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Electron contamination modeling and skin dose in 6 MV longitudinal field MRIgRT: Impact of the MRI and MRI fringe field

2012; Wiley; Volume: 39; Issue: 2 Linguagem: Inglês

10.1118/1.3676181

2473-4209

Bradley M. Oborn, Peter Metcalfe, Martin J Butson, Anatoly Rosenfeld, Paul Keall,

Advanced MRI Techniques and Applications

Medical PhysicsVolume 39, Issue 2 p. 874-890 Radiation therapy physics Electron contamination modeling and skin dose in 6 MV longitudinal field MRIgRT: Impact of the MRI and MRI fringe field B. M. Oborn, B. M. Oborn Illawarra Cancer Care Centre (ICCC), Wollongong, NSW 2500, Australia and Centre for Medical Radiation Physics (CMRP), University of Wollongong, Wollongong, NSW 2500, Australia Author to whom correspondence should be addressed. Electronic mail: bmo03@uow.edu.auSearch for more papers by this authorP. E. Metcalfe, P. E. Metcalfe Centre for Medical Radiation Physics (CMRP), University of Wollongong, Wollongong, NSW 2500, AustraliaSearch for more papers by this authorM. J. Butson, M. J. Butson Illawarra Cancer Care Centre (ICCC), Wollongong, NSW 2500, Australia and Centre for Medical Radiation Physics (CMRP), University of Wollongong, Wollongong, NSW 2500, AustraliaSearch for more papers by this authorA. B. Rosenfeld, A. B. Rosenfeld Centre for Medical Radiation Physics (CMRP), University of Wollongong, Wollongong, NSW 2500, AustraliaSearch for more papers by this authorP. J. Keall, P. J. Keall Sydney Medical School, University of Sydney, NSW 2006, AustraliaSearch for more papers by this author B. M. Oborn, B. M. Oborn Illawarra Cancer Care Centre (ICCC), Wollongong, NSW 2500, Australia and Centre for Medical Radiation Physics (CMRP), University of Wollongong, Wollongong, NSW 2500, Australia Author to whom correspondence should be addressed. Electronic mail: bmo03@uow.edu.auSearch for more papers by this authorP. E. Metcalfe, P. E. Metcalfe Centre for Medical Radiation Physics (CMRP), University of Wollongong, Wollongong, NSW 2500, AustraliaSearch for more papers by this authorM. J. Butson, M. J. Butson Illawarra Cancer Care Centre (ICCC), Wollongong, NSW 2500, Australia and Centre for Medical Radiation Physics (CMRP), University of Wollongong, Wollongong, NSW 2500, AustraliaSearch for more papers by this authorA. B. Rosenfeld, A. B. Rosenfeld Centre for Medical Radiation Physics (CMRP), University of Wollongong, Wollongong, NSW 2500, AustraliaSearch for more papers by this authorP. J. Keall, P. J. Keall Sydney Medical School, University of Sydney, NSW 2006, AustraliaSearch for more papers by this author First published: 25 January 2012 https://doi.org/10.1118/1.3676181Citations: 49 Read the full textAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat Abstract Purpose: In recent times, longitudinal field MRI-linac systems have been proposed for 6 MV MRI-guided radiotherapy (MRIgRT). The magnetic field is parallel with the beam axis and so will alter the transport properties of any electron contamination particles. The purpose of this work is to provide a first investigation into the potential effects of the MR and fringe magnetic fields on the electron contamination as it is transported toward a phantom, in turn, providing an estimate of the expected patient skin dose changes in such a modality. Methods: Geant4 Monte Carlo simulations of a water phantom exposed to a 6 MV x-ray beam were performed. Longitudinal magnetic fields of strengths between 0 and 3 T were applied to a 30 × 30 × 20 cm3 phantom. Surrounding the phantom there is a region where the magnetic field is at full MRI strength, consistent with clinical MRI systems. Beyond this the fringe magnetic field entering the collimation system is also modeled. The MRI-coil thickness, fringe field properties, and isocentric distance are varied and investigated. Beam field sizes of 5 × 5, 10 × 10, 15 × 15 and 20 × 20 cm2 were simulated. Central axis dose, 2D virtual entry skin dose films, and 70 m skin depth doses were calculated using high resolution scoring voxels. Results: In the presence of a longitudinal magnetic field, electron contamination from the linear accelerator is encouraged to travel almost directly toward the patient surface with minimal lateral spread. This results in a concentration of electron contamination within the x-ray beam outline. This concentration is particularly encouraged if the fringe field encompasses the collimation system. Skin dose increases of up to 1000% were observed for certain configurations and increases above Dmax were common. In nonmagnetically shielded cases, electron contamination generated from the jaw faces and air column is trapped and propagated almost directly to the phantom entry region, giving rise to intense dose hot spots inside the x-ray treatment field. These range up to 1000% or more of Dmax at the CAX, depending on field size, isocenter, and coil thickness. In the case of a fully magnetically shielded collimation system and the lowest MRI field of 0.25 T, the entry skin dose is expected to increase to at least 40%, 50%, 65%, and 80% of Dmax for 5 × 5, 10 × 10, 15 × 15, and 20 × 20 cm2, respectively. Conclusions: Electron contamination from the linac head and air column may cause considerable skin dose increases or hot spots at the beam central axis on the entry side of a phantom or patient in longitudinal field 6 MV MRIgRT. This depends heavily on the properties of the magnetic fringe field entering the linac beam collimation system. The skin dose increase is also related to the MRI-coil thickness, the fringe field, and the isocenter distance of the linac. The results of this work indicate that the properties of the MRI fringe field, electron contamination production, and transport must be considered carefully during the design stage of a longitudinal MRI-linac system. 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