This is breast+lymph nodes breast DIBH, treated with AlignRT. In the Image 1 accuracy in the tangential image ribs and breast looks good in the image acquisition position. Image 2 is exactly the same image as 1. but now I show you vertebra and shoulder joint visible. There is 7 mm offset in the vertebra and shoulder joint in LNG. Situation was systematic based on Offline review. Since this is lymph+ DIBH, tangential image vertebra should be inside 5 mm in LNG. On the same day, we acquired orthogonal images right after that tangential image to see the reason for LNG offset in vertebra. In the orthogonal Image 3 in the acquisition position, there is 7 mm displacement in the spine and shoulder joint in LNG, just like we noticed from open kV-tangential image. When we match those same orthogonal images based on vertebra (Image 4), we see that sternum is 2 mm too dorsally, indicating slightly too shallow BHL. This is also visible in the Image 5, still same tangential image is now matched on vertebra. Finally, we were not able to totally solve this LNG discrepancy between tangential image ribs and vertebra in the upcoming fractions. We had to create setup reference surfaces which led to location where both vertebra and ribs were daily inside 5 mm in LNG in tangential image, so we created setup surfaces which in practice led to acceptable accuracy in both supraclavicular- and tangential fields.
The purpose of this case is to show, that matching location in orthogonal images which leads to good accuracy in the tangential image may sometimes be surprisingly individual, like in this case and therefore we need a combination of AP+LAT+tangential during the first fractions until the systematic error in the images is solved. On the other hand I wanted to show that relying only on good tangential image ribs accuracy (Image 1) may lead to 7 mm systematic uncertainties in the lymph node area, or would we accept the accuracy in the Image 3? Third thing to say is that with daily open kV-tangential image, used for quick verification only (Image 2), we were able to detect the 7 mm offset in the lymph node (vertebra and shoulder joint) LNG and get even some tips that the BHL is too shallow.
Have a good week at work