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

Now we have 6 DIBH- patients with SKIN DICOM, created 2 mm outside BODY structure, with HU value -350. Based on the first fraction, 2/6 of the patients, BODY would be better in VRT, since sternum located 2 mm too ventrally in the LAT image with SKIN. For those two, new reference BH- surface was acquired. 3/6 of the patients had zero error in the sternum VRT on the first fraction with SKIN. 1/6 of the patients sternum located 1 mm too ventrally, indicating that the structure in between the BODY and SKIN would have been optimal. Group mean is +0.8 mm (sternum needs to be more dorsally), indicating that the 1 mm structure outside BODY would be optimal in general, not 2 mm outside BODY. For 4/6 of the patients DICOM was used during the whole course of treatment. If I take account all the treatment fractions from those patients and evaluate the residual error to the sternum in the images in the acquisition position (with SKIN DICOM), group mean error to the sternum in VRT is only 0.1 mm, indicating that finally SKIN (2 mm outside BODY) is optimal in the group. This is may be due to increased swelling in the ROI area during the whole treatment course. But, this finding will not entirely eliminate the need to acquire new setup surfaces, if we want to eliminate the small systematic errors individually. One possibility would be to setup with SKIN on the first fraction and on the second fraction switch to BODY, if it seems to work better concerning VRT isocenter in the images and if we want to stick with the DICOM and if we are interested in the 1-2 mm systematic errors in VRT. With the +-2 mm BH VRT threshold we need to pay good attention to check where the AlignRT VRT delta is exactly, at the time LAT image was acquired in the online match (patient in BH), if we want to compare that VRT delta reliably to the sternum VRT in the image acquisition position. This is just what I have found here in Tampere. There may be several things in the entire workflow, which affects on VRT accuracy and there is a possibility that this conclusion is not relevant in other hospitals and thereby this kind of finetuning will not work in other hospitals. This is why I suggest you to test these things on your own. On the other hand I suppose you have also found some kind challenges with this topic, since you ask this question.