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I continue this VMAT DIBH bolus case with one more reply. One possibility is to setup the patient inside treatment room as said earlier that is to create “bolus+treatment” BH reference surfaces for treatment, but acquire only CBCT with TrueBeam. For that we need once again additional field at AlignRT, so finally we have four fields for 1-2)FB+BH setup without bolus, 3) for treatment with bolus and 4) for CBCT. Full arc CBCT is for example taken accordingly: once the patient is in BH (with 1mm/1° accuracy) in first isocenter BH reference surface (we name it “bolus+treatment” field in AlignRT), we perform center couch, acquire new reference surface under AlignRTs “CBCT field “and let the patient breath again. ROI must draw on the fly. CBCT is performed to that reference surface. If there is not need for small couch shifts based on that CBCT, we can (relax), restore couch and treat the patients with BH- reference surfaces that we created before imaging with bolus.
If there is need for couch shifts which exceed IGRT tolerances based on that CBCT, we are in the middle of challenges again. We need to acquire new BH reference surface in “CBCT field” after those small isocenter shifts. Then we should ask patient again to BH to that acquired BH reference surface and restore couch, patient in BH, and acquire new reference surface for the treatment in “bolus+treatment” field and let patient breath again. For the second isocenter we perform planned LAT shift, patient in BH and acquire new reference surface for the treatment, with the workflow as written in the previous reply, since the reference surface which was created before CBCT for that isocenter was not ok. Tangential image before treatment may be recommendable before treatment now from that side. Now we have updated BH reference surfaces for the treatment on both sides after isocenter corrections which exceeded IGRT tolerances based on CBCT.
Since the reference surface is taken under “bolus+treatment” field daily, there is a need to modify that ROI weekly to keep its original size and shape.
There may be other ways to do this two-isocenter VMAT DIBH with bolus, but as you assumed in some cases there is challenges and you may need to consider several things.