ESTRO 2020 Abstract book

S202 ESTRO 2020

Figure 1A shows the variation between the observers in L- R translation per patient, Figure 1B shows the distribution in L-R translation of seven observers pooled over 20 patients. The other registration parameters showed similar behavior. The results of the statistical tests in Table 1 demonstrate the significantly reduced IOE for the MRCAT- TRA registration compared to the CT-TRA registration. In general, the IOEs of MRCAT-TRA were about three times smaller for both translations and rotations compared to CT-TRA.

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