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A Novel Doubly-Focused Multileaf Collimator Design for MR-Guided Radiation Therapy

H Li

H Li1*, S Mutic1 , D Low2 ,O Green1, G Fought3 , I Kawrakow3 , A Sharma3 , S Shvartsman3 , J Dempsey3 , (1) Washington University School of Medicine, St. Louis, MO, (2) UCLA, Los Angeles, CA,(3) ViewRay, Inc., Oakwood Village, OH


TH-AB-BRA-1 (Thursday, August 4, 2016) 7:30 AM - 9:30 AM Room: Ballroom A

Purpose: To describe the physical and dosimetric properties of a novel double-stack multileaf collimator (MLC).
Methods: One of the compromises made in the MLC design has been to employ linear-motion singly-divergent shapes. Because the MLC leading edge moves linearly, it is rounded to provide a consistent, albeit compromised penumbra. The MLC employed in the new linac-based MR-IGRT unit is designed to be doubly focused in that each leaf moves in an arc centered at the source, and the sides of the leaves are machined such that they lie parallel to a line between the leaf edge and the source. The curvature of the MLC keeps motors and encoders in lower magnetic field. However, high spatial-resolution leaves are difficult to manufacture to sufficiently tight tolerances and difficult to move due to restricted space on the gantry. Wider leaves alleviate this problem with less moving parts but the coarse resolution disallows treating very small lesions. This compromise has been overcome by splitting the MLC leaf bank into two sets, stacked one upon the other and offset half of a leaf width. The dosimetry has been simulated using Monte-Carlo and a 6 MV linac in a 0.35 T magnetic field.
Results: The combined MLC leaf set has a spatial resolution of effectively half of the leaf width, 4mm here. The dosimetry resolution and conformality are consistent with 4mm wide MLC assisted by inverse fluence modulation. Also, because each leaf junction is backed up by the stacked leaf that lies over the junction, the problem of tongue-and-groove dosimetry has been greatly reduced. The novel MLC design allows the use of more powerful leaf motors than would be otherwise possible if a single MLC bank is employed.
Conclusions: The stacked MLC will provide highly conformal dose distributions suitable for stereotactic radiation therapy of small lesions.

Funding Support, Disclosures, and Conflict of Interest: The research was funded by ViewRay, Inc.

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