SGRT Forum

Hello,

We are using OSMS during SRS treatments and we've set the tolerance at 1 mm and 0.5 degrees to indicate if we should pause and re-image. We've been using the system for about a year now, and pretty consistently we see deviations around 1mm at couch angles, but when we re-CBCT, there are no shifts. The offending couch angles are not, however, consistent (sometimes it's on the 90 side, sometimes the 270).  Sometimes once a certain camera gets blocked by the gantry, the deviation will settle back down to closer to 0.5 mm. Again, the offending camera is not consistent either. Our Winston Lutz tests also do not show couch walk out corresponding to the deviations we see during treatment.

It's becoming frustrating to continue pausing SRS treatments to re-image when OSMS shows a deviation, only to have no shifts. Has anyone else run into this?

We have a truebeam, 6D couch, Q-fix Ecompass immobilization (open mask from upper forehead to under the nose and to the temples).

Hey Kileigh,

My institution has not been doing frameless SRS as long as yours, but we have had a very similar experience. We've reached out to an number of other clinics and the issue of real-time delta deviations for larger couch rotations is fairly common. There seems to be two interrelated issues:

As the patient rotates, the view that the cameras have of the tracking surface changes. This results in growth in the real-time delta readouts at extreme couch rotations (near 90 and 270). Like you, our imaging typically shows that there is no real deviation. The deviations are usually in the longitudinal and lateral directions. For us, they are usually on the order of 1 mm or less, but we have seen 1.5 mm on a couple occasions.
As the gantry rotates, the camera opposite of the couch rotation direction can become blocked. Depending upon the patient and couch angle, this can result in a change in real-time deltas. We often see changes on the order of 0.5 to 1 mm.

In speaking with other clinics, many have noted that the phenomenon has an isocenter depth dependence too: When isocenter is more posterior (deeper relative to the surface), the deviations are larger.

The clinics I have spoken to have responded differently to the issue. One clinic has chosen to limit their couch rotations to +/- 45 degrees to avoid problematic angles. Some repeat their CBCT or orthogonal imaging for all deviations that result in an out-of-tolerance level. Initially, we performed AP kV planar imaging at the problematic couch angle to verify patient position. If shifts were not required, we capture a VRT at that angle and continue treatment. Lately, we have transitioned to driving the couch back to zero degrees to verify the position rather than using imaging, as the real-time deltas at couch zero had corresponded well to what we see on imaging.

To my knowledge, no one has been able to eliminate the issue entirely. Vision RT is set to release an advanced camera optics calibration, I believe later this autumn. They have some beta test sites who already have the calibration, and these sites report significant improvement. From what I've heard, it sounds like the new calibration cuts the real time delta deviations by about half and significantly reduces the effect of one of the camera pods getting occluded.

We have also had Vision RT remote into our system to review our tracking ROIs and observe treatment. They have made some recommendations regarding the ROIs that have reduced the frequency of large deviations. One thing that is especially important is to review your ROI on the VRT capture used for treatment to make sure it transferred from the DICOM surface with high fidelity. If the ROI splashes onto the mask, you can get some unexpected deviations. For us, sometimes the top of the nose ends up looking very jagged or spiky, and we will remove the ROI here. I would recommend having the vendor remote in at least once to look for something easy to fix.

Kind regards,

Dustin

Kileigh,

A number of things can be happening here and understanding which is important. We have been using Vision RT's AlignRT (same thing as OSMS) for SRS for over 5 or 6 years now and have characterized the system performance under many SRS conditions. There is a white paper and at least 1 presentation from Ben Waghorn Ph.D. DABR (a physicist who works for Vision RT) that discusses the aspects of the ACO camera calibration that Dustin referred to above. The whitepaper covers the expected improvements it can bring to a system and the results of that have been seen at my institution as well as documented by the data out of UAB. With you being an OSMS customer and not a AlignRT customer I do not know what the logistics are in getting the ACO camera calibration for your site and I would recommend reaching out to Varian to see what that might take.

