Reply To: DIBH LNG findings

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Marko Laaksomaa

…Some additional thoughts about DIBH LNG. One thing that we used to do earlier with AlignRT DIBH was to do small couch adjustments in LNG (and LAT) at time patient takes a BH at the setup to reach zero errors in deltas in BH. This workflow includes a risk that we shift vertebra out of its correct location in LNG if we trust the patient and her ability to reproduce the daily BH at the setup. One improvement that we have done in n+ and mastectomy DIBH to keep the eye on the vertebra accuracy in LNG (and thereby going to more safely to tangential images only + weekly AP+LAT workflow) has been to write down the BH amplitude values on the paper at the treatment room. We have a list of DIBH patients inside treatment room, to know the correct chest wall movement in VRT+LNG in DIBH. Then at the time we setup the patient on the first fractions, we write down both vertical and longitudinal delta movement between FB and BH surfaces from the fractions when images have been ok (BHL, isocenter etc.). This LNG value on the list tries to demonstrate the correct vertebrae location in LNG at the setup. At the setup, to optimize accuracy in the vertical direction, we use the planned couch vertical (vertebrae matched couch value) to keep the vertebra location daily correct and we use as well AlignRT FB surface vertical (zero error), to keep the BH amplitude correct. If there is a need to acquire new reference surfaces for the setup: the more patient has taken BHs before we capture the references for the setup, the more there is a risk that those reference surfaces will fail for the next day setup (in the images). These are some of improvements, which have led to clear improvements in the AlignRT DIBH- setup accuracy.

If we do small couch shifts based on how patient takes a BH in LNG without understanding what we do, there is a risk that we will shift the vertebra out of its correct location in that direction. For the chest wall accuracy this thing has not much to do with. To say it exaggerated, it seems possible to accidentally setup DIBH patient in FB with BH field, all the BH field deltas inside thresholds and in the tangential image MID_PTV chest wall accuracy may seem good enough, if vertebrae is not visible in the images and only lateral part of the ribs and breast is visible. This chest wall accuracy is what we have known and noticed to be excellent in the first place with AlignRT due to predefined BH- reference surface. The challenge in the workflow has been to improve the accuracy to the vertebrae and arm position (lymph node area) with DIBH.

This is an example what sometimes happens with DIBH: we have reference BH amplitude at the list (chest wall delta movement between FB and BH based on the accurate reference surfaces, which have led to accurate posture and isocenter position in the images) in VRT -1.0 cm and in LNG 1.0 cm. Next we setup patient to FB with the AlignRT accuracy of 1mm/1° and with plan couch vertical. Then we start monitoring BH field and for reason or other deltas are now  -1.0 cm in vertical (as it should be) and 0.5 cm in LNG (as it should not be). What we nowadays do is that we manually shift the couch/patient in LNG 0.5 cm caudally (to reach 1.0 cm LNG delta value at the BH field) before patient takes any BHs (now on the background FB isocenter goes with 5 mm offset in LNG). Usually with this workflow deltas goes to zero at the time patient takes a BH and vertebra locates in the images with better accuracy than without the shift based on the list. If we do not do this small couch shift in LNG to correspond the value at the list before patient takes the first BH at the setup, it is much possible that she will do BH similarly anyway and such that we will finally shift the couch that same amount (5 mm) in LNG, patient being in BH (which was the case earlier). Anyway, we should be somehow wiser than the patient and not let her do anything she wants in LNG and follow that with the couch shifts at the BH setup with a risk that we will shift the vertebrae out of its correct location in LNG.

Some days ago there was a patient, whose BH amplitude at the list was -1 cm VRT and +1 cm LNG. For reason or other she did a breath hold to delta -1 cm LNG (too cranially) starting from those named values. In the images vertebra location was ok in LNG, but sternum location in the images was 1 cm too cranially (as supposed). At the setup we did not trust the patient and her ability to reproduce the BH, we trusted the 1) AlignRT and 2) value on the list. We finally asked patient not to move her chest wall in LNG that much at the BH and she changed the way to take a BH kindly without couch shifts and tangential image was ok. We have been able to acquire kV- tangential images for a year now without problems. We have not used MV tangential images for a year.

Some days ago there was another problematic DIBH patient, which was imaged with tangential image before treatment and there was everything ok in the images (vertebrae and chest wall accuracy good). During the treatment, LNG delta went to -1cm in BH, too cranially, and we imaged tangential image at that point in the middle of fraction. Indeed, there was 0.8 cm need to shift the couch in LNG, based on chest wall match. Since we earlier during the fraction tried to guide the patient how to do BH in LNG direction without any improvements, we decided to shift the couch in LNG, based on chest wall and accepted the 8 mm weaker accuracy in the lymph node area. The rest of the fraction went well with excellent accuracy on the chest wall, but in theory with 8 mm weaker accuracy in the lymph node area.

Finally about the DIBH setup, if we have good FB and BH reference surfaces taken from CT or taken based on image guidance (and plan vertical value), the BH amplitude and thereby vertebrae location should remain daily the same in both vertical and longitudinal directions without the need for any small couch corrections (LNG, LAT) at the FB/BH setup or without any lists, which mostly is the case in our hospital also.