ESTRO 36 Abstract Book

S283 ESTRO 36 2017 _______________________________________________________________________________________________

Conclusion The remote EPID based digital phantom method is feasible for centres equipped with Varian linear accelerators either Clinac or Truebeam. The methods to date for Elekta systems require improvement before widespread use in the remote audit. OC-0538 A virtual dosimetry audit – towards transferability between global QA groups in clinical trials M. Hussein 1 , D. Eaton 2 , P. Greer 3 , A. Haworth 4 , C. Hurkmans 5 , S. Ishikura 6 , S. Kry 7 , J. Lehmann 4 , J. Lye 8 , A. Monti 5 , M. Nakamura 6 , C. Clark 2 1 Royal Surrey County Hospital NHS Foundation Trust, Medical Physics, Guildford, United Kingdom 2 NCRI Radiotherapy Trials Quality Assurance Team, RTTQA, London, United Kingdom 3 Calvary Mater Newcastle, Radiation Oncology Physics, New South Wales, Australia 4 The Trans Tasman Radiation Oncology Group, TROG, New South Wales, Australia 5 European Organisation for Research and Treatment of Cancer, EORTC, Brussels, Belgium 6 The Japan Clinical Oncology Group JCOG, National Cancer Center, Tokyo, Japan 7 Imaging and Radiation Oncology Core, IROC, National Cancer Institute, USA 8 Australian Clinical Dosimetry Service ACDS, ARPANSA, Victoria, Australia Purpose or Objective Quality assurance for clinical trials is important. Lack of compliance can affect trial outcome. Different international QA groups have different methods of dose distribution verification and analysis, all with the ultimate aim of ensuring compliance. As some clinical trials are open to international recruitment, it is important to understand how different analysis techniques and tolerances translate between different groups. Therefore the aim of this study was to gain a better understanding of different process to inform potential future dosimetry audit reciprocity. Material and Methods Six international radiotherapy clinical trial QA groups participated. A treatment plan, using the 3DTPS test virtual phantom (previously used in UK VMAT and Tomotherapy Audit [1, 2]), was created using a 2 Arc VMAT technique in Varian Eclipse 13.6 calculated using AAA. This phantom has similar complexity to a head & neck cancer case [1]. Each group was supplied with four datasets. The first was a TPS reference dose cube. The remaining three were simulated ‘Measured’ dose cubes, labelled 1 to 3. These datasets were the original TPS plan with deliberate errors introduced; including dose difference and MLC positional errors. All data was exported in DICOM format to be readable by any software. Users were blinded to the measured data detail. Each group was requested to perform an analysis on the datasets using their standard technique for a typical head & neck plan (e.g. gamma index analysis, Distance-to-Agreement (DTA), dose difference, etc.) and to create their standard audit report, including how the analysis was performed and whether the ‘measurements’ pass or fail. Results Table 1 summarises analysis technique, software used, acceptance criteria, and results for each ‘measured’ dataset. All but one group used the global gamma index (γ) analysis. The remaining used the DTA analysis. For the global γ there were differences in the normalisation method (i.e. whether to use the maximum dose, a point in a high dose low gradient region, etc.) and in pass/fail criteria. Two groups had three decision levels for analysis; optimal pass, mandatory pass and fail. The remainder had straight pass/fail decision criteria. All groups passed Measured 1 and 3. However, for Measured 2, four groups

recorded a pass, 1 passed but with further investigation necessary, and 1 recorded a fail. Figure 2 shows an example analysis of Measured 2.

Conclusion For the same dataset, different international audit groups had different analysis approaches and results. The results of this study will lead to a better understanding of methods and variability between audit groups and is informative for future work focussing on analysis techniques that are transferable between different groups. References [1] Tsang Y et al BJR 2012;86:1022. [2] Clark CH et al Radiother Oncol 2014;113:272-8. OC-0539 A multicentre QA study on 4DCT and IMRT/VMAT techniques for lung SBRT using a respiratory phantom M. Lambrecht 1 , J.J. Sonke 2 , M. Verheij 2 , C.W. Hurkmans 1 1 Catharina Ziekenhuis, Physics/Radiotherapy, Eindhoven, The Netherlands 2 Netherlands cancer institute, Radiotherapy, Amsterdam, The Netherlands Purpose or Objective The EORTC has launched a phase II trial to assess efficacy of SBRT for centrally located NSCLC: The EORTC— LungTech trial. Due to neighbouring critical structures, these tumours remain challenging to treat. To guarantee accordance to protocol and treatment safety, an RTQA procedure has been implemented within the frame of the EORTC RTQA levels. To determine SBRT accuracy, we have performed end-to-end tests investigating both 4D-CT and IMRT/VMAT under static and respiratory conditions. Material and Methods All centres audited performed 3D-CTs and 4D-CTs on the same phantom. It was successively scanned using two film inserts, one with a 15mm diameter target (15mmd) and one with a 25mm diameter target (25mmd). Three motions were tested: 20bpm/15mm amplitude (A), 10bpm/15mmA, and 15bpm/25mmA. A test procedure was developped to evaluate the impact of motion on the target volume and motion as determined using the binned CT data. These results were compared to the true volumes and motion amplitudes. Regarding the credentialing of