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MRI-Linac Systems: Can a Standard MLC Be Incorporated Into Such a Device?

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S Kolling

S Kolling1*, B Oborn2, P Keall1, (1) University of Sydney, Sydney, Australia, (2) Illawarra Cancer Care Centre, Wollongong, Australia

SU-F-500-5 Sunday 4:00PM - 6:00PM Room: 500 Ballroom

Purpose: Integrated MRI-linac systems for real-time image guidance are currently under development to master intrafraction tumor motion. As the multileaf collimator (MLC) is a crucial component of such systems, this study is aimed to answer if a standard MLC can be incorporated based on (a) its impact on the MRI field homogeneity and (b) the electromagnetic force acting on the MLC in the MRI field.

Methods: A 1.0 T split-bore magnet and the key ferromagnetic components of a Varian Millennium 120 MLC, namely the leaves and motors, were modelled using the finite element method (FEM). In this work both the parallel and perpendicular configuration for such a system were investigated, i.e. the linac treatment beam was orientated parallel and perpendicular to the MRI field direction. Simulations were performed for isocentre distances in the range from 100 to 200 cm. From the resultant magnetic field distributions, the introduced field distortion over the MRI imaging volume, a 30 cm diameter sphere volume (DSV) around the isocentre, and the net electromagnetic force acting on the MLC were evaluated.

Results: For all distances of 140 cm or larger, the field distortion was below 300 ppm which is commonly referred to as maximum pre-shim inhomogeneity of MRI devices. The total force pulling the MLC towards the MRI magnet was found to be maximum at a 120 cm isocentre distance with 1706 N and 3499 N in parallel and perpendicular configuration, respectively, and to decrease for larger distances.

Conclusions: The theoretical results suggest that the distortion introduced by a standard MLC can be corrected for by shimming for isocentre distances of 140 cm or larger. Appropriate mounting of the MLC to withstand the electromagnetic forces is considered mechanically challenging and may require an isocentre distance larger than 140 cm.

Funding Support, Disclosures, and Conflict of Interest: NHMRC Australia Fellowship, Australian Endeavour Research Fellowship

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