ESTRO 36 Abstract Book

S242 ESTRO 36 _______________________________________________________________________________________________

Three hollow plexiglass cubes filled with VIPAR polymer gel were produced and used in this study. Planning CT scans of each one of the gel filled cubes and arbitrary RStructures have been used for treatment planning. Cube- 1 was planned to be irradiated with mono-energetic proton beams (90MeV & 115MeV) avoiding overlapping of the irradiated gel areas (Max Dose : ~ 15 Gy). Cube-2 was planned to be irradiated with a multi-energetic beam forming a spread-out Bragg peak (SOBP) (Max Dose : ~ 13 Gy). Cube-3 was planned to be irradiated with two opposing beams (Max Dose : ~ 13 Gy) each delivering an overlapping and uniform SOBP. Set-up and irradiation of each cube followed. One day post-irradiation each cube was MRI scanned in order to derive high spatial resolution 3D-T2 maps that were subsequently co-registered to the corresponding planning-CT scans and DICOM-RT Dose and Structure data. Assuming a linear gel dose response, 1D, 2D and 3D dose measurements were derived and compared against corresponding TPS data. Results VIPAR gel response seem to be non-dependent on LET for LET values < ~6 keV/µm implying that their use for most clinical cases is acceptable. No matter their LET dependence, the protons range can be well verified. Even if uncertainties related to imaging, set-up, beam delivery, dose calculations, co-registration, gels LET dependence were incorporated, the range measured by the proposed method was within ~ 1 mm to that calculated by TPS. Moreover, the corresponding ranges at the 80% value of the maximum dose point for both TPS and polymer gels derived percentage depth dose profiles (pdds) were equal within ~1 mm. Additionally, for the opposed beams experiment (cube-3), the proposed methodology results in even more accurate dosimetry due to the reduced LET values inside the SOBP compared to the high LET values present in the irradiated schemes of cubes 1 and 2. Conclusion The proposed End-to-End Quality Assurance method based on polymer gel dosimetry, provides valuable outcomes for proton range verification and 3D proton dosimetry.

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

A. T2-map of the irradiated polymer gel cubic phantom, co-registered to the corresponding planning-CT scans and TPS calculated dose. B. Pdd measurements C. Isodoses in an arbitrary 2D plane D. GI (5%dose/ 2mm criteria) calculated by the data presented in C First row: SOBP irradiation. Second row: Mono-energetic 115 MeV irradiation

Poster Viewing : Session 10: RTT

PV-0456 Volumetric Modulated Arc Therapy for patients with bilateral breast cancer S. Lutjeboer 1 , J.W.A. Rook 1 , G. Stiekema 1 , A.P.G. Crijns 1 , N.M. Sijtsema 1 , E. Blokzijl 1 , J. Hietkamp 1 , J.A. Langendijk 1 , A.J. Borden van der 1 , J.H. Maduro 1 1 UMCG University Medical Center Groningen, Radiation Oncology, Groningen, The Netherlands

has been implemented. Material and Methods