I suppose you have AlignRT in use. In the multi-iso DIBHs we imaged and treated the first isocenter normally. When it was time to treat the second isocenter, inside treatment room we asked patient to inhale inside first isocenter thresholds, manually did planned LNG couch shifts, stopped monitoring, switched to the second isocenter BH, took the reference capture and allowed patient to breath again. If you are uncertain that what did you or the patient did during described procedure between the isocenters, check from the second isocenter BH DICOM that the small displacements in the RTDs are pretty much similar with your online matched small isocenter corrections done at the first isocenter. If the first isocenter went to DICOM, we only performed planned couch shifts between the isocenters and treated the second isocenter to BH DICOM. I think AlignRT is more accurate than the tattoos also in this case, needs to be studied to say for sure. Thus, you can always acquire AP+LAT images also after treatment for example at the first three fractions to see the position of the arm, th1-2 and the isocenter to verify the realized accuracy and succesful of the workflow for the lymph node area and the breast/chest wall.
Best regards Marko