Document Type

Conference Proceeding

Publication Date


Publication Title

Medical Physics


Purpose: Maintaining a linac within tight stereotactic isocentricity tolerances requires minimizing errors in individual components of the linac. However, while isocentricity can be measured using a Winston-Lutz test, contributions from individual components can be difficult to discern. The disconnect between how individual components contribute to isocentricity can lead to lengthy troubleshooting exercises when isocentricity is outside tolerance. To provide more info rmation to physicists and increase the speed of isocentricity adjustments, a new tool that uses optical tracking to investigate and adjust individual linac components that contribute to isocentricity was tested on three clinical linacs. Methods: The optical tracking tool was used to measure mechanical movement of the gantry, collimator, and couch of three Elekta linacs. The tool consisted of three illuminated balls attached along the central axis of the collimator that were monitored by a stereoscopic camera placed on the treatment couch. The camera tracked motion of the balls during a series of gantry, collimator, and couch rotations. Camera results were used to make recommendations for mechanical adjustments to improve isocentricity. Results: All mechanical components were within TG-142 guidelines, however multiple adjustments were identified to improve isocentricity. Gantry axis movement in the gun-target direction after a 360 rotation for the three linacs were 0.28mm, 0.25mm, and 0.03mm. Two linacs required jockey wheel adjustments to reduce this movement. Deviations between couch and linac isocenter were 0.03mm, 0.25mm, and 0.27mm, thereby requiring adjustments of two couch isocenters. Lateral inaccuracies in gantry arm position were 0.06mm, 0.31mm, and 0.13mm. Arm positions were adjusted to reduce these inaccuracies. Conclusion: The optical tracking tool identified mechanical machine adjustments to improve isocentricty that would not have been easily discoverable with a routine Winston-Lutz test. Future work will focus on using the camera to position a ball-bearing for beam steering, followed by long -term monitoring of improved isocentric





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American Association of Physicists in Medicine 65th Annual Meeting & Exhibition, July 23-27, 2023, Houston, TX