ESTRO 2022 - Abstract Book

S1557

Abstract book

ESTRO 2022

acquisition time, but only one series is needed. Using EPI, a distortion correction algorithm must be used, which requires the acquisition of two series, lengthening the process. Well-vascularised areas of heterogeneous tumours and small tumours show similar ADC values, greater than poorly vascularised areas. Funded by ISCIII PI17/01735 grant (co-funded by FEDER) and SINFONIA project (EURATOM 2019-2020 under GA No.945196).

PO-1754 Delineation-related ADC variation between centres in the Elekta Unity MR linac Consortium

A. Bisgaard 1 , A. van Lier 2 , C. Coolens 3 , R. Keesman 4 , A. Tree 5 , A. Wetscherek 6 , P. Romesser 7 , N. Tyagi 8 , M. Lo Russo 9 , J. Habrich 10 , D. Vespirini 11 , A. Lau 12 , S. Mook 13 , P. Chung 3 , L. Kerkmeijer 14 , M. Nowee 15 , T. Schytte 16 , E. Lorenzen 1 , C. Brink 1 , U. Van der Heide 17 , F. Mahmood 18 1 Odense University Hospital, Laboratory of Radiation Physics, Department of Oncology, Odense, Denmark; 2 University Medical Centre Utrecht, Department of Radiotherapy, Utrecht, The Netherlands; 3 Princess Margaret Cancer Center, University Health Network, Radiation Medicine Program, Toronto, Canada; 4 Radboud University Medical Centre, Department of Radiation Oncology, Nijmegen, The Netherlands; 5 Department of Radiotherapy, The Institute of Cancer Research and The Royal Marsden NHS Foundation Trust, London, United Kingdom; 6 Joint Department of Physics, The Institute of Cancer Research and The Royal Marsden NHS Foundation Trust, London, United Kingdom; 7 Memorial Sloan Kettering Cancer Center, Department of Radiation Oncology, New York, NY, USA; 8 Memorial Sloan Kettering Cancer Center, Department of Medical Physics, New York, NY, USA; 9 Unversity Hospital and Medical Faculty, Eberhard Karls University, Department of Radiation Oncology, Tübingen, Germany; 10 University Hospital and Medical Faculty, Eberhard Karls University, Section for Biomedical Physics, Department of Radiation Oncology, Tübingen, Germany; 11 Sunnybrook Odette Cancer Center, Department of Radiation Oncology, Toronto, Canada; 12 Sunnybrook Research Institute, Department of Physical Sciences, Toronto, Canada; 13 University Medical Center Utrecht, Department of Radiotherapy, Utrecht, The Netherlands; 14 Radboud University Medical Center, Department of Radiation Oncology , Nijmegen, The Netherlands; 15 The Netherlands Cancer Institute, Department of Radiotherapy, Amsterdam, The Netherlands; 16 Odense University Hospital, Department of Oncology, Odense, Denmark; 17 The Netherlands Cancer Institute, Department of Radiation Oncology, Amsterdam, The Netherlands; 18 Odense University Hospital, Laboratory of Radiation Physics, Department of Oncology, Odense , Denmark Purpose or Objective The apparent diffusion coefficient (ADC), derived from Diffusion-Weighted MRI (DWI), is a promising radiotherapy response biomarker. However, lack of consistency in ADC measurements hampers its translation into clinical use. ADC measurements require delineation of a region of interest (ROI), and both choice of ROI type and inter-/intra-reader variation introduces ADC variation. The aim of this study is to investigate the impact of delineation variation on associated ADC measurements between centres in the Elekta Unity MR-linac consortium utilizing the same patients. Materials and Methods Nine centres in the Elekta Unity MR linac consortium participated in this study using data acquired on MR linac (Unity, Elekta) by one of the participating centers (OUH). MRI scans from four clinical cases were used: adrenal gland metastasis, oligo metastasis in pelvis, pancreatic cancer relapse and prostate cancer. T2-weighted images and repeated DWI images (test-retest) were acquired using b-values of 150 and 500 s/mm 2 . Delineation of GTV and CTV (prostate only) was done for each case by oncologists from the nine centres in a mutually blind manner using ProKnow (Elekta), otherwise adhering to local workflow. A so-called viable tumour volume (VTV), defined as viable tumour (i.e. excluding necrotic and cystic parts), was delineated as well. Deliberately, we did not give detailed delineation guidelines. For each type of ROI, delineations were compared pair-wise (for each permutation), to calculate the median Dice Similarity Coefficient, maximum Hausdorff Distance, and Mean Surface Distance (MSD). ADC values were reported as the median values within each ROI using ADC maps generated on MRL Philips MR console (Release 5.3) at OUH. Retest ADC values were calculated using rigid contour propagation of GTV between test and retest scans. Test-retest ADC variation (based on GTV) was estimated as the difference between the 25 th and 75 th percentile of ADC differences between test and retest scans. Results Delineation variation was different between the four clinical cases and types of ROI (Figure 1). The largest variation was observed for pancreas and prostate (GTV and VTV), possibly due to small volumes of ROI for pancreas, and the fact that GTV and VTV are not normally delineated on prostate. The between-centre ADC variation followed the same trend as the delineation variation (Figure 2). The test-retest ADC variation was estimated as 0.06*10 -3 (adrenal gland), 0.01*10 -3 (oligo), 0.09*10 -3 (pancreas), and 0.23*10 -3 mm 2 /s (prostate), comparable to the between-centre ADC variation, and possibly overestimated since contour propagation was used instead of re-contouring on retest scans.