ESTRO 2020 Abstract book

S409 ESTRO 2020

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.

Conclusion A vacuum mattress can reduce intra-fraction motion during SBRT for pelvic or para-aortic lymph nodes, with a median absolute reduction of 0.3 mm in the anterior- posterior (AP) direction (bony anatomy). More importantly, it reduced the needed PTV margin in AP direction with 0.8 mm (for 30-min. time interval) which may be especially relevant for further hypo-fractionation

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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