ESTRO 2020 Abstract Book

S187 ESTRO 2020

identify FM positions. In the second automatic step, the six voxels with the lowest grey value within the areas were determined and burned-in on the pCT. Slice-thickness of CT and mDIXON was 2.5 mm, in-plane resolution 1.7x1.7x2.5 mm. For this study, 226 CBCTs were retrospectively registered (Figure 1) on the pCT with burned-in FM. These registration results were compared to the 226 clinical CBCT to CT registrations to evaluate the burned-in FM position verification accuracy. For a proper comparison the different patient position on pCT and CT need to be taken into account. We corrected CBCT-pCT registrations with an FM based registration between CT and pCT. The residual error after this correction was caused by prostate deformation and inaccuracies in determining center-of-mass (COM) positions on the different modalities, and is quantified by calculating the root mean square (RMS) of the components of the difference vector of the COM positions of all markers. After correcting for the different patient position on pCT and CT, for each strategy an average and standard deviation in translations and rotations of the FM match on the available CBCTs was calculated. These were used to determine population mean, systematic, and random error and compared to each other. Wilcoxon Signed Ranks Test was used to indicate the significantly difference of the two strategies.

Conclusion The use of an MR-only workflow for prostate cancer radiotherapy significantly reduces the inter-observer registration error compared to a CT/MR-based workflow due to reduced times between acquisitions. OC-0350 Registration accuracy of pseudoCT with burned-in markers with CBCT for MR-only prostate radiotherapy K.N. Goudschaal 1 , J. Visser 1 , F. Beeksma 1 , M. Boon 1 , M. Bijveld 2 , K. Hinnen 1 , Z. Van Kesteren 1 1 Amsterdam UMC- University of Amsterdam, Radiation Oncology, Amsterdam, The Netherlands ; 2 Catharina hospital, Radiation Oncology, Eindhoven, The Netherlands Purpose or Objective The benefit of an MR-only workflow compared to the current CT-based workflow for prostate radiotherapy, is elimination of errors introduced by registrations between CT and MR. Nowadays commercial software is available to create a pseudo-CT (pCT) based on MR that can be used as a reference for both treatment planning and IGRT. IGRT based on registration of fiducial markers (FM) on CBCT is not straightforward for an MR-only workflow, because FM are not visible on the pCT. Therefore, a semi-automatic method was created at our department for burning in FM on the pCT. The aim of this study was to compare the accuracy of FM position verification between the current CT/MR workflow and the MR-only workflow using burned-in FM. Material and Methods Ten prostate cancer patients who underwent radiation treatment with curative intent, gave informed consent for scanning an additional MR sequence (mDIXON FFE) to create a pCT. Four 1x5mm gold FM were implanted. In the first manual step of the semi-automatic method, the four areas containing a FM were delineated on the water- weighted reconstruction of the mDIXON FFE with a 2 mm margin. A dedicated BTFE SPAIR MR was used to help

Results The group mean, systematic and random errors did not differ significantly from each other (p> 0.05) (Table 1). The residual error of the FM registration between CT and pCT was below 1 mm, the largest component is in the slice direction.

Conclusion The accuracy of IGRT in our MR-only workflow using burned-in FM is comparable to our current CT-based workflow and therefore pCT with burned-in FM is suitable as a reference image for IGRT using CBCT in clinical use.