ESTRO 36 Abstract Book

S969 ESTRO 36 _______________________________________________________________________________________________

Material and Methods The leaf position is defined as the position of the 50% of the dose profile. This measurement depends on relative position of the beam source and of the leaves. In order to validate the EPID measurements of the absolute leaf positions, 10 dose profiles, at the center of different leaf pairs of the same MLC field, were acquired with an Elekta iViewGT EPID and with a diode positioned in a water phantom. The comparison between the two detectors was performed by Matlab. Garden Fence (GF) was chosen as test of the leaf position accuracy and a preliminary study on the gap width was conducted. Leaf position accuracy was checked automatically with DC by acquiring GF at the 4 cardinal gantry angles and with all the beam energies (6, 10 and 15MV), while the reproducibility was tested with 5 GF repeated in one day and 6 repeated in a time interval of 70 days. Results The difference between EPID and diode absolute measurement of the leaf positions was less than 0.8mm for all the analyzed leaves, resulting from the summation of an error due to the isocenter identification (0,5mm) plus the leaf positioning error (0.2mm). The gap width study revealed that, because of the penumbra widening observed in small fields, the leaf position could be accurately measured as the 50% of the edge profile, only if the gap width is equal or larger than 16mm with 6MV beam. Therefore, GF with 20mm gap was chosen as leaf position accuracy test for all the energies in order to distinguish the effect of beam source from that of leaf positioning. For the GF at different gantry angles the difference between the measured and the prescribed position was well within 1.0mm for all the leaves. Moreover, reproducibility of each leaf position resulted to differ from its average value less than 0.4mm. Conclusion This work permitted to assess the accuracy and the repeatability of the Elekta Agility MLC leaf positioning by the combined use of the Elekta IviewGT EPID and the Dosimetry Check software through the acquisition and the analysis of Garden Fence test. This system was validated comparing the EPID with a diode in a water phantom and assessing the minimum gap width necessary for an accurate leaf position measurement at all energies which is useful to distinguish issues related to beam symmetry from those related to leaf positioning. EP-1760 A simple method for estimating the longitudinal isocentre shift due to gantry motion R. Hudej 1 , D. Brojan 1 , S. Pulko 2 , P. Peterlin 1 1 Institute of Oncology Ljubljana, Department of Radiophysics, Ljubljana, Slovenia 2 University Clinical Centre Maribor, Department of Oncology- Radiotherapy Unit, Maribor, Slovenia Purpose or Objective The isocentre as a point of intersection of the three rotational axes (gantry, collimator and treatment couch) ideally remains fixed in space during the rotation of gantry, collimator, or the treatment couch. Due to the mechanical limitations, gantry sags slightly, and consequently the radiation isocentre shifts slightly towards the treatment couch when the gantry rotates from the uppermost to the lowermost position. The purpose of this study is to assess this shift. Material and Methods A strip of radiochromic film embedded in a suitable water- equivalent phantom is irradiated with a cross-line half-slit field from the top (0°). Then the gantry is rotated to the lowermost position (180°) without moving the jaws and the phantom is irradiated again. The film is scanned and analysed with an image analysis script. The central lines of both half-slit images are determined, then the

intersection angle between them is calculated, and finally the distance between the intersections of extrapolated lines with the 'sagittal” plane is calculated. Results This method was tested on 7 linacs of different makes and models (Elekta Synergy Platform, Elekta Versa HD, Varian Unique, Novalis Tx) in the authors' radiotherapy centres. The average distance by which the isocentre moves between both gantry positions was found to be 1.04 mm (SD 0.30 mm), with the whole range covered by the [0.53, 1.48] interval. The two lowest values were achieved on the two single energy Varian Unique linacs. It was found out that the longitudinal isocentre shift is largely independent of the gantry isocentre wobble determined by the star-shot test. We also tested the alignment of the collimator 0° setting with the gantry rotation plane. The average deviation was found to be 0.16° (SD 0.10°), range [0.04°, 0.31°]. Conclusion The results appear consistent, but it would be helpful to test the method on a wider pool of treatment machines over a longer period of time. The longitudinal isocentre shift during gantry rotation is a non-negligible parameter which needs to be incorporated into the uncertainty budget which is the basis for the CTV-PTV margin. EP-1761 Workflow development for the clinical implementation of an MR-guided linear accelerator T. Stanescu 1 , A. Berlin 2 , L. Dawson 2 , J. Abed 2 , A. Simeonov 2 , T. Craig 2 , D. Letourneau 2 , D. Jaffray 2 1 Stanescu Teodor, Radiation Physics- PMH, Toronto, Canada 2 Princess Margaret Cancer Centre, RMP, Toronto, Canada Purpose or Objective Development of clinical workflows for the implementation of a new external beam radiation therapy environment which relies on hybrid MR-CBCT in-room imaging guidance. Material and Methods A standard radiation therapy 6X linac (TrueBeam, Varian Medical System, Palo Alto, CA) was integrated with a 1.5 T diagnostic MR scanner (IMRIS, Minnetonka, MN). The MR can move on rails and was tuned up to perform optimal imaging inside the treatment room in the proximity of the linac. The patient load is transferred directly between the MR diagnostic table and the linac IGRT couch via a hovercraft system (Zephyr XL, Diacor, Salt Lake City, UT). No special MR safety requirements were employed regarding the curation of the linac room – the linac/couch can be freely operated mechanically when the MR magnet is present - only typical MR room screening for ferromagnetic content was implemented. Comprehensive testing was completed to confirm negligible magnetic field coupling between the MR and the TrueBeam system (linac and patient table). Since the linac retains its default features and an MR imager is available in the linac vault a combined MR-kV approach can be employed for the patient setup verification and treatment delivery. A new software tool was developed in collaboration with Varian to provide the computation and implementation of treatment couch shifts based on soft-tissue information, i.e. image matching between plan MR and guidance MR. In this study, clinical workflows for liver and prostate were developed and tested. Each site posed challenges from patient image data planning and acquisition to RT planning and in-room guidance. The approach was to integrate the capabilities offered by the new technology in existing processes. Results MR imaging protocols for planning and guidance were established. The guidance scans were optimized to minimize session time with negligible penalty on the accuracy of the image matching process (planned vs. on