ESTRO 37 Abstract book

the best parameters quantitative metrics were used. Shannon entropy was firstly computed on the phantom images. Then, SNR and CNR were extracted from the differences of successive volunteer images. In addition, a subjective metric called the Mean Opinion Score, was proposed and obtained from 8 radiotherapists and 3 physicists for volunteer T1 images. Results For the phantom, the minimum entropy value found is 2.05 bits in v3 T1. For all volunteers, SNR and CNR give better results for v3 T1 (Table 1). The native v4 T2 axial images have better resolution and contrast compared to the T2 sagittal acquisitions (Fig 1). The maximum Mean Opinion Scores obtained is for v3 T1 (label score: good). In total, v3 T1 and v4 T2 sets of parameters provide the best image quality.

phase. The extensional margin was calculated by the formula: Results (1) The centroid coordinates displacement of left CA ostia for X, Y, Z three coordinates were: LM 7.5±2.6mm, 6.2±2.3 mm, 6.2±2.0mm, OM 11.8±4.0mm, 10.8±4.1mm, 11.8±5.5mm, D 1 5.7±1.9mm, 6.5±4.4mm, 6.7±2.1mm, D 2 5.3±1.9mm, 6.3±3.6mm, 7.1±2.5mm, APX 6.4±3.4mm, 4.6±2.4mm, 7.7±3.3mm. The centroid coordinates displacement of right CA ostia for X, Y, Z three coordinates were: V 15.4±4.0mm, 18.4±4.9mm, 17.4±6.1mm, AM 15.4±3.9mm, 21.8±4.8mm, 18.7±6.2mm. (2) Left CA ostia extensional margin for X, Y, Z three coordinates caused by heart beat were: LM 4mm,3mm,4mm, OM 7mm, 7mm, 7mm, D 1 and D 2 4mm, 4mm, 5mm, APX 4mm, 3mm, 4mm, right CA ostia extensional margin for X, Y, Z three coordinates caused by heart beat were: V 11mm, 15mm, 11mm and AM 10mm, 13mm, 10mm, can include the whole range of ostia motion. Conclusion Conclusion: In view of the heterogeneity of different ostia, we suggest chose the individually extensional margin when considering the coronary artery as a separate organ at risk. EP-2350 Dosimetric impact of daily variations to OAR during prostate SABR treatment delivery L. Devlin 1 , S. Currie 1 , D. Dodds 1 , A. Sadozye 1 , P. McLoone 1 , A. Duffton 1 1 Beatson West of Scotland Cancer Centre, Radiotherapy, Glasgow, United Kingdom Purpose or Objective Prostate stereotactic ablative radiotherapy (SABR) is a highly conformal radiotherapy technique. Due to large fraction sizes and steep dose gradients, the effect of geometric uncertainties could be greater on organs at risk (OAR). There is little published evidence on the true dose received by OAR in conventional prostate radiotherapy and SABR. Currently the dosimetric impact of organ motion on OAR planning constraints is not well understood. With the increased conformality and dose per fraction, it is essential to evaluate the dose received by OAR. The UK SABR consortium guidelines recommend OAR doses to be as low as achievable.This study retrospectively evaluates dose delivered using CBCT images for patients who have completed treatment within a local safety, feasibility and efficacy study. Aim Is the delivered dose on treatment in agreement of the planned dose to organs at risk. Material and Methods 41 patients treated Prostate SABR linear accelerator based technique 35Gy/ 5, 10X FFF. Prostate tracked pre delivery matching to fiducial markers on CBCT. Retrospective delineation of bladder and rectum on 205 pre-treatment CBCT image sets by 1 observer. CBCT registered to planning CT at fiducial markers. Daily CBCT rectum and bladder contours overlaid on planning CT for dosimetric analysis. Recalculation of original plan to evaluate the dose volume histogram for each structure. Assessing the impact of inter motion of internal organs. The dose received by organs at each fraction measured using ratio of structure at the planning constraint. Total delivered dose received by each organ evaluated to ensure planning constraints met despite organ motion. Results In 8 patients 35% of the rectum received >18Gy. In 17 patients 10% of the rectum received >28Gy. In 17 patients 5% of the rectum received >32Gy and in 15 patients 1% of the rectum received >35Gy. In 18 patients 1% of the Bladder received >35Gy(Table 1).

Conclusion We obtained suitable 3D T1 and T2 images for RTP within a protocol time of 15 minutes at 1.5 T. Our institution will propose this imaging protocol for dose calculation for patients suffering from cancers of the head or neck. EP-2349 Quantification and analysis of coronary artery major ostia motion based on ECG-gated 4D-CT Q. Li 1,2 , Y. Tong 1,2 , G. Gong 1 , Y. Yin 1 1 Shandong Cancer Hospital Affiliated to Shandong University, Department of Radiation Oncology, Ji'nan, China 2 University of South China, School of Nuclear Science and Technology, Hengyang, China Purpose or Objective To quantify and analyze the motion of coronary artery (CA) major ostia which held high risk of radiation- induced injury. Material and Methods 37 female volunteers were selected into our trials, all volunteers underwent intravenous contrast electrocardiography gated four-dimensional computed tomography scanning with thinkness 0.5mm in inspiration breath hold state, which were sorted into 20 phases. The left main coronary artery (LM), the obtuse marginal (OM), the first diagonal branch (D 1 ), the second diagonal branch (D 2 ) ostium and the left anterior descending branch which close to the apex (APX) in the left CA, the first right ventricular artery (V) and the acute marginal (AM) ostium in the right CA according to CA segmentation criteria of American College of Cardiology. Four adjacent slices of all ostia from 0% to 95% phase in cardiac cycle were contoured. The CA major ostia motion was quantified by calculating mean displacement in three co- ordinates: Left-Right (X), Anterio-Posterior (Y) and Cranio-caudal (Z). Using the centroid coordinates of cardiac systolic phase as standard phase, we calculated the displacement of all structures refer to the standard