ESTRO 37 Abstract book

ESTRO 37

S562

PO-1006 Geometric acceptance testing for a hybrid MRI-Linac system J.W.H. Wolthaus 1 , J.J. Bluemink 1 , S.J. Woodings 1 , J.H.W. Vries- de 1 , T. Soest- van 1 , J.G.M. Kok 1 , H.M. Zijp- van 1 , S.L. Hackett 1 , B. Asselen- van 1 , B.W. Raaymakers 1 1 UMC Utrecht, Department of Radiation Oncology, Utrecht, The Netherlands Purpose or Objective In our department an Elekta Unity MRI-Linac, combining a 7 MV FFF linac with a 1.5T diagnostic MRI, was installed. Machine acceptance and commissioning comprise geometrical beam (alignment) verification, reference dosimetry, beam characterisation and MR imaging. We have followed international guidelines on reference dosimetry and on linac QA to comply with clinical standards. However, specific tests have been adapted to incorporate the magnetic field effects on delivered dose and measurement. The presented work will focus on QA of beam alignment. Material and Methods No light field and laser system are available on the MRI- Linac for alignment. Therefore, we use the on-board EPID, rigidly mounted on the gantry opposite to the beam generating system, for alignment of the measurement equipment (e.g. instruments, detectors, watertank, film). On the EPID images the isocenter pixel is identified from multiple projection images of a ball bearing. Since no collimator rotation is possible, beam centres cannot be derived directly from field edges. Therefore, beam alignment has to be determined from multiple tests. (1) Beam orthogonality is verified using the alignment of profiles at different depths measured in a watertank. (2) Focal spot alignment in the lateral direction is determined using two longitudinal opposing half fields with opposing gantry angles irradiated onto a film (fig 1a). The film is placed between copper plates to mitigate the effect of electron deflection due to the Lorenz force. The difference in centre position (defined by the edges) of the two profiles extracted through each of the two half fields is related to the misalignment of the focal spot. (3) Alignment of the MLC bank in the lateral direction is determined by comparing the profile peak position of a 30 x 20 cm 2 field to the centre defined by the field edges. (4) MLC leaf position accuracy is determined using a stripe/picket-fence-test in which the film is registered to simultaneously acquired EPID images to define the isocenter in the film (see fig 2). (5) Gantry isocenter accuracy is determined using a spoke-test on a film, sandwiched between copper-rings (fig 1b). (6) A general check of the beam alignment and field-size is performed by evaluating the congruence of two opposed fields of different sizes for 0/180° and 90/270° (fig 1c).

Results Isocenter positioning uncertainty on the EPID was < 0.1 mm. Beam angle alignment accuracy was 0.3° for both directions. Focal spot alignment accuracy was < 0.5 mm. MLC absolute position accuracy was < 0.5 mm. Gantry isocenter locus had a radius of < 0.4 mm. Conclusion The presented results are a subset of all performed tests and measurements. All were within tolerances as defined by standard protocols. Most of these tests are easy to perform and will therefore be repeated periodically to establish sufficient statistics for evaluation of the long- term performance. In conclusion, the Elekta MRI-linac geometric alignment is suitable for clinical treatments. PO-1007 Monte Carlo modelling of the ImagingRing System – a new method for realistic X-ray distribution N. Reisz 1 , P. Kuess 1,2 , H. Fuchs 1,2 , P. Steininger 3,4 , I. Messner 3 , A.D. Law 3 , H. Deutschmann 3,4 , M. Stock 5 , A. Ableitinger 5 , D. Georg 1,2 1 Medical University of Vienna, Department of Radiotherapy, Vienna, Austria 2 Medical University of Vienna, Christian Doppler Laboratory for Medical Radiation Research for Radiation Oncology, Vienna, Austria 3 medPhoton GmbH, Medical Physics, Salzburg, Austria 4 Paracelsus Medical University, Institute for Research and Development on Advanced Radiation Technologies radART, Salzburg, Austria 5 EBG MedAustron GmbH, Medical Physics, Wiener Neustadt, Austria