ESTRO 36 Abstract Book

S957 ESTRO 36 _______________________________________________________________________________________________

planning process. The aim of this study was to assess the feasibility of MRI-based atlas-based autosegmentation for organs-at-risk (OAR) and prostate target volumes, and to compare the segmentation accuracy with CT-based autosegmentation. Material and Methods Images were retrospectively selected from 6 prostate patients who received whole field T2 weighted 3D SPACE MRI and CT in the radiotherapy treatment position (at the Northern Centre for Cancer Care, Newcastle). Organs at risk (Bladder, rectum, seminal vesicles, left and right hips, penile bulb) and the prostate were manually delineated on the CT and MRI separately. A ‘leave one out’ approach was used to automatically segment structures onto the remaining images separately for CT and MRI. Contour comparison was performed using the DICE index and mean distance to conformity (MDC) positional metrics. MDC, DICE and absolute volume were used to assess the performance of the contouring by comparing the automatic to the manual contours. A paired t test was used to determine the statistical significance between MRI The volume analysis (data not presented) showed that manual and automatic contouring on MRI gave smaller contours than CT (significantly so for the hips, prostate and seminal vesicles). The positional analysis results are shown in table 1. MRI autocontouring was more accurate than CT for the bladder (MDC significantly so) and the prostate/penile bulb (although not significantly). There was little difference in accuracy between CT/MRI autocontouring for both hips, rectum and seminal vesicles. and CT. Results Conclusion Accurate atlas-based automatic segmentation of structures for prostate radiotherapy is feasible using T1- MRI; segmentation of the penile bulb and seminal vesicles was found to be poor. Comparison with CT-based automatic segmentation suggests that the process is equally or more accurate using MRI. Although this study was on a small sample size these results support further translation of MRI-based segmentation methodology into clinical practice. EP-1740 Nationwide audit of multileaf collimators performance K. Chelminski 1 , W. Bulski 1 1 The Maria Sklodowska-Curie Memorial Cancer Center, Medical Physics Department, Warsaw, Poland Purpose or Objective The delivery of accurate intensity-modulated radiation therapy (IMRT) or stereotactic radiotherapy depends on a multitude of steps in the treatment delivery process. The proper intensity modulation depends on the proper functioning of a multileaf collimators (MLC). The aim of this audit was the control of the proper collimator leafs positioning. Material and Methods The methodology of the audit of small field output performance was established within the framework of the CRP E2.40.16 project "Development of Quality Audits for Radiotherapy Dosimetry for Complex Treatment

Conclusion Preliminary image quality tests of a prototype MV-CBCT imaging system that utilizes a 6X-FFF beam and a fast acquisition mode revealed acceptable performance, but as expected, worse than those of kV-CBCT and diagnostic CT. The fast acquisition may potentially be beneficial for motion management treatments such as DIBH for breast and lung tumors. The MV-CBCT also has superior contrast linearity for higher density and metallic materials because of reduced beam hardening, and could be utilized to supplement diagnostic CT images for treatment planning in such cases. EP-1739 The feasibility of atlas-based automatic segmentation of MRI for prostate radiotherapy planning C. Morris 1 , M.J. Gooding 2 , A. Henry 3 , R. Speight 4 1 University of Leeds, Department of Medicine, Leeds, United Kingdom 2 Mirada Medical Ltd, Mirada Medical Ltd, Oxford, United Kingdom 3 St James Institute of Oncology, Department of Clinical Oncology, Leeds, United Kingdom 4 St James Institute of Oncology, Medical Physics and Engineering, Leeds, United Kingdom Purpose or Objective Atlas-based autosegmentation is an established tool for segmenting structures for CT-planned prostate radiotherapy. MRI is being increasingly integrated into the