SGRT Forum

I have two questions:

(1) Is everyone simulating brain SRTs with chin neutral or down? If not, why not?

(2) What is your process when RTDs exceed 1 mm after the couch has been rotated, particularly to 90 or 270? To exclude patient motion, we return the couch to zero and if the RTDs are negligible, we conclude that the patient has not moved. Then, we rotate the couch back to 90/270 and acquire an AP or PA MV image at the couch angle. At this point we must make a decision: is the RTD real or artifact of the VisionRT system at extreme couch angles? Do we shift or not?

Thank you,



Thanks,

Hi Jose

(2) Important question with no easy answer. First you must find out if your shift e.g. >1mm at 90/270 is due to patient shift or due to couch run out. I.e. your machine QA should give an answer to the amount of couch runout when rotating from 0° to 90/270. If this value is known, then you can test AlignRT with a head phantom where you know that you have no patient movement. Here the delta should be no larger than the known geometrical couch runout. I did these kind of tests both with a head phantom and 2 volunteer RTTs. Translational deltas where <0.5mm for the full range from 90 to 270 for both phantom and volunteer. rotational deltas where up to 0.4°. So I don't have any indication for artifacts of AlignRT at extreme couch angles.

Best regards
Daniel

Hi José,

I agree to Daniel, that it is not easy to answer. I would additionally recommend to do MV calibration procedure for Couch roataions on the AlignRT system immediately before doing SRS. We also find deviations <0.5mm and >0.5° in all Couch angles. So these results make the decision easier how to handle such situations .

Best regards Kirsten

Hi.  Our physicists here also identified the discrepancies you are describing here.  What we were told from VisionRT was:

"The new upgrade will come with our Advanced Camera Optimization that will fix the phantom errors you are seeing with your couch rotations"

Hi Joanne

I would be cautious with the statement that AOC will fix the errors seen under couch rotation. This only holds true when you have a perfect mechanical couch behaviour under rotation and as we know this is seldom the case. So before you make judgments on the accuracy of the RTDs under rotation you must acquire knowledge on your mechanical couch runout under rotation by other means than AlignRT. By saying that I don't question that AOC will improve accuracy.

Best regards
Daniel

How do you do the MV calibration procedure for couch rotations?

Thanks,

We have only treated three brain SRT patients since the upgrade to Advanced, but I must say that I am noticing the RTDs to be a lot tighter so I am no longer seeing a >1 mm LNG shift with extreme couch angles (i.e. 90, 270). It is a small sample size, so we will see if this trend remains true.

Here is latest, interesting and important publication again to this topic. Surface guided imaging during stereotactic radiosurgery with automated delivery, Elizabeth L. Covington et al., Journal of applied clinical medical physics, 23 October 2020.