Room: Exhibit Hall | Forum 9
Purpose: The purpose of this project is to characterize hysteresis effects of gantry and table rotational direction on isocentric accuracy for linac-based stereotactic treatments.
Methods: Winston-Lutz images were obtained from each of three Elekta linacs using their respective iView portal image systems. One of the three linacs included a Hexapod 6D robotic tabletop. The effect of table rotational direction on Winston-Lutz deviation was determined by obtaining a series of images over the full range of table angles in a clockwise sequence, with a vertical beam, and comparing with a similar set of images taken in a counterclockwise sequence. Similarly, the effect of gantry rotational direction and collimator rotational direction were determined.
Results: A directional dependence on couch and gantry rotation is observed, with deviations for each axis ranging from 0.3 to 0.5 mm. Table directional dependence is observed using either automated table axis motion, or with manually applied couch rotation, and is very reproducible. For table rotation, the hysteresis is in the lateral direction relative to the table coordinate system and not a rotational shift. Collimator directional dependence was observed to be minimal (< 0.1 mm). Results were similar for all three machines.
Conclusion: The observed hysteresis of up to 0.5 mm is significant compared to the standard of sub-millimeter accuracy for stereotactic radiosurgery. This effect should be accounted for when characterizing and optimizing the isocentric accuracy of a linac for stereotactic treatments. It is important to understand how individual machine components contribute to the overall treatment accuracy, but it is not known at this time which machine components are responsible for the directional dependence.