Program Information
Linac X-Ray Beam Spot Measurement Using MLC Or Jaws
M Cassese*, A Purwar , J Star-Lack , Varian Medical Systems, Palo Alto, CA
Presentations
MO-F-FS1-5 (Monday, July 31, 2017) 4:30 PM - 6:00 PM Room: Four Seasons 1
Purpose: To evaluate a new method of measuring x-ray beam spot images using linac MLC/jaws and EPID.
Methods: Using a TrueBeam system (Varian Medical Systems, Palo Alto, CA) in Research mode, XML plans were delivered to acquire 6 MV EPID images with either the MLC bank or jaw at beam axis. Single projection image was acquired at each collimator rotation angle over 360 degrees in 1.0 to 10.0 degree steps. Line spread functions (LSF) obtained from the Edge spread functions (ESF) were used to generate sinograms that then were used to reconstruct the spot images. Beam spot metrics such as: spot size (FWHM), spot circularity and ellipticity were used to compare the MLC/jaw results with reference spot images obtained using a 2 mm tungsten plate (edge method). Sensitivity of the beam spot measurement was investigated with respect to the position of the EPID, MLC, Jaws, number of images and dose per image.
Results: Our measurements indicate that using the MLC or jaws can produce beam spot measurements comparable to the edge method, while requiring fewer images.
Conclusion: MLC or jaws can be used for rapid beam spot measurement without need of custom equipment or film.
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