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Verification of the MRI-Linac Beam Alignment

JWH Wolthaus*, JJ Bluemink , SJ Woodings , JHW Vries , TL van Soest , JGM Kok , HM van Zijp , SS Hackett , B van Asselen , BW Raaymakers , University Medical Center Utrecht, Utrecht, The Netherlands


(Monday, 7/30/2018) 9:30 AM - 10:00 AM

Room: Exhibit Hall | Forum 3

Purpose: A major aspect of MRI-Linac acceptance testing is geometrical beam (alignment) verification. International guidelines on dosimetry and linac QA were adopted but specific tests have to be adjusted to incorporate the magnetic field effects on measurements and to overcome the absence of collimator rotation. The submitted work will focus on QA of beam alignment.

Methods: Elekta’s MRI-linac has a rigidly mounted on-board EPID to align the measurement equipment (detectors, watertank, film). The isocenter pixel is identified from projection images of a ball-bearing.First, beam orthogonality is verified in the watertank by alignment of profiles at different depths. Secondly, lateral focal spot alignment is determined using two longitudinal half-fields of opposing gantry angles irradiated onto a film (sandwiched between copper plates to mitigate the effect of electron deflection due to the Lorenz force). The difference in center position (defined by the edges) of the two profiles extracted through each of the two half-fields is related to the misalignment of the focal spot. Subsequently, lateral MLC-bank alignment is determined by comparing the profile peak position of a 30x20 cm2 field to the center defined by the field edges. MLC leaf position accuracy is determined using a stripe/picket-fence-test in which the film is registered to simultaneously acquired EPID images. Finally, the gantry isocenter accuracy is determined using a Starshot-test on a film, sandwiched between copper-rings. Congruence of two opposed fields of different sizes for 0/180° and 90/270° are verified as a general geometrical check.

Results: EPID Isocenter uncertainty was <0.1 mm. Accuracies of beam alignment (x/y) <0.3°, Focal spot alignment <0.5 mm, MLC absolute position <0.5 mm. Gantry isocenter locus radius <0.4 mm.

Conclusion: The presented work shows a subset of all performed tests and measurements but were all within tolerance. The Elekta MRI-linac geometric alignment is suitable for clinical treatments.

Funding Support, Disclosures, and Conflict of Interest: The research in the presented work has been supported by Elekta


MR, Linear Accelerator, Quality Assurance


TH- External beam- photons: Development (new technology and techniques)

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