ESTRO 2021 Abstract Book

S1481

ESTRO 2021

Conclusion This is the first study quantifying the impact of different 4D CT derived reference scans for IGRT. Automatic registration was surprisingly robust independent of the reference image, except when using MIP for tumours close to the diaphragm. Observed biases show it is important to use the same scan for planning and IGRT reference. Future work should include comparison between 4D and 3D analysis methods and relating failure modes to scan parameters, tumour size and amplitude. An optimised workflow for 4D CT processing as IGRT reference should be provided by industry with high priority. PO-1756 Real time motion-including dose reconstruction on the MR-Linac E. Goodwin 1 , R. Nilawar 2 , S. Nill 1 , U. Oelfke 1 1 Institute of Cancer Research, Radiotherapy Physics Modelling, London, United Kingdom; 2 Institute of Cancer Resarch, Radiotherapy Physics Modelling, London, United Kingdom Purpose or Objective The Unity MR Linac (Elekta AB, Stockholm, Sweden) combines a 1.5 T MR scanner with a radiotherapy linac. The MR images provide excellent soft tissue contrast for daily treatment adaptation. During treatment, planar cine MR images depicting the tumour are acquired at a frequency of 4 Hz. The intrafraction tumour motion observed in the cine images is used to calculate 3D PTV-motion-including