Why does my couch iso size always turn out so large? If I do a couch walkout with lightfield, it is <1mm. Is there anything I can troubleshoot?
Thanks,
Landon
Hi,
How are you testing the couch? What angles are you using?
Here are the angles. I run with gantry and collimator at 0: 0, 90, 270
Do I need to add 315 and 45? We are Varian IEC scale.
Thanks
Did you try only couch to separate the variables?
The couch iso size done with a Winston-Lutz is expected to deviate from a couch walkout with lightfield.
Have a look at this paper :
https://aapm.onlinelibrary.wiley.com/doi/full/10.1120/jacmp.v15i2.4682
We implemented this method at our center and it works great. I don’t know if there is an equivalent publication for Varian linacs.
I am not sure if your problems is caused by BB position, but to continue Michel's point, position the BB into the average MV isocenter of the gantry and collimator. You do this by taking at least 8 images with the following angles g180c90, g180c270, g270c270, g270c90, g0c90, g0c270, g90c270, g90c90, or similar, as long as you use opposite collimator angles to average out the asymmetry of Mlcs and jaws. After that calculate BB shift that minimizes deviations, and apply the shift to the BB. Then repeat the procedure until the BB is in the average isocenter.
After that, turn g and c to 0 and acquire images at several couch angles. In theory, if your couch axis of rotation is matched with the average gantry/collimator isocenter, then the BB will remain stationary when couch is rotated. This is important if your linac has a large gantry sag, like all Elektas do. The imaging system is normally calibrated to gantry and collimator isocenter, so the axis of the couch should match this.
If you position the BB away from the couch axis the resulting wobble will be large.