ESTRO 2021 Abstract Book

S1262

ESTRO 2021

Results Contours propagated by MIM showed higher geometrical accuracy than those propagated by Elastix (see figure). While the DSC values based on the Elastix registrations ranged down to 0.80 for the bladder and 0.73 for the rectum, the DSC values for MIM registrations were significantly higher (see table), and the distribution tighter with values ranging down to 0.93. A similar tendency was observed for the MSD and HD metrics with significantly lower values for MIM than for Elastix. The Elastix registrations were significantly less accurate than the IOV in terms of DSC and MSD, while no significant difference was observed between MIM registrations and IOV.

Conclusion For MRI-based bladder and rectum contours, the MIM DIR algorithm provides higher geometrical accuracy and precision than Elastix and is comparable to the IOV. The algorithm is thus preferable for dose accumulation purposes for such highly deformable organs. PO-1537 Dose accuracy for Nasopharyngeal Carcinoma patients under MR-guided online adaptive radiotherapy S. Ding 1 , H. Liu 1 , B. Wang 2 , Y. Li 1 , B. Liu 1 , X. Huang 1 1 Sun Yat-sen University Cancer Center, Department of Radiation Oncology, Guangzhou, China; 2 Sun Yat-sen University Cancer Center, Department of Radiation Oncology, Guangzhou, China Purpose or Objective MR-guided radiotherapy relies on the daily assignment of a relative electron density (rED) map to acquire the fraction specific dose calculation. The approach to acquire the rED map is used bulk rED assignment based on the rED values of the delineated region of interests (ROIs) to generate a synthetic CT (sCT). This study aims to evaluate the dose accuracy of this approach for application in 1.5 T MR-Linac, to assess the reliability of this