We also verify the BH- level with tangential images and check the heart position after three first fraction in n0 DIBH treatments. We have found that tangential image may give false positive signals for BH- correction, because of daily position variation of the heart and poor image quality to check the real position of the heart. Additionally not much corrections can be done based on tangential images. Therefore we check and when needed correct the BH- level based on the information of displacement error between vertebra and sternum (VRT,LNG) at the beginning of the treatment and if systematic errors in the heart position is seen in the tangential images during the treatment. Problematic cases are nowadays rare, but the workflow should be known if the heart is systematically nearer the treatment field in tangential image and BHL is too shallow in LAT image. I think these problematic cases should be taken more into account already at the AlignRT training. If anyone is acquiring AP+LAT images more than once during the treatment in DIBH, I am interested in to compare the feedback and results with ours. Please e-mail, if you are.
Best regards Marko