ESTRO 2020 Abstract Book

S186 ESTRO 2020

four gold fiducials on the water-weighted reconstruction of the mDixon sequence. Seven experienced observers executed two manually registrations based on gold fiducials. Each observer registered the CT, via the BTFE SPAIR to the T2W image (CT-TRA) and the mDixon sequence was registered to the T2W image (MRCAT-TRA). The registration yielded rotations and translations defined with the center of mass of the fiducials as rotation point. The IOE of the CT-TRA registration was compared to the MRCAT-TRA registration on patient and on group level. The IOE was quantified by the inter quartile range (IQR) of the registration results per patient, and by the standard deviation (SD) of the registration results of the group. To be able to pool the data between patients, the mean of the registration values per patient was set to zero. Significance of difference in IOE on patient level was tested using a paired two-sided Wilcoxon signed rank test. A non-parametric Levene’s test was used to test the difference in IOE for the whole group. Results 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 When contours are separated by structure type, models with higher PAs are seen. This suggests the suitability of algorithm model is dependent on structure type. The liver GTV models produced the lowest PAs. This could be due to difficulties in delineating the liver GTV boundaries due to low contrast, resulting in larger variation in outlining and of the scoring of a pass/fail. These models can reduce the time required for benchmark case reviews, but cannot replace a clinical review entirely. Further work is required to refine and test these models before they can be used routinely. OC-0349 Inter-observer variation of image registration in an MR-only versus CT/MRI-based workflow M.A. Boon 1 , J. Visser 1 , F. Beeksma 1 , A.A. Goedhart 2 , M.M.C. Bijveld 3 , K.A. Hinnen 1 , K.N. Goudschaal 1 , Z. Van Kesteren 1 1 Amsterdam UMC, Radiation Oncology, Amsterdam, The Netherlands ; 2 Vrije Universiteit Amsterdam, Medical Natural Sciences, Amsterdam, The Netherlands ; 3 Catharina hospital, Radiation Oncology, Eindhoven, The Netherlands Purpose or Objective In current prostate cancer radiotherapy MRI is used for target definition whilst CT is needed for treatment planning. Co-registration of CT and MRI introduces a systematic error in the treatment, which might be reduced by introducing an MR-only workflow for target definition. In an MR-only workflow an additional sequence is required and registrations between MRI sequences are needed because of inter-sequence movement within the patient. The systematic error can be reduced by removing the time between MRI and CT acquisition. The aim of this study was to quantify the inter-observer registration error (IOE) of a CT/MR-based and an MR-only workflow for prostate cancer radiotherapy. Material and Methods Twenty prostate cancer patients, treated with external beam radiotherapy at our institute, were included in this study after giving informed consent. CT and MRI data were acquired on a RT-couch top in RT position, with less than two hours between the CT and MRI. The MRI scan consisted of a transversal T2-weighted (TRA) image for target definition and a dedicated acquisition for gold fiducial identification (BTFE SPAIR). In addition, as part of this study, an extra (mDixon) scan for pseudo-CT (MRCAT) generation was acquired. It was possible to identify the Proffered Papers: Proffered papers 20: Ensuring precision and accuracy in RT