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Reproducible Beam-Position Hysteresis Observed with Direction of Linac Gantry Rotation

D Pinkham1*, S Kamath1 , S Hancock3 , (1) Yale School of Medicine, New Haven, CT, (2) Southeast Missouri Hospital, Cape Girardeau, MO


(Sunday, 7/14/2019)  

Room: ePoster Forums

Purpose: The purpose of this project is to investigate a reproducible linac hysteresis effect observed during gantry rotation along the crossline axis of a C-arm linac radiation beam.

Methods: Winston-Lutz images were obtained of a static BB phantom for an Elekta linac using the iViewGT portal imaging system. The gantry was rotated in a clockwise manner over its full range of motion, with a 2cm x 2cm, 6MV projection image captured every 45 degrees. This was followed by a counterclockwise sequence. The full bidirectional sequence was repeated with collimators swapped (using collimator rotation angles of 0 and 180 degrees). The full sequence was also repeated for the 6MV FFF beam energy.

Results: A hysteresis in crossline (AB) beam position of up to 0.6 mm is observed between clockwise and counterclockwise gantry motion. Smaller hysteresis effects (less than 0.3mm) were observed in the inline (GT) direction. Hysteresis along the AB direction was observed for both 6MV and 6FFF Winston-Lutz sequences, and it was also present in either collimator orientation.

Conclusion: The observed rotation directional dependence of up to 0.6 mm is significant relative to sub-millimeter tolerances required for stereotactic radiosurgery. Although it is unclear what linac components are responsible for this hysteresis, it should be accounted for when characterizing the isocentric accuracy of a linac for stereotactic treatments.


Stereotactic Radiosurgery, Quality Assurance


TH- External beam- photons: Quality Assurance - Linear accelerator

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