SGRT Forum

We have recently purchased the Vision RT system with the head adjuster at our center. I am in the process of commissioning the system and I am using a Styrofoam head with a thermoplastic mask into which I have inserted the Varian morning QA cube so that I have a target for CBCT. After acquiring the CBCT, we must perform the rotational corrections calculated by the OBI at the head adjuster. In order to do so, we need to add or subtract the calculated rotational shift from the RTD shown on Align RT. This would be straight forward if the RTDs were stable, which they are not. They have a tendency to fluctuate by up to +/- 0.3 degrees, which is of not much consequence for the Styrofoam head patient, but could be for some of the patients we will be treating for multiple metastases with a single isocenter.</span>

I need to know if the instability is an artifact from using a Styrofoam white head. Since I have no way to test this system with a real patient yet, I would like to obtain feedback from those centers that are.

How unstable are the RTDs you are observing for our brain SRS/SRT patients with open masks, particularly regarding rotations? Do you ever get your RTDs to become stable, or do they keep flickering between two or more values?



Thank you

Jose C Pichardo, Ph.D, DABR

Miami Beach, FL

We typically don't see this type of variation in the rotational deltas with the variability in output typically sub 0.1 degree.

A few things to look into…

It may be due to the phantom surface but it would be very difficult to say without seeing the raw sensor data. You can look at the sensor data in the phantom patient folder and see if there are large over exposed (i.e. hot white spots) in the image which may indicate you need to have the apertures readjusted and the histograms rebalanced.

You can’t test the system with a real patient but you or a volunteer may lay on the table in a mask and evaluate the RTD exports for monitoring that person for say 5 minutes and compare the stats of that person to the phantom to make sure the phantom is a proper representation of what you will see on the patients you will treat (please use a reference capture and not a CT of the volunteer and please don’t image them during the process…I only say something because people have asked me). If the RTD stats differ between the volunteer and the phantom you can try adjusting some of your exposure settings for the surface settings in the software, change the phantom complexion (using masking tape works well) or call service to look into the camera settings.

If you place isocenters all over the cranial vault and monitor you should see some spread of the noise in the signal which I have always attributed to the decrease in focus of the speckle patter away from some optimal point but again it is always sub 0.1 degree for my systems and others I have commissioned in the past. I have seen it around 0.1 on a one or two of them.

Procedurally, you want to be careful to stand off the head of the table with you back to the gantry while adjusting the head adjuster so that you aren't shadowing the ROI with your body or arm while adjusting the head plate. The surface for an SRS in high resolution and a larger ROI combined with the higher resolution surface will produce some "jumpier" deltas if you are obstructing large sections of the ROI while adjusting it.

Hope some of that gets you down the road to finding the solution

Best Regards,
Mike