Michael Tallhamer, MSC, DABR
At our institutions we always setup using the DICOM for setup unless there is some type of systematic offset from sim to treatment that we see during our initial 3 days of imaging. If an offset is detected we would take a new VRT reference image for all future setups once approved by the physician. There are a number of items one needs to understand when using the DICOM surface to setup a patient many of which are based on how that surface is generated in you TPS and by what process you have validated it as appropriate for setup.
That being said if you are using an acquired reference for setup based on your approved first day imaging using a CBCT or kV/kV image pair you will have that reference throughout the rest of treatment for setup if you isolate it to a setup field in the current version of the software. If you do not the reference you take each day for monitoring the intrafraction motion will override it as the most recent reference surface.
When we import patients into AlignRT we get the ISOCENTER FIELD by default which we call the setup field. This field has the DICOM RT surface imported into it and the starting ROI. We also create a separate empty field called the MONITORING FIELD which will be the field under which we capture the daily reference images for monitoring intrafraction motion of the patient. The MONITORING FIELD doesn’t have to be empty, you can import the DICOM and ROI into that field as well but it can cause confusion at the machine if the therapists are not careful. If the field is left empty the result is a need to draw the ROI on the fly the first day of monitoring as there will not be a predefined ROI under that field to propagate on the first day.
If you import patients in this fashion you can save the reference surface you take on the first day after imaging under the ISOCENTER FIELD with the original DICOM surface for use in setup while taking all other reference surfaces under the MONITORING FIELD to simple monitor daily intrafraction motion. This way the “approved” VRT reference surface from the first day remains available each day for setup purposes.
There is currently no way I’m aware of to “replace” the DICOM surface with a reference surface taken at the time of sim for the first day setup.