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

I did some additional calculations for this topic today, since I feel this interesting and important in practice. I calculated 48 DIBH- patients first fractions sternum VRT location in image acquisition position from the time BODY structure was used, with -350 HU value. The 48 different DIBH patients first fraction sternum VRT errors in LAT image, in the image acquisition position, with +- 2 mm AlignRT BH threshold in VRT were (mm): -2.4, -3.1, -1.9, -0.5, -2.2, -1, 0.9, -1.1, -1.4, 0, 3.2, -0.9, -4.1, 1.3, -5.4, 0, -4.6, -2.9, 6.1, 0, -0.3, -0.8, -0.5, -2.6, 0.5, 0.2, -1.3, 2.2, -1.1, 1.1, -4.2, -4.1, -0.8, -3.1, 1.1, -2.4, -1.3, 0.8, -2.2, -1.9, -3.1, -1.1, -1, -0.6, -4.2, -2.1, 4.7, -1.6. The group mean for those first fraction values is -1 mm. 71% of all the values are in minus, sternum in the images too dorsally compared to DRR. 23% of all the values are positive, sternum too ventrally. 40% of all the values are larger than 2 mm, indicating that DICOM needs to be replaced with new reference surface that often, due to unsuccessful VRT in the images, if the IGRT action level for sternum VRT error is 2mm. Corresponding percentage for 3 mm or larger sternum VRT displacement was 23% (8/11 of these values are minus). If I add in Excel 1 mm to those each individual values listed above (indicating 1 mm margin outside BODY= SKIN= BODY+1mm), 31% of all the values are still larger than 2 mm, even though the group mean is 0. 3 mm or larger sternum displacement exists with BODY+ 1 mm in 21% of the cases (6/10 of these values are minus). If the BH window is +-2 mm, that provides for its part an explanation for 0-2 mm errors in sternum VRT. With BODY+1mm sternum sternum VRT error sign is in minus in 52% of the cases and in plus in 44% of the cases. With this 1 mm margin outside the BODY, we can only slightly drop down the needs to acquire new setup reference surfaces for the setup. Our physicists noticed that there is a possibility to adjust DICOM isocenter in VRT,LNG and LAT in the AlignRT workstation offline, if we want to keep DICOM in the setup, in the cases where we notice small systematic isocenter errors in the images. Have you taken that into consideration? We have found it easier to take a new reference surfaces for the setup, at the time there is a need to correct systematic error. I have handled only VRT in my answer, since this question has most to do with the VRT I suppose. On the other hand, there is also different issues which leads us to give up DICOM surface, such as systematic errors in the images concerning rotations, individual structure such as shoulder joint etc. I think that there is not much possibility to know beforehand, which ones will be those “3 mm or larger cases or problematic cases” and forecast that with adjusting the HU- values. Isocenter/monthly calibration is one issue, which may shift this accuracy with 1 mm and some may think that this is our issue. It is difficult to believe that this is our issue, that long time I have had this mean error in the results. With BODY+1mm we can go on with DICOM in about 80% of the cases concerning sternum VRT in DIBH (with our 3 mm AL for sternum VRT) and I think that is good result, taking consideration how much can happen in between the CT and first treatment fraction in the patient anatomy for example. I hope there was something useful for you also.