To me this seems like patient can not relax her chest wall after BH. This has been noticed to be a common case already with RPM (where baseline may sneak nearer the BH window when the amount of breath holds increases during the treatment, n+ and ablation). With SGRT we see this much better (more deltas visible than only vertical). Some patients may leave their chest wall to tensed position (residual air or patient position has changed) already after one or two BH:s (which seems to be the fact in your case) but usually they get tired after several BH:s. We have noticed that patient guidance to relax their chest wall to setup position is very important in several points of entire workflow to avoid the errors in the images. I think bolus has nothing to do with this, difficult to say exactly.