ESTRO 38 Abstract book

S215 ESTRO 38

OC-0414 Intrafraction displacement of breast tumor (bed) and individual axillary lymph nodes on cine MRI M. Groot Koerkamp 1 , H.J.G.D. Van den Bongard 1 , M.E.P. Philippens 1 , J.J.W. Lagendijk 1 , A.C. Houweling 1 1 UMC Utrecht, Radiotherapy, Utrecht, The Netherlands Purpose or Objective The purpose of this study was to evaluate intrafraction displacement of breast tumor (bed) and individual axillary lymph nodes (LN) on cinematic MRI (cine) scans. Evaluation of displacement in local and regional target regions will help us to assess the potential impact of these displacements on future MRI-guided (stereotactic) RT for 2D cine scans (0.3-0.6s/slice) of 25 breast cancer patients participating in an MR imaging study were acquired on a 1.5T MRI scanner. Transverse-sagittal interleaved imaging of the breast was performed in both prone and supine position. Coronal-sagittal interleaved imaging of regional LNs was performed in supine position only. Additionally, sagittal cine scans (0.3s/slice) of LNs of 7 healthy volunteers were assessed. All cine scans were acquired during 1-3 minutes of free breathing. Intrafraction displacement in the cine scans was investigated with an optical-flow algorithm. With this method we applied a deformable registration of the cine scan frames to a reference frame of the same cine series. The mean displacement of pixels within a region of interest was calculated to determine tumor (bed) or LN displacement with respect to the reference frame. In preoperative patients the visible tumor or biopsy marker was chosen as the region of interest; in postoperative patients the best visible surgical clip was used. The maximum peak-to-peak difference (Figure 1) was determined in left-right (LR), superior-inferior (SI), and anterior-posterior (AP) direction. Results In total 40 cine scans of LNs (17 patients, 7 volunteers), and 21 cine scans of the breast (6 pre- and 4 postoperative patients) in prone (12) and supine (9) position were analyzed (Figure 1). The median maximum displacement for LNs was around 2mm in all directions (Table 1). Tumor or surgical clip position in prone position was very stable (median maximum displacements around 1mm in all directions). In supine position median maximum displacements were 1.5mm in LR, 2.6mm in SI, and 3.7mm in AP direction. breast cancer patients. Material and Methods

Conclusion Intrafraction displacement of both the tumor (bed) and axillary lymph nodes was small. With the current PTV margins (5-8mm) these displacements are not clinically relevant. However, the displacements should be considered for future MRI-guided stereotactic (single fraction) breast RT with smaller PTV margins, e.g. for tumorpositive LNs or partial breast irradiation. For local stereotactic breast RT, prone positioning might be beneficial, since the displacements in the breast were smaller in prone than in supine position, especially in AP direction. OC-0415 MERINO study: Defining a standardised delineation method for repeated GTV assessment using DW MRI A. Duffton 1 , I. McCrea 2 , S. Allwood-Spiers 3 , L. Hay 4 , L. Devlin 4 , M. Sankaralingam 3 , M. Thomson 4 , P. McLoone 5 , M. McJury 3 , J. Foster 3 , C. Paterson 6 1 Inst. of Cancer Sciences-Univ. Glasgow, Research & Development Radiographer, Glasgow, United Kingdom ; 2 Queen Elizabeth University Hospital, Department of Radiology, Glasgow, United Kingdom ; 3 Beatson West of Scotland Cancer Centre, Department of Clinical Physics and Bioengineering, Glasgow, United Kingdom ; 4 Beatson West of Scotland Cancer Centre, Radiotherapy, Glasgow, United Kingdom ; 5 University of Glasgow, West of Scotland cancer surveillance unit, Glasgow, United Kingdom ; 6 Beatson West of Scotland Cancer Centre, Clinical Oncology, Glasgow, United Kingdom Purpose or Objective Diffusion-weighted MRI (DW MRI) enhances anatomical imaging obtained from MR, by providing information on the cellularity of tumours by measuring Brownian motion. Studies show promising results in defining a threshold change in ADC that predicts non-responders to treatment. However, these studies have; low patient numbers, include a number of sub-sites, and often don’t describe the delineation method. The overall aim of the MERINO study is to determine threshold change in ADC that determines responders from non-responders, which may allow intensification of RT treatment for those who require it [1] . The aim of this work is to:  Quantify primary gross target volume (GTV) change between; baseline MR (GTV_T1_Base) to repeat MR (GTV_T1_Rpt) as defined on T1 post contrast fat sat (T1PCFS) anatomical imaging, and baseline MR (GTV_b0_Base) to repeat MR (GTV_b0_Rpt) on DW MRI. Describe a standardised method of defining a region of interest (ROI) to allow repeated measurements within clinical studies Material and Methods This is a prospective observational imaging study (REC approval (15/WS/0159). Patients with intermediate and high risk, locally advanced SCC oropharynx (OPSCC) Proffered Papers: RTT 4: Reducing uncertainties in volume definition 