ESTRO 2020 Abstract Book

S374 ESTRO 2020

PD-0673 MRI vs CBCT image guidance when treating lymph nodes in patients with locally advanced (LA)- NSCLC S. Brown 1 , M. Dubec 2,3 , R. Chuter 4 , C. Eccles 5 , R. Hales 5 , J. Parker 5 , J. Rodgers 5 , L. Whiteside 5 , M. Van Herk 2,3 , C. Faivre Finn 2,3 , D. Cobben 2,3 1 Clatterbridge Cancer Centre, Radiation Oncology, Bebington, United Kingdom ; 2 The Christie NHS Foundation Hospital Trust, Radiotherapy Related Research, Manchester, United Kingdom ; 3 Division of Cancer Sciences, Manchester Cancer Research Centre- The University of Manchester, Manchester, United Kingdom ; 4 The Christie NHS Foundation Hospital Trust, Christie Medical Physics and Engineering CMPE, Manchester, United Kingdom ; 5 The Christie NHS Foundation Hospital Trust, Radiotherapy, Manchester, United Kingdom Purpose or Objective CBCT is routinely used to guide curative radiotherapy (RT) for lung cancer. This is limited by poor soft tissue discrimination. Consequently anatomical surrogates for lymph nodes (LNs) are used to facilitate registration and generous PTV margins are needed. MRI can improve soft tissue discrimination, and may allow a more accurate set- up. The aim of this study was to compare MRI to CBCT for the identification and localisation of LNs in patients undergoing curative RT for LA-NSCLC. Material and Methods Four experienced RTTs rigidly registered the outlined LNs on planning CT (pCT) with the CBCT and MR images of 10 patients with LA-NSCLC. MR and CBCT images were acquired 2 weeks into RT. The MR sequences used were: T2 turbo spin echo (TSE) non-fat saturated (Seq1) and T1 DIXON water (Seq2). Observers were asked to localise each LN on the study image and register this to the LN contour on the pCT (study images had been offset in advance by 5cm in the X, Y and Z planes). The resulting X, Y, Z translations were recorded. Observers then rated their confidence in performing the registration and the quality of the image in permitting visualisation of the LN, with a view to contouring (Fig 1). The process was repeated for each individual LN using all study images. For each LN, translations from all observers were reviewed for each plane and the standard deviation (σ) was calculated. The σ from all planes were used to calculate the vector value (σV). The mean σV for each image modality was compared to assess inter-observer variation.

quality, compared to CBCT, both Seq1 (2.66 vs 1.75, p<0.001) and Seq2 (2.74 vs 1.75, p<0.001) had significantly greater scores. No difference was seen between Seq1 and Seq2 (p=0.5).

Conclusion Observers scored the image quality of MR higher than CBCT and had significantly greater confidence using MR to perform direct LN registrations. T1 DIXON outperformed the T2 TSE MR sequence in providing more accurate registrations. The T2 TSE shows fat and LNs as bright contrast. This might make it difficult to differentiate the edges of mediastinal LNs, whereas the T1 DIXON provides a fat suppressed image which may aid LN identification in this scenario. However no difference was seen between MR and CBCT.. This work suggests MR may be useful in increasing the accuracy for set-up correction during RT for LA NSCLC. Further evaluation of its potential role is needed. PD-0674 The role of radiation therapists in the area of online and adaptive treatment on the MR Linac G. Sikkes 1 , E.N. De Groot - van Breugel 1 , N.G.P.M. Vissers 1 , G.H. Bol 1 , S.L. Hackett 1 , P.S. Kroon 1 , B. Van Asselen 1 , I.M. Jϋrgenliemk-Schulz 1 1 UMC Utrecht, Radiation Oncology Department, Utrecht, The Netherlands Purpose or Objective The role of radiation therapists (RTTs) in multidisciplinary treatment teams can change when innovations are implemented into clinical use. Here we present how the new online adaptive treatment workflows as present in the 1.5T MR Linac (MRL) (Unity, Elekta, Veenendaal) influenced RTTs work and responsibilities at our institute using SBRT for lymph node oligometastases (LNO) as an example. Material and Methods Our institute started in August 2018 with MRL treatments with LNO in the pelvis being the first clinical site. Patients were treated with fractionated SBRT (5x7Gy) using online planning on the actual anatomy as provided within the Elekta Adapt to Shape workflow (ATS). This process includes MRI scanning, image fusion, contour propagation, delineation, planning and plan QA. A multidisciplinary team of 1 radiation oncologist (RO), 1 medical physicist (MP) and 3 RTTs is responsible for the individual treatment. Prior to start treating patients with the MRL, RTTs are trained for MR scanning and online adaptive workflows. They further specialize in two profiles with competence for: 1. image registration and contouring; 2. treatment planning/plan evaluation/plan QA. To further optimize the workflow and reduce the workload for RO and MP, additional steps have been taken. A training program was started to achieve full competence for target en organ at risk delineation by RTTs. Contouring by RTTs started under

Results 24 LNs distributed between hilar (station 10-11) and mediastinal regions (1-7) were assessed. Fig 2 shows the distribution of individual LN σV values for each imaging modality. The mean σV for CBCT was not significantly different than either MR Seq1 (0.22 vs 0.27, p= 0.09) or Seq2 (0.22 vs 0.20, p= 0.47). However the mean σV for Seq 1 was significantly greater than Seq2 (0.27 vs 0.20, p= 0.04). Compared to CBCT the mean confidence scores for Seq1 (2.5 vs 1.8, p<0.001) and Seq2 (2.7 vs 1.8, p<0.001) were both significantly better. Seq2 had a significantly greater mean score than Seq1 (p=0.041). Regarding image