be considered.