ESTRO 36 Abstract Book

S228 ESTRO 36 2017 _______________________________________________________________________________________________

SABR treatments. The potential benefits may include real- time, good quality image guidance without additional radiation. We are optimistic that improvements in the detector and collimation system, will lead to better image quality, more accurate tumor localizations, and possible 3D reconstruction of patient’s anatomy within the irradiated region.

0.5 Gauss line, needed to be adjusted in order to get beam parameters within tolerances. Adjusting the LUTs fully corrected the influence of the magnetic fringe field of the MRL. In case of an unexpected ramp down of the magnet (i.e. quench) both neighbouring accelerators cannot be used clinically before the LUTs are adjusted to the new situation. Adjustment of the LUT can be done in a short time by experienced personnel, without a dedicated measurement device.

OC-0438 The impact of a 1.5 T MR-Linac fringe field on neighbouring linear accelerators. T. Perik 1 , J. Kaas 1 , F. Wittkamper 1 1 The Netherlands Cancer Institute, Department of Radiation Oncology, Amsterdam, The Netherlands Purpose or Objective In our institution a clinical prototype of the MR- Linac(MRL)(Elekta AB, Stockholm, Sweden) was installed in an existing treatment room. The MRL, which has a field strength of 1.5 T, is neighboured by 3 clinical Elekta accelerators at a distance (Isocenter MRL to gun linac) of 7.5 and 5.5 and 11 meters. The peripheral magnetic field outside of the magnet core of the MRL, the fringe field, may influence the beam steering of accelerators in adjacent treatment rooms. This influence for a pre- clinical prototype was described by Kok et al. in 2009. The aim of this study is to investigate the influence of the significantly reduced fringe field of the clinical prototype on the beam steering of its neighbouring accelerators. Material and Methods A STARCHECK MAXI detector array (PTW Freiburg, Germany) was mounted on all the neighbouring accelerators with a frame that puts the array on isocentre height. An inclinometer was attached to the gantry to acquire a gantry rotation signal. For every available energy, two 360 degree arcs (clockwise and counter clockwise) were irradiated with a 40x40 field. A measurement of beam profile was acquired in movie mode at a frame rate 2.5 Hz. Beam symmetry (IEC) was determined for every frame. These measurements were done before and after ramping up the MRL magnet, and a 3 rd time after adjusting the look-up tables (LUT) which correct the beam steering by applying a gantry-angle dependent current to the steering coils (2R and 2T). These LUTs were adjusted using the accelerator internal monitor chamber. Results A change in beam symmetry as a function of gantry angle, before and after ramping the magnet, of up to 4% (Linac A) and 1% (Linac B) is observed, causing beam symmetry on both linacs to be out of tolerance (IEC 102%). Linac C did not show any significant change. Figure 2 shows the LUT before ramping the magnet (pre) and after adjustment (post) and the difference for Linac A for the 10 MV beam. After adjustment of the beam steering on Linac A and B, the symmetry was within tolerance for all gantry angles. Adjusting the LUTs took 1.5 hours per linac. Conclusion The influence of the MRL fringe field is less than described by Kok for the pre-clinical prototype, but does still influence the beam steering of the accelerators in adjacent treatment rooms. The LUTs of 2 accelerators, that were situated the closest to the MRL, but outside the

References: Kok et al., Phys. Med. Biol. 54 (2009) N409–N415 OC-0439 Treating patients with Dynamic Wave Arc: first clinical experience M. Burghelea 1 , D. Verellen 2 , J. Dhont 1 , C. Hung 3 , K. Poels 4 , R. Van den Begin 1 , M. Boussaer 1 , K. Tournel 1 , C. Jaudet 1 , T. Reynders 1 , T. Gevaert 1 , V. Simon 5 , M. De Ridder 1 1 Universitair Ziekenhuis Brussel- Vrije Universiteit Brussel, Department of Radiotherapy, Brussels, Belgium 2 GZA Ziekenhuizen- Sint Augustinus – Iridium Kankernetwerk Antwerpen, Radiotherapy Department,