Room: AAPM ePoster Library
Purpose: To investigate the dose distribution due to electron streaming effect (ESE) during MRI guided lung stereotactic body radiotherapy (SBRT) treated on the 1.5T MRI Linac.
Methods: Two lung SBRT cases with prescription doses of 30Gy/5fx and 60Gy/4fx were retrospectively investigated. The curative treatment plans were designed using system specified Monaco (v5.40.01) with step-and-shoot intensity modulated radiotherapy (IMRT) technique under the 1.5T magnetic field. To display the in-air dose outside the body in Monaco system, an auxiliary structure was created by extending the external. Another comparable treatment plan without magnetic field was created using the same planning parameters. The two plans with or without magnetic field were then compared in terms of in-air dose distributions outside the body and dose to the chin.
Results: Electron streaming effects outside the body were obviously observed for the two cases with magnetic field, while not observed in the absence of magnetic field. From sagittal view, the electron streams can travel a long distance in air along the magnetic field direction, even can reach to the patient’s chin. The chin surface doses for the two MR-Linac plans were 2.443Gy (8.14% of 30Gy) and 1.763Gy (2.94% of 60Gy) respectively, depending on how far the distance from tumor site to the chin. While for the plans without magnetic field, the corresponding chin surface doses were much lower with 0.326Gy (1.09% of 30Gy) and 0.213Gy (0.36% of 60Gy) respectively.
Conclusion: ESE outside the body were also observed in MRI guided lung SBRT plans and which could lead to a higher chin dose. It is therefore recommended that the chin region of lung SBRT patients should be scanned and included in planning images for ESE assessment. For protection, suggestions like shielding with a bolus or keeping the patient’s head tilted to the opposite side could be considered.
Funding Support, Disclosures, and Conflict of Interest: National Natural Science Foundation of China (No. 11805292); Natural Science Foundation of Guangdong, China (No. 2018A0303100020)
IM/TH- MRI in Radiation Therapy: MRI/Linear accelerator combined (general)