…the purpose of the bony match to ribs in the tangential image is addition to verify the named radiated lung and heart area accuracy of course for its part to verify the target accuracy itself…
With MV tangetials I would shift only LNG direction and only in the group of n0 patients treated with opposing tangential fields. In the MV tangential images I would use planned field size and thereby only ribs are visible. To open MV tangential field and daily acquire such images increases heart dose, and that is what I would not do. With the n+ patients I would not perform the couch shifts at all if there is needs for couch shifts in tangential MV image ribs in LNG, because lymph node area is not visible. When you have more than 5 mm error in the ribs in that group and you use MV tangential images, I think you need to acquire AP+LAT images before treatment and verify that the named LNG shift is adequate in the lymph node area also.
In the n0 group with opposing tangential fields I would shift the couch based on tangential image online match in LNG if the residual error in that direction is between 5mm-1cm and there is only 0-3 mm error in the ribs in AP/LAT directions. If the error in the ribs is larger than 1 cm in LNG or larger than 3 mm in LAT/VRT, I would acquire AP+LAT images and do the couch shifts based on those images before treatment. If the 5mm- 1cm error in the ribs exists mostly in LNG and AP/LAT error is less than 3 mm in the tangential images in n0 group, I choose only LNG active for the couch shifts and do not choose vertical or lateral value active for applying the couch shifts or leave them zero in the online match when couch is not moving in those directions. So ask patient to BH to the surface used in the imaging, which led to 5mm-1cm LNG error in the tangential image, apply only LNG couch shift, acquire new surface for the treatment and let patient breathe again. Acquire new tangential MV image with the new surface to see is the image ok now. I would certainly not use that BH surface tomorrow. I would go to record mode to approve former BH surface active and start with that and AP+LAT images on the next day. This kind of possibility to have to shift the couch because the tangetial image ribs are outside of the 5 mm AL in LNG is possible in 7% of the fractions in our latest n0 data without preceding orthogonal images and couch shifts, with AlignRT setup only. Not often thus.
Best regards Marko