- Tue Dec 13, 2022 8:40 am #1369
Some words near this topic. In our practice, with whole breast in free breathing FB, it is allowed and with accurate AlignRT very possible to acquire only daily tangential images after three first fractions. Our action level in tangential images is 4 mm in AP/LAT and 5 mm in CC directions for ribs in whole breast RT in FB. Exceeding of those ALs causes the need to acquire orthogonal images, since we do not typically do couch shifts based on tangential images only. However, in practice 3 mm in AP/LAT in tangential images ribs causes the need to acquire orthogonal images and do the couch shifts based on those images. This is because 3 mm isocenter displacement in tangential image ribs (and in sternum in LAT image) in AP looks already quite bad. “Several times” AlignRT shows small AP delta displacement due to relaxation in AP direction at the time of tangential image acquisition in FB. If we acquire such tangential image, we naturally see the needs for couch shifts in AP direction in that image, just like AlignRT shows. What we could do in this case is to perform the couch shifts with send to couch function based on AlignRT and acquire second tangential image for verification, typically image is ok then. The better thing to do in those cases is to perform AlignRT send to couch already just before (first) tangential image acquisition. Then we need to be sure that the deltas are not shifted due to camera blocking and look stabile and reliable. Of course, the same baseline drift challenge is visible in orthogonal images in sternum AP showing systematic needs for couch shifts in AP, due to relaxation between setup and LAT image acquisition and causing the unnecessary needs to continue with orthogonal images, since the problem would have been solved with performing the AlignRTs send to couch just before image acquisition. Typically, we do "send to couch" just before leaving the treatment room, but however there are FB breast patients showing 2 mm displacement in AP delta, at the moment of tangential image acquisition, due to baseline shift. Some of us may however forget to finalize these send to couch corrections (just before leaving the treatment room and before imaging) and relaxation is visible in the images. If the couch shifts are determined to be done based on daily image guidance (orthogonal images, CBCT, AP+tangential image…), this is not an issue. However, I have noticed the baseline drifts that are discussed in the studies mentioned earlier, in setup practice with AlignRT.