Camera blocking with VMAT

SGRT - Best Practice & Applications
1

We are a new user and are currently treating without beam control. We are noticing that the displayed deltas will be out of tolerance only when some cameras are blocked by the rotating gantry. Once the camera is no longer blocked the deltas are again within acceptable range. The duration of out of tolerance deltas is many seconds. How are people treating VMAT patients with active beam control?

2

We have been treating SRS (VMAT, HyperArc) with AlignRT with active beam control without any camera obstruction issues. A few things come to mind while reading your post. First, I would check to make sure the monthly physics camera calibration looks good (also good to check the daily for any drifts). Secondly, I would check the ROI you are using with the plan, make sure it’s the right size and location for the treatment you are doing. We (and other people) have evaluated out AlignRT system to have submillimeter accuracy even with camera obstruction. At our clinic we see a mac of 0.5mm deviation with camera obstruction, which is well under out 1mm tolerance.

I’d be happy to look at your setup, feel free to contact me

Adi

3

Thanks for the reply Adi, I will contact you off line. Yes our calibration is good and recent. I am going to evaluate our ROIs more carefully today and document which parameter is out of tolerance and for which gantry angles. The imaging arms are retracted so at most we would be blocking one camera only.

4

If it is happening on a patient and not on a phantom, I would check

  • See if the isocenter location is in a similar location relative to the surface of the phantom and patient (i.e. is the iso really posterior in the patient or far from the surface and centered in the phantom)

  • Are the monitoring surfaces similar in the number of features (i.e. monitoring in a phantom with nice square edges vs on a patient with few contours across the ROI)

  • Is the patient iso to surface distance larger putting the surface in an unfocused or less focused portion if the speckle pattern

  • Do testing with the phantom but place iso so that the phantom is in different regions of the speckle pattern to make sure you get consistent results on the phantom at different distances surface to iso

  • Use an anthropomorphic phantom and test with the phantom in an orientation similar to the patient you have problems with (i.e. chin down or chin up) to make sure you are not dealing with self shadowing effects.

    • 5

    One additional item you can look at is if the ROI is “deep” into the gantry (i.e. beyond isocenter plane moving into the gantry) while the arms are out for imaging (e.g. triggered imaging) and in some instances even without imaging arm obstruction.

    You may need a second ROI that is inferior the isocenter plane. Some patients have a self shadowing area in the camera 3 perspective where the chest slopes toward the head and neck area.

    This area is not an issue with full visibility from both lateral cameras and in fact is not detectable when taking an unobstructed reference image. Try blocking one or both lateral cameras and take a reference of the patient in question and if there is a significant hole in the surface in that region, and the ROI extends into it, that is essentially what your cameras are dealing with when obstructed even if the reference surface that is being monitored is complete as a result of taking the reference in an unobstructed orientation.

    6

    Hi, hopefully you have solved this issue already - we rarely experience this but there are a few things we trouble shoot and always solve the issue.

    Are the lighting levels ok?

    Does the patient have a darker skin tone - adjust setting?

    Is the breast itself (or any anatomy) blocking the inferior camera view of the set ROI? This can leave only 1 camera with real ‘eyes on’. Increase the ROI size.

    Surface capture to improve quality of the reference surface.

    If this doesn’t solve it, increasing the ROI generally helps especially if you have been conservative in the first place. We

    leave the tolerances where they are and try everything else instead.

    Hope this helps. Ben