ESTRO 36 Abstract Book

S239 ESTRO 36 2017 _______________________________________________________________________________________________

TV coverage. To avoid the common mistakes encountered, a new approach to training including verbal instructions is being developed for testing. Our aim is to demonstrate benefits in clinical workflow and expand validation to other RT centres. OC-0453 Stereotactic radiosurgery for multiple brain metastases: Results of multi-centre benchmark studies D.J. Eaton 1 , J. Lee 1 1 Radiotherapy Trials QA group RTTQA, Mount Vernon Hospital, Northwood, United Kingdom Purpose or Objective Stereotactic radiosurgery (SRS) is strongly indicated for treatment of multiple brain metastases. Various treatment platforms are available, and comparisons have been made between modalities, but mostly in single centre studies. A pre-requisite for all providers selected as SRS/SRT centres in England was to participate in a quality assurance process, informed through collaboration between the national trials QA group and a multidisciplinary expert advisory group. All clinical centres undertook planning benchmark cases, providing a unique dataset of current practice across a large number of providers and a wide range of equipment. This was used to facilitate sharing of best practice and support centres Two brain metastases cases were provided, wit h images and structures pre-drawn, involving three and seven lesions respectively. Centres produced plans a ccording to their local practice, and these were reviewed centrally using metrics for target coverage, selectivity, gradient fall-off and normal tissue sparing. Results 38 plans were submitted, using 21 differe nt treatment platforms, including Gamma Knife, Cyberknife, Varian (Novalis / Truebeam STx / 2100), Elekta (Synergy / Versa HD using Beam Modulator / Agility MLC) and Tomotherapy. 6 plans were subsequently revised following feedback, and review of 4 plans led to a restriction of service in 3 centres. Prescription doses were typically 18-25Gy in 1 fraction (or 27/3fr), except for a lesion within the brainstem, which was prescribed 12-20Gy in 1 fraction (or 18-30Gy/5fr). All centres prioritised coverage, with the prescription isodose covering ≥95% of 208/209 lesions. Selectivity was much more variable, especially for smaller lesions, and in some cases this was combined with high gradient index, resulting in large volumes of normal tissue being irradiated. Both Tomotherapy submissions were outliers in terms of either selectivity or gradient index, but all other platforms were able to give plans with relatively low gradient indices for larger lesion volumes, although there was more variation among Varian and Elekta plans, than for Gamma Knife and Cyberknife (first figure). There were also larger differences for the smaller volumes, with increasing variation both inter- and intra- treatment-platform. Doses to normal brain and brainstem were highest when PTV margins were applied, but substantial improvements were possible by re-planning, even without changing margin size (second figure). with less experience. Material and Methods

Conclusion These benchmarking exercises give confidence in the safe and consistent delivery of SRS services across multiple centres, but have highlighted areas of different priorities, and potential for service improvement. The data can be used to progress standardisation and quality improvement of national services in the future, and may also provide useful guidance for centres worldwide. OC-0454 End-to-end QA methodology for proton range verification based on 3D-polymer gel MRI dosimetry E. Pappas 1 , I. Kantemiris 2 , T. Boursianis 3 , G. Landry 4 , G. Dedes 4 , T.G. Maris 3 , V. Lahanas 5 , M. Hillbrand 6 , K. Parodi 4 , N. Papanikolaou 7 1 Technological Educational Institute of Athens higher education, Radiology/Radiotherapy Technologists, ATHENS, Greece 2 Metropolitan Hospital, Medical Physics Department- Radiation Oncology Division, Athens, Greece 3 Medical School- University of Crete, Department of Medical Physics, Heraklion, Greece 4 Ludwig-Maximilians-Universität München, Department of Medical Physics, Munich, Germany 5 National and Kapodistrian University of Athens, Medical Physics Laboratory - Simulation Center-, Athens, Greece 6 Rinecker Proton Therapy Center, Department of Medical Physics, Munich, Germany 7 University of Texas Health Science Center, Department of Radiation Oncology-, San Antonio- Texas, USA Purpose or Objective In clinical proton therapy, proton range measurements are associated with considerable uncertainties related to : a) imaging, b) patient set-up, c) beam delivery and d) dose calculations. A sophisticated QA process that can to take into account all the mentioned sources of uncertainties is required in clinical practice. In this work, cubic phantoms filled with VIPAR polymer gels have been used towards this aim. An investigation of the gels dosimetric performance and their potential use for proton dosimetry purposes and as an end-to-end QA method for proton range verification

has been implemented. Material and Methods