However, you don’t have ACO and I have found a non ACO system can maintain a compound uncertainty with all variables under the worst conditions of around 0.7-0.8mm across all couch positions. That was with throwing every unfavorable characteristic (i.e. poor head position, obstruction, extreme couch angles, etc.) at the system and letting the errors compound at the absolute edge of the FOV for the cameras. I have listed a few items you can consider below and referenced a couple white papers and talks as well.

You should be characterizing your system for SRS by looking at the rotational stability and obstruction characteristics of your cameras and isolating various confounding variables. I have written a whitepaper (pre ACO) outlining a number of tests you can use and the expected results of those tests under a full range of isocenter placement positions in the cranial vault. It also covers the impact of cranial positioning and ROI effects that I can send to you if you are interested.

The long and short of it is obstruction characteristics of the cameras are, or should be, on the order of 0.1mm - 0.2mm (can vary with isocenter location). The rotational walkout of the deltas followed our couch walkout within 0.1mm when the radiographic WL images are corrected for gantry sag effects (very important - I can send you the slide from my AAPM 2018 talk that shows this). Head position and isocenter position can have a large impact on the deltas in a non ACO system as the couch rotates over the full range of possibilities for SRS.

Head tilt should be kept to neutral or chin down for SRS as chin up orientations result in shadowing or obstruction of the ROI by the patient’s own anatomy at couch angles approaching 90 and 270. With treatment arcs often passing over the head and obstructing the opposite camera this obstruction effect can cause serious issues with the camera’s ability to see the ROI and result in poor performing deltas.

If the patient has a poor head position the mask opening and ROI size becomes much more important and in the case of Qfix I have seen on a number of my consulting cases where the opening in the mask is far too small for the orientation of the patient’s head during delivery. This seems very common for Qfix users but is by no means limited to them. I would encourage some good mask making training to make sure the mask opening is of the proper size going into treatment planning.

The calibration of the camera is also important. The conditions of the calibration should be controlled and there is in fact a Customer Information Bulletin from back in 2017 (ID # CIB2017SN0005) that discusses this and makes recommendations related to the conditions under which the cameras should take place and gives an outline of the procedure.

I cannot say I have ever seen things like what Dustin mentioned in his points 1 and 2 without some of these other issues being at the root cause nor have we ever had to limit our couch rotation angles for SRS deliveries. I wouldn’t venture to say we have eliminated ALL delta discrepancy issues entirely but we can predict with a very high degree of success the very small fraction of patients that are going to potentially have one during delivery because 1) we are operating at the extreme limit of the system or 2) the patient’s position is very unfavorable well before we see it during treatment. We can do this because the rest of the system is fairly easy to characterize and control even without ACO.

If you need a copy of anything I have mentioned my email is mike.tallhamer@gmail.com. I would be happy to share what I have with you or others.

Thanks for your reply, Dustin. I do not think we've been very diligent in updating the ROI on the VRT to edit out any parts of the mask that may be caught in the transfer, so we will definitely try that. That's also a great recommendation about having them remote in during a treatment, I believe we will try that as well.

Thanks again.

Thanks for your reply, Mike. I will definitely be inquiring with Varian when we can expect the ACO to come out, that sounds like it may help tremendously. As for your white papers, I'd be very interested in your pre-ACO paper on testing. I'll email you separately about that.

Pertaining to your experience with Qfix users, have you also seen that they've had difficulty getting the chin in neutral/downward position? The system only comes with one headrest, and the size and shape encourages patients to tilt the head back so that the chin is up. We've actually been trying to come up with a good solution for this for a few weeks now, as another user I spoke with mentioned the importance of lowering the chin. I'd love to hear if you or anyone else has suggestions for this.

Thanks again.

Hi Mike, these are all great tips and certainly could only be gained through experience and time in testing. Thank you for sharing.

We are about to treat our first patient this week and the physics team has been asked to provide tolerances for re-imaging. I have provided values based on what was achievable with an anthropomorphic phantom (0.5mm translations and 0.3 degree rotations), stating clearly that clinical tolerances should be loosen.

Would something like what Kileigh is using (1mm, 0.5 degree) be a realistic starting point? If you don't mind me asking, what tolerances are you guys using and how does this compare to the tolerances you had when you first started?

Cheers