It is much possible that with the fixation without tilt (wingboard) there is a possibility that with “averaged couch vertical instead of FB surface vertical”, wingboard is not so sensitive to cause pitch errors/BH level errors in the images as tilted breast board at the situations where patient is in wrong position at the fixation in SI direction without remarkable posture errors. This is because in those situations the vertebra location with wingboard in vertical direction does not change. The advantage of tilted board is for example that the center pod covers larger area of the chest wall in SI direction.
When comparing the workflows between averaged couch vertical and FB surface vertical, the advantage of averaged couch vertical is in the situations when at the FB setup patient chest wall/sternum position in vertical direction is for reason or other as a matter of fact nearer the BH surface than planned. This is what we see in the FB breast images sometimes, please check out AlignRT results in the SGRT forum. With the AlignRT we see this kind of situation between the BHs if patient normal breathing has not recovered back to original values. This knowledge is significant at the CT workflow or at the time FB setup reference is acquired. This kind of situation is possible also with the DIBH at the FB setup and in those cases setup based on surface vertical does not lead to optimal BHL in the images. I think these errors are still mostly systematical and possible to correct at the beginning of the treatment. The advantage of using surface vertical instead of averaged couch vertical is that with both fixation devices it does not matter if the patient is slightly at the wrong position at the fixation if there is no significance posture errors.
Challenges working with the both setup methods are still the situations when at the FB setup there is more than 3 mm displacement in the FB surface vertical or couch vertical. Is it for example because of swollen area on the ROI or is it because patient is at wrong location in the SI- direction on the tilted fixation or should we still setup the patient arms or chest wall…? So far this exceeding of 3 mm threshold in FB surface vertical with our averaged couch vertical setup workflow needs AP+LAT images. Fortunately these situations are rather rare. The optimal case is of course that after good surfaces for FB and BH setup are saved, neither acquired couch vertical or FB surface vertical does not vary at the daily setup.
Lastly there was VMAT DIBH AlignRT patient at this week, whose treatment is at 10th fraction now. We took CBCT some days ago and noticed that there is 7 mm swelling in the entire breast. With the upside down T- ROI patient setup has been accurate now in FB and in DIBH and with whole breast ROI we see that 5-7 mm swelling with the AlignRT also. So in this case additional ROI for the soft tissue gives exactly the same information as CBCT and daily verification is not needed with CBCT. Theoretical, the more our FB setup ROI includes that kind of swelling soft tissue, the more we could have displacement error to the sternum in the isocenter vertical and possibilities to fail in the bony chest wall setup with the roll increases.
If you are interested in to study these things together and get a longer aswer, please send me an e-mail.
Best regards Marko