Hello Paul
Interesting and important finding. Relevant questions with individual patients may be that how systematical those diaphragm level displacements are, is there a heart shift towards the treatment field at the time diaphragm has shifted superiorly, does SGRT give you any extra information of wrong type of BH at the time diaphragm has shifted so that you could correct the error with the aid of SGRT? Here is one publication where the writers have among other things investigated the displacements between apex and diaphragm and they also found more than 2 cm errors between those structures in SI direction and additionally demonstrated this findings significance for the heart location: Geometrical uncertainty of heart position in deep-inspiration breath-hold radiotherapy of left-sided breast cancer patients, Koivumäki et al, 2017.
Marko