Our policy in VMAT DIBH breast RT is to acquire daily orthogonal + tangential images. CBCT is taken at least once during the first three fractions to verify the diaphragm level and to ensure correct soft tissue accuracy more detailed than based on daily tangential images. There is 7 mm line drawn as an action level outside soft tissue in tangential image (DRR) and if it is exceeded, we acquire CBCT for more detailed analysis when needed. With AlignRT user can see the breast position and posture already at the setup with deformation workspace https://www.sciencedirect.com/science/a ... 2421000457
or with (additional) breast ROI https://journals.viamedica.pl/rpor/article/view/75829
or with video feature.
When we acquire CBCT, the workflow with the TrueBeam is following: we make sure that gantry is at zero and imagers are not blocking the cameras or in the danger of collision with the couch, then we say to patient that “next the couch is moving during your BH, but keep your BH all the time”, then we activate the center couch, ask the patient to BH to current BH surface, then we do center couch, acquire new BH surface and let patient breathe again. Then we acquire full thorax CBCT and let patient breathe in a halfway. If there is a need to do small couch shifts as there typically is, those are applied in BH, then the couch is moving back to correct isocenter and finally there is a need to acquire new BH surface for the treatment and let patient breathe again. CBCT is in our case typically acquired after treatment for offline evaluation, but if it is taken before treatment, I would take large sized tangential kV-image before treatment fields just to make approximately sure that BH level and thereby chest wall and spine are in their correct location. This is because we have done relatively large couch shift patient in BH without informative monitoring and without knowing exactly what patient has done during that time. One possibility to ensure the succeeding of the BH during center couch and restoring it (the process of CBCT in DIBH), is to pop in to original FB setup surface to check that are those deltas nearly as much out of zero as we shifted the couch based on CBCT match, since this is how those should be. However, we have to take into account that patient baseline can be additionally slightly changed. With CBCT in DIBH, we finally make copy out of copy concerning BH surfaces and it may cause inaccuracies and we need to be sure what we are doing, we need co- operative patient which is not changing the BH when BH window at the monitor moves while couch is moving (same challenge with small couch shifts in VRT direction in BH without CBCT) and we need to brief the patient what will happen. I suppose, there are hospitals which are doing these CBCT shifts in a daily basis with excellent accuracy.