VMAT Module and ROI Placement


Our clinic does the Varian VMAT testing on 10FFF beams as well as both on gated and non-gated treatments.

When doing the analysis of our EPID images it seems as if the ROIs aren’t always being placed correctly, particularly in the instance of the gated 10FFF delivery, which is very noisy.

So, some questions:

  • Do other clinics do VMAT testing of 10FFF beams in this manner? If not, what is your rationale in not doing so?
  • Is there a way for me to override the ROI placement in instances where the error is occurring?

Please see the attached reports to see an example.

Thanks in advance,

DRGS_Good_Placement.pdf (195 KB)

DRGS_Bad_Placement.pdf (247 KB)

The issue is probably having to due with where the center of the image is calculated to be. If indeed that’s the problem then you could override the method that determines that: https://github.com/jrkerns/pylinac/blob/698254258ff4cb87812840c42b34c93ae32a4693/pylinac/vmat.py#L147

If this is a relatively uncommon problem and you want a one-off solution you can simply override the x_offsets: https://github.com/jrkerns/pylinac/blob/698254258ff4cb87812840c42b34c93ae32a4693/pylinac/vmat.py#L36

Here’s a gist doing that:https://gist.github.com/jrkerns/ce16acc9b7135dbbfc30e535806cec20

Let me know how it works out.


we notice the same issue on 6FFF however the ROIs are placed properly on 6X, 10X and 10FFF (Varian TrueBeam STx).