ESTRO 2021 Abstract Book

S1663

ESTRO 2021

Clinical Target Volume (CTV) was also analysed in relation to GTV. The variation of these volumes was analyzed in a descriptive way according to the definition of volume and percentage changes. In the assessment of GTV 3T , the following sequence was taken into account: - brain district, DWI with 7 different intensity of diffusion gradients (b-value = 0, 50, 100, 150, 200, 400, 1000 s/mm2). At the end of the treatment, patient underwent clinical and instrumental examinations according to internal PDTA and national/international guidelines. MRI was performed using multichannel phased-array coils, according to tumor location and the same was used for the pre-RT baseline. Results In this preliminary descriptive analysis,it was observed that: in the brain district the GTVrifPET was volumetrically higher than the GTV 3T (55.88%). 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-1952 Quantification of MRI distortion in patients with metallic hip prosthesis: a feasibility study L. McDaid 1 , A. Clough 1 , R. Benson 1 , C. Nelder 1 , J. McMahon 1 , M. Aznar 2 , A. Choudhury 1 , S. Jackson 3 , C. Eccles 1 1 The Christie NHS Foundation Trust, Radiotherapy, Manchester, United Kingdom; 2 The University of Manchester, Centre for Radiotherapy Related Research, Manchester, United Kingdom; 3 The Christie NHS Foundation Trust, Christie Medical Physics and Engineering, Manchester, United Kingdom Purpose or Objective The presence of metallic hip prostheses will cause distortion in MR images that may impact dose calculation and treatment delivery on the MR Linac. The aim of this feasibility study is to quantify perceived visual distortion on MR images of patients with both prostate cancer and hip prosthesis. Materials and Methods Four patients undergoing radical prostate radiotherapy between October 2019 and February 2020 were recruited to an REB approved study. All patients underwent standard of care planning CT (pCT) and MRI on a 1.5 T MR Linac (Elekta, Sweden). Three T2-weighted turbo spin echo 3D volume (T2 TSE 3D) MRI sequences were acquired, of which two were