Forums SGRT – Best Practice Determination of HU threshold to generate the DICOM reference surface Reply To: Determination of HU threshold to generate the DICOM reference surface

Marko Laaksomaa

Hi Jose
If you use FB and BH- surfaces in DIBH setup which are generated at the CT (without daily the same couch value in vertical at the setup), HU- values should not have any effect on BHL amplitude itself, neither the BHL you see in AlignRT from FB to BH surface, nor BHL in the LAT image (vertebra vs. sternum). This is because I suppose the same named possible error in the BODY exists in both FB and BH reference surfaces if the BODY is created with the same HU- values in both surfaces. Then in the setup, from zero VRT error in FB to zero VRT error in BH, the BHL itself should be ok. For the sternum VRT accuracy, and thereby PTV VRT isocenter accuracy, this thing may have significance and is therefore important anyhow. Additionally, if in the DIBH, you use averaged (daily the same) couch value in the setup, and acquire FB reference surface during the first fractions like we do, also BHL with described unsuccesful BODY in BH DICOM is finally slightly different than planned. Then there is a need to acquire also new DIBH reference surface for the setup. With -350 HU, which is use in all the treatment sites here, I have noticed -1.5 mm group average error in the sternum VRT in DIBH DICOM, with +- 2 mm BHL window in VRT. Sternum remains too dorsally in the DIBH kV- LAT images. The case is the same in other sites also concerning bony structures. All the group averages in couch shifts in VRT in the IGRT studies are in minus, indicating the need to raise the couch to meet the DRR bony structures. This knowledge is not that significant in the treatments where we shift the couch VRT based on online image match daily, and daily acquire new reference surfaces for the treatment. As you mentioned in DIBH for example, this may have significance, if we daily setup the patient with such DICOM, which leads to errors in sternum VRT in the acquisition position and we do not correct the residual error with image guidance. Small errors are then possible in treatment, if we do not ask patient to take more or less air as much as the sternum VRT error is, or if we do not shift the couch in VRT, patient in BH. This small systematic residual error in sternum VRT is not necessarily visible in the tangential image ribs in VRT direction. To me it seems like we should create the BODY after scaling down the HU- values that much that the external surface in planning CT begins to be that white that it begins to pixelate and is not that accurate any more. Better solution would be to create external structure 1-2 mm outside the BODY. In the FB cases it should be taken into account that patients may relax right after setup and therefore that is the reason for need to raise the couch VRT based on image guidance, and it is possible that AlignRT DICOM surface finally agrees with the couch shifts in VRT. Therefore, if you want to evaluate this in practice with FB patients, it could be useful to check the AlignRT VRT in FB cases with DICOM after the couch shifts from the large number of patients, in the cases where the online image matching is done based on bony structures near the surface. If the AlignRT VRT is still on average +1.5 mm after the couch shifts is done in online match, before acquiring the new surface for the treatment, you have similar findings with me. KV-setup images and patients are not possible the best way to evaluate these things, because there may be for example pitch in the sternum and it causes uncertainties to select correct matching location, but if the offline data of hundreds of patients shows the same result, it improves the reliability. However, please test this on your own. We are talking about 1- max. 2 mm errors now, but certainly about systematic errors, which should be taken seriously if those exist in the group and user wants to go on with the DICOM without daily couch VRT corrections or even without IGRT, since possible swelling in the ROI area during the treatment may even worsen the accuracy systematically. Once we have the results of group average with SKIN, 1 or 2 mm outside the BODY, I will let you know.

BR Marko