I have an example about pitch from this week with AlignRT DIBH. There was no need to do any small adjustments to couch in LNG and LAT at the time patient was at BH in BH DICOM and arm position was perfectly aligned in the treatment capture in the first two fractions. AlignRT guided the isocenter near sternum and shoulder joint based on AP+LAT images, leading to good accuracy in the tangential image. In LNG direction there was 7 mm residual displacement error to vertebra, indicating pitch. For the third fraction offline review (OLR) evaluation was done. After matching the images in OLR to vertebra, both sternum and shoulder joint were 7 mm too cranially as was also the soft tissue in the tangential image in both two first fractions. This ment that patient should relax her arms and chest wall in LNG direction. On the third fraction we asked her to do so and slightly lowered the arms and pressed the chest wall towards her knees. We went with the same couch vertical and with the same FB and BH surfaces (with tight thresholds of +- 1mm/°, vrt +-3 mm for FB surface setup and +-2mm vrt,+-3mm/° in other directions for BH surface) as former days and patient fell into her thresholds well again. Now there has been only minor displacement betwen the named structures at the AP+LAT or tangential setup images after FB- setup correction was done based on OLR, so possible systematic 7 mm residual error for the vertebra in SI was then mostly corrected. My point was that it is possible that the patient falls into BH DICOM thresholds with excellent accuracy, tangential images are prime but still there can be more than 5 mm systematic error in the spine in SI for the reason of certain kind of pitch. First question is that how much do we finally have to care about that possibility and how willing we are to hunt that systematic error? Second question is that can you beat the displacement errors between vertebra and sternum in SI (one aspect of pitch) in the images entirely even with the SGRT without offline review? Most of our DIBHs goes well, but based on results so far I can not be sure that the realization of the lymph node area goes that well that we could go to weekly IGRT protocoll and leave fractions without imaging in WBLN and mastectomy DIBHs. This pitch error is very diverse and patient-specific/individual topic particularly in DIBH and therefore maybe none of us can not give exact answer to solve it entirely. We just bring different kind of aspects from different kind of units to this forum. Personally I consider successful CT workflow more important in this issue than the modern fixations itself, for the reason that I have get that often identical results for the structure displacements near PTV and isocenter variation at clinical studies for the breast patients with different kind of (test) fixation devices.
See: Optimization of whole breast irradiation setup: comparison between two different positioning systems,E. Sanfilippo, 2016.
Best regards Marko