SGRT Forum

I attended the SBRT virtual presentations yesterday morning and I was surprised to find that in Europe it is common to take a CBCT for breast DIBH cases. I was wondering if anyone in the US is using CBCT for breast DIBH, and if you are, what is the rationale i.e., what are you gaining that you cannot get from orthogonal kVs? Also, do you do a CBCT daily?

We use Mv/Kv orthog set (you can take both of these in one breath on the true beam) then confirm with tangential film. We check to make sure spine and sternum are matching our to ensure correct breath, check the contour of the heart on tangential film and compare the difference in Free Breathe and DiBH ap ssds vs what was planned. We have had much success with this and don't feel the need to CBCT these breast setups.

We just started treating with SGRT, and our first patient is a left chest wall DIBH we are treating with VMAT arcs. We did the setup with FB Contour, and and treating with DIBH. We confirmed the setup with orthogonal images.

Now the physician has found out from another facility, that they do an CBCT for vmat patients, and she is insisting we do an CBCT on this patient as well. Patient also is on breastboard. If we do CBCT and make moves based on the CBCT, and doesn't match the DIBH contours, what happens?

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.

BR Marko