ESTRO 36 Abstract Book

S920 ESTRO 36 2017 _______________________________________________________________________________________________

of volumes according to the Dice similarity coefficient (DSC) and the distance between contours as average Hausdorff distance (AHD) were calculated to estimate the inter-observer variability. The size of volumes obtained with visible and hidden surgical clips was also evaluated. Results A summary of the calculated metrics is reported in Fig.1, with the boxplot representing the distribution of the metrics obtained from all the possible pairwise operator comparisons. A relative median increase of 48.7% in DSC (absolute difference 0.21) and a relative median decrease of 50.7% and 57.1% in CMD and AHD (absolute difference - 0.68 mm and -2.21 mm), respectively, were observed when surgical clips are visible. The differences between “w/o clips” and “with clips” contours were always statistically significant (p<0.001), with the surgical clips showing an important contribute in diminishing the inter- operator variability. Greater volumes were obtained when surgical clips are visible (relative median increased volume +46.4% with visible surgical clips, p-value<<0.001). However, a larger variability in segmented volumes between operators is observed when surgical clips are not visible, as shown in Table 1.

artery (RCA), and circumflex artery (CX)) – which are difficult to image due to influence of respiratory and cardiac motion – as well as the brachial plexus, body contour, and chest wall, using MRI in supine treatment position. Material and Methods Ten healthy volunteers were scanned, arms abducted, on a 1.5 T scanner (Ingenia, Philips). An anterior receive coil was placed on an adjustable bridge, from the abdomen until the mandible without touching the patient; a posterior receive coil was located in the scanner table. No contrast agent was administered. Three MRI sequences were optimized and evaluated qualitatively for structure identification in RT planning: - Coronal 3-dimensional (3D) T2-weighted (T2w) fast spin echo with short-tau inversion recovery fat suppression (T2-STIR), covering the supraclavicular and axillary lymph node regions and the apex of the heart as caudal edge - Transverse 3D balanced spoiled gradient (bSPGR) covering the heart using: spectral inversion recovery for fat suppression (SPIR); electrocardiography cardiac triggering on the diastolic heart phase; respiratory gating on a 1-D MRI navigator placed on the long-liver interface. - Transverse 3D T1w SPGR, with mDixon water-fat separation (T1-SPGR) covering the body contour. Results The heart and the RCA, LAD, CX arteries were clearly identified on bSPGR, with high contrast to the surrounding tissues, without apparent motion artifacts [fig 1A]. Small (4 mm diameter) IMLNs located close to the heart were also identified on bSPGR. The chest wall was visualized in bSPGR [fig 1C] and T1-SPGR; the latter distinctly showed the body contour [fig 2B]. The brachial plexus was clearly visualized with T2-STIR, showing intermediate signal intensity (SI), while main axillary arteries showed high SI [fig 2A]. The plexus could be followed from the spinal cord to the axillary nerves into the arm, on both sides.

Conclusion Discrepancies in contouring target structures can undermine the precision of ERT. The correct identification of tumour bed is important to avoid target miss in the subsequent ERT, to spare this area from the overdosage if the patient underwent IORT and to allow for reliable multi-centric studies. The use of surgical clips supports the radiation oncologist during the contouring process. In this study, we demonstrated that the inter-operator variability decreases with the guidance of surgical clips. A dosimetric analysis will be performed as further development in order to estimate possible underdosage of the target or overdosage of the surrounding normal tissue. [The study was partially supported by AIRC and Accuray Inc.] EP-1705 MR imaging of internal mammary lymph nodes and organs at risk in supine breast radiotherapy T. Van Heijst 1 , E.M. Aalbers 1 , E. Alberts 1 , H.J.G.D. Van den Bongard 1 , J.J.W. Lagendijk 1 , B. Van Asselen 1 , M.E.P. Philippens 1 1 UMC Utrecht, Radiotherapy, Utrecht, The Netherlands Purpose or Objective Standard radiotherapy (RT) for breast cancer patients is performed with CT guidance. Addition of MRI in breast RT planning is being investigated. We have developed MRI sequences to visualize individual axillary lymph nodes (LNs) [van Heijst et al. 2016, BJR]. The next step is to optimize visualization of internal mammary LNs (IMLNs) and organs at risk (OARs). The purpose of this study is to image the IMLNs, the heart including the coronary arteries (left anterior descending artery (LAD), right coronary