ESTRO 36 Abstract Book

S944 ESTRO 36 2017 _______________________________________________________________________________________________

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 Techniques", run by the Health Section of the International Atomic Energy Agency (IAEA). The

participants of the audit were obliged to irradiate provided dosimetric films, in a slab phantom, for a specific leaf arrangement, producing a pattern of five stripes, commonly called a picket fence. The participants had to programme such a pattern so that the stripes are 5 mm wide and are 3 cm distant between themselves. The Gafchromic EBT2 radiochromic films were placed in a slab phantom close to maximum dose depth. The irradiation was 250 MU per stripe. Results Thirty two Polish radiotherapy centres took part in the audit. They were equipped with various accelerator types and various treatment planning systems. In all cases the 6 MV quality beams were used. The discrepancies between measured and expected stripe positions were in the range 1.2 mm. For particular participants, the leaf position discrepancies were in the range -0,5 mm to 0,5 mm. For particular participants, the mean opening width measured with films for each pair of leafs was between 6 and 8 mm. Conclusion In the audit, the best performance showed the new type multileaf collimators with 120-160 leafs, The worst performance showed collimators MLC80 from Elekta. The results of the audit are very useful for the participants who should carefully investigate the performance of their multileaf collimators. EP-1741 Commissioning of a robotic patient positioning system equipped with an integrated tracking system A. Ableitinger 1 , A. Utz 1 , A. Zechner 1 , S. Vatnitsky 1 , M. Stock 1 1 EBG MedAustron GmbH, Strahlentherapie, Wiener Neustadt, Austria Purpose or Objective Robotic patient positioning systems (PPS) used in clinic must consider weight-induced couch bending and must show high reproducibility and stability to achieve the required positioning accuracy. Extensive commissioning of these robotic systems is therefore crucial. The aim of the current work is to determine the positioning accuracy of the PPS, that is equipped with an integrated optical tracking system. Material and Methods Three different aspects of the PPS were investigated in this study: the basic characteristics including couch bending, reproducibility and stability; the relative spatial deviation in terms of rotation and couch height and the absolute accuracy of the treatment couch. The treatment volume of the PPS has a dimension of 115cm x 50cm x 40cm. The robotic system enables couch rotations of more than 190°, pitch and roll of ±3° and non- isocentric treatment positions. A photogrammetric camera tracks the treatment couch of the PPS via reflecting markers mounted on the bottom side of the couch (see Fig.1). An iterative position correction loop aligns the couch to the prescribed position. The reference instrument was a laser tracker with reflecting probes. Drilling holes near the indexing positions (H4-F9) located laterally along the couch every 14cm served as measurement positions for the evaluation of the basic characteristics and the absolute couch position. For the relative deviation the drift of one measurement point on the couch was evaluated. To determine the bending of the couch it was loaded with six different weights up to 156kg. The reproducible positioning for the same couch position (different axis setting) and the couch stability after 1 hour were measured with the highest payload. The evaluation of the absolute spatial deviation was based on six measurement points being closest to the room isocenter. They were compared with their expected coordinates for 1020 different robot positions, poses and payloads.