ESTRO 2021 Abstract Book

S1660

ESTRO 2021

and the same was used for the pre-RT baseline. Results

In this preliminary descriptive analysis,it was observed that: in the head-neck district the GTVrifPET was volumetrically lower than the GTV3T (68%). GTV were used by the radiotherapist in defining the CTV.

Conclusion In this preliminary analysis, it has been demonstrated the contribution of the PET-CT method for the contouring of the volumes of interest. This improvement in the definition of GTV leads to an optimization and a clear change in the definition not only of the GTV, but especially of other target volumes such as CTV and Planning Target Volume (PTV)for the radiotherapist. It is also important for a more adequate planning in the distribution of dose, also for the sparing of organs at risk, placed in proximity to target volumes. GTVrifPET and GTV 3T are not always overlapping. Both are useful in a more accurate definition of target and CTV. PO-1949 Image registration guidelines impact on radiographer inter-observer variability for cervical cancer R. Benson 1 , J. Rodgers 1 , C. Nelder 1 , A. Clough 1 , J. Parker 1 , L. Davies 1 , R. Bailey 1 , J. McMahon 1 , H. Kolbe 1 , E. Pitt 1 , B. Dobby 1 , A. Cree 2 , M. Dubec 3 , A. Choudhury 4,5 , C. Eccles 1,5 1 The Christie NHS Foundation Trust, Radiotherapy, Manchester, United Kingdom; 2 The Clatterbridge Cancer Centre NHS Foundation Trust, Clinical Oncology, Liverpool, United Kingdom; 3 The Christie NHS Foundation Trust, Medical Physics and Engineering, Manchester, United Kingdom; 4 The Christie NHS Foundation Trust, Clinical Oncology, Manchester, United Kingdom; 5 University of Manchester, Division of Cancer Sciences, Faculty of Biology, Medicine and Health, Manchester, United Kingdom Purpose or Objective This group has previously reported the impact of an educational tool on MR-CT inter-observer image registration variation using a cohort of 4 radiographers (RT I ). The introduction of an MR-MR workflow required an extension of this work to evaluate the impact of experience with MR guided Radiotherapy (MRgRT) compared with the use of an image registration guide across three imaging modalities, CBCT-CT, MR-CT and MR-MR. Materials and Methods Eight radiographers participated and were divided into two cohorts, 4 with experience in MRgRT and Monaco planning system (RT MR ), and 4 with limited MRgRT and Monaco experience (RT CT ). Observers completed offline retrospective image registration in Monaco (v5.11.02) for 10 patients with cervical cancer including 3 CBCT-CT, 3 MR-CT and 2 MR-MR