I wanted to correct my answer in the terms of attachment. Based on the discussions with the physicist and onkology, we may at some point use this technique for selected patients. Then we could begin with the VMAT and FB/DIBH. In the literature both VMAT and DIBH are recommended for the APBI (Accelerated partial breast irradiation (APBI): Are breath-hold and volumetric radiation therapy techniques useful? 2014, Essers et al.). I could use DIBH with accurate AlignRT- setup and not that much take care of entire patient posture in the orthogonal images after it has been verified in the first fraction. I could setup the patient with AlignRT normally in FB, with ROI mostly on the rigid surface to achieve accurate patient posture. In these cases I could pay extra attention in setup to get the arm position and soft tissue in good order with additional whole breast- ROI and treatment capture for the arm to avoid the rotation/displacement of the clips. When both ROIs are in tolerances of ±1-2mm/degree, I could do daily IGRT, AP+LAT (or half arc CBCT) to verify the position of clips, then do the couch shifts based on clips match in BH and acquire new reference surface in WB- ROI for the treatment.
In our unit 7 mm margin is used for SIB clips with daily IGRT and clips are drawn on the DRR with that 7 mm margin. Clips should be inside those circles after bony match. In APBI I could concentrate more in clip- based match instead of bony match, based on literature. This makes AlignRT- DIBH easy: trust the SGRT-system and acquire small sized AP+LAT images, exclude out of the field vertebra etc, include clips. If some of the clip(s) is outside of drawn circles, CBCT is needed and ongology evaluates the significance of the displacement. During the treatment intrafractional movement of the clips is possible and AlignRT is excellent tool to follow the soft tissue location. Therefore I could use whole breast- ROI for intrafractional monitoring with the thresholds of ±3 mm/degree and DIBH with described easy setup workflow and for the reason of accurate treatment delivery. Hope someone that is already using this technique also answers, they may have better or other solutions. Have a good week-end.