ESTRO 37 Abstract book

S1214

ESTRO 37

Material and Methods A questionnaire was designed by three medical physicists expert of SBRT using Google Forms. Questions regarded technology: image-guidance solutions, respiratory management, delivery mode, treatment planning system (TPS) commissioning, and QA approach. A large number of medical physicists from 53 centers with heterogeneous technology was invited to participate and completed the survey. Experts were asked to provide suggestions based on personal experience and specificity. Results Many comments from the panel were provided. After two webcall and two versions, the final questionnaire was developed. Preliminary results from the 46 centers shown great heterogeneity in terms of technologies, image- guidance solutions, respiratory management, delivery mode, TPS commissioning and QA approach. The most available delivery system was conventional linacs with volumetric modulated arc therapy modality; the most common energy used was 6 MV; for the 50% of centers the smallest measured field size for output factors and lateral beam profiles was ≤1x1cm 2 ; there was a great heterogeneity for the minimum OF inserted into the TPS (1x1cm 2 - 4x4cm 2 ). As far as pre-treatment verification is concerned, there were a lot of different approaches in terms of QA devices and evaluation (gamma and passing rate percentage). Conclusion This survey on SBRT could allow to improve the QA procedure and to define minimum requirements for SBRT dosimetric verification. EP-2192 Developing a national SBRT spine dosimetry audit J. Lee 1 , R. Patel 1 , C. Clark 2 , D. Eaton 1 1 Mount Vernon Cancer Centre, National Radiotherapy Trials QA, Northwood, United Kingdom 2 National Physical Laboratory, Medical Physics, Teddington, United Kingdom Purpose or Objective An end-to-end dosimetry audit was designed as part of a national commissioning programme for spine SBRT. The objective of this work was to establish an audit method for verifying the accuracy of dose delivery to both a target and organs at risk (OARs) in an anthropomorphic phantom. The method was required to be compatible with a variety of coplanar and non-coplanar treatment modalities and suitable for measuring in the steep dose gradients produced in SBRT plans. Material and Methods A CIRS E2E SBRT phantom (model 036A) was used. Film can be placed in this phantom in the axial plane. Metal pins were placed in the phantom to pierce the film and allow registration of measured and calculated dose distributions to ~1mm accuracy. The phantom was also modified to position either a PTW Semiflex 0.125cc chamber (sensitive volume 5.5mm diameter, 6.5mm length) or PTW Microdiamond detector (sensitive volume 2.2mm diameter, 1µm length) adjacent to the film plane within the phantom’s spinal cord and vertebral body. Both detectors were tested in the phantom with a range of co-planar VMAT plans on Varian linacs, with clinical

SBRT spine plans (9Gy/#, cord D0.1cc 7.3Gy/# constraint). As the audit would include non-coplanar treatment modalities, the angular dependence of the microdiamond was also investigated using a custom cylindrical mini-phantom. Pilot audit measurements were taken at two clinical centres; a Varian Truebeam linac (coplanar VMAT) centre and a Cyberknife (non-coplanar) centre. Results Steep dose gradients at the chamber location in the spinal cord caused substantial measurement error with the 0.125cc chamber. Measurement of the same plans using the Microdiamond detector showed no trend with dose modulation (figure 1).

The Microdiamond detector showed substantial angular dependence for non-coplanar fields, up to 3.5% with 50° inclination. For the pilot audit, the non-coplanar angle of irradiation was limited to 20° from the axial plane in the direction of the detector stem to keep the range of this error to <1% (no limitation was required in the opposite direction). The difference between the Microdiamond measured dose and the calculated dose in the vertebral body and spinal cord were -0.1 and -5.5%, and 2.4 and -4.2% for the for the linac and Cyberknife delivery plans respectively. Conclusion The PTW Microdiamond detector is suitable for use with an adapted CIRS E2E SBRT phantom for a national SBRT spine dosimetry audit, with restrictions on the non- coplanar angle of irradiation. The audit protocol can be extended to all clinical centres in the national commissioning programme. EP-2193 Clinical implementation of MR-only prostate radiotherapy excluding CT E. Persson 1,2 , P. Ambolt 1 , C. Gustafsson 1,2 , S. Bäck 1 , S. Engelholm 1 , A. Gunnlaugsson 1 , L.E. Olsson 1,2 1 Skåne University Hospital, Hematology- Oncology and Radiation Physics, Lund, Sweden 2 Lund University, Medical Radiation Physics, Malmö, Sweden Purpose or Objective Based on our previous experience from implementing and running an MR-only workflow, we present a description specifically for MR-only Prostate RadiOTherapy Excluding CT (MR-PROTECT). This is a presentation of the first ten patients treated with MR-only prostate radiotherapy at our clinic. Material and Methods From studies of the present CT-based workflow, the needed actions for an MR-only workflow implementation were identified. The workflow design was adapted to the MR-only treatment technique and quality assurance needed for the implementation was integrated.