ESTRO 37 Abstract book

S870

ESTRO 37

Conclusion In clinical intermediate-risk localized prostate cancer, apart from the well-known risk factor of percentage of positive cores on biopsy, the presence of extraprostatic extension in pre-treatment mpMRI is an independent predictor biochemical failure and metastatic failure. A. Pathmanathan 1 , M. Schmidt 1 , D. Brand 1 , L. Delacroix 2 , C. Eccles 2 , A. Gordon 2 , T. Herbert 2 , H. McNair 2 , N. Van As 1 , R. Huddart 1 , A. Tree 1 1 The Institute of Cancer Research and The Royal Marsden NHS Foundation Trust, Radiotherapy and Imaging, Sutton, United Kingdom 2 The Royal Marsden Hospital NHS Foundation Trust, Radiotherapy, Sutton, United Kingdom Purpose or Objective MR-guided radiotherapy (MRgRT) of the prostate requires sequence optimisation for accurate and timely prostate definition for planning and intrafractional imaging. There has been interest in magnetic resonance imaging (MRI) sequences enhancing the signal void of fiducials (1, 2), required for accurate MR and computed tomography (CT) fusion (3). T2*-weighted (T2*W) MRI uses multiple echo times (4) also resulting in a more defined prostate capsule and reliable depiction of fiducials. This study assesses the variability and accuracy of prostate delineation by radiographers using CT, T2-weighted Five radiographers experienced in delineation or registration of the prostate on CT, delineated the prostate on CT, T2W and T2*W MRI using Monaco v.5.19.02 (research version, Elekta AB, Stockholm, Sweden) for ten patients scanned within the PACE trial (NCT01584258), following a training session. Time taken for delineation was recorded and images were scored (0- 10) for ‘image quality’ and ‘confidence in contouring’. There was a minimum of two weeks between contouring EP-1613 Comparison of prostate delineation on multi- modality imaging for MR-guided radiotherapy (T2W) and T2*W MRI. Material and Methods (1) Interobserver variability (IOV) by comparing individual contours to a simultaneous truth and performance level estimation (STAPLE) contour created from all radiographer contours (2) Accuracy by comparison of radiographer contours to a ‘gold standard’ created from the STAPLE of three physician contours. Contours were assessed using Monaco ADMIRE software v.2.0 (research version, Elekta AB). For each comparison, the overlap measures Dice similarity co-efficient (DSC) and Cohen’s kappa were recorded, (higher values indicate greater agreement). In addition, distance measures of Hausdorff distance (HD) and mean distance between contours were recorded (lower values indicate greater agreement). SPSS statistics v.23 was used to perform separate Friedman’s tests for each comparison between modalities. Where significant, pair-wise group comparison was undertaken using Wilcoxon’s signed rank (Bonferroni corrected). to minimise recall bias. Assessment was made of;

Results

Median comparisons for each imaging type, delineation times and imaging scores are summarised in Table 1. Overall there is good agreement between radiographers and between radiographers and the gold standard. T2*W shows significantly reduced IOV and significantly higher agreement with the gold standard compared to CT, for all comparison metrics. In addition, there is significantly decreased IOV for prostate contours delineated using T2*W compared to T2W MRI (DSC and mean distance) and significantly improved accuracy (DSC, Cohen’s kappa and mean distance) when comparing to the gold standard (Figure 1). Greater quality images and confidence in contouring were reported for T2*W MRI, reflected in the shorter time to complete contours. Conclusion Radiographer prostate contours are more accurate, show less IOV and are more confidently and quickly outlined using a T2*W MR sequence compared to T2W or CT imaging. EP-1614 Inter-observer contouring variation of multiple pelvic structures on CT and MR for prostate cancer D. Roach 1,2 , M.A. Ebert 3,4,5 , M. Jameson 2,6 , J.A. Dowling 1,4,7,8 , A. Kennedy 3 , P. Greer 7,9 , L. Holloway 1,2,4,6 1 University of New South Wales, Faculty of Medicine, Sydney, Australia 2 Ingham Institute for Applied Medical Research, Medical Physics, Liverpool, Australia 3 Sir Charles Gairdner Hospital, Radiation Oncology, Nedlands, Australia 4 University of Wollongong, Centre for Medical Radiation Physics, Wollongong, Australia 5 University of Western Australia, School of Physics and Astrophysics- Faculty of Science, Crawley, Australia 6 Liverpool and Macarthur Cancer Therapy Centres, Department of Radiation Oncology, Liverpool, Australia