IMRT 2018

MLC position Quality Control

• Mechanical sag compensation (between Gantry/MLC and EPID ) • Correct for sag during MLC position analysis • Both in plane and cross plane • Absolute position and rotation of MLC • MLC may be mounted a bit off => correction • Requires: Absolute position of radiological iso-c known

Determine position of MLC on bank A and bank B eg from derivative Compare to position of radiol iso-c => Abs position and rotation

4 3 2 1

Static 5x5cm 2 MLC field delivered in an arc. Always measure perpendicular to MLC tip.

3 closed MLC on each side

Courtesy of L Præstegaard

Rowshanfarzad, MedPhys39, 2012. Detection and correction for EPID and gantry sag during arc delivery using cine EPID imaging Zwan, J Appl Clin Med Phys 17,2016. An EPID-based system for gantry-resolved MLC quality assurance for VMAT