ESTRO 37 Abstract book

S1205

ESTRO 37

EP-2176 Compatibility test of a newly-designed patient transfer system with a 1.5T MR-simulator O.L. Wong 1 , K.F. Cheng 2 , J. Yuan 1 , Y.H. Zhou 1 , G. Chiu 2 , S.K. Yu 1 1 Hong Kong Sanatorium & Hospital, Medical Physics and Research Department, Hong Kong, Hong Kong SAR China 2 Hong Kong Sanatorium & Hospital, Department of Radiotherapy, Hong Kong, Hong Kong SAR China Purpose or Objective Patient transfer system using hover technique has been introduced to accommodate the use of MRI for patient positional verification prior to each radiotherapy treatment fraction. The previous design of such a system isolates all electronics outside the MRI scan room, and only the hose passes through the MRI scan room and connects to the hover board. Recently, a new design integrates the electronics and power button at the hose end in the MRI room for better user friendliness and efficiency, while raising a potential concern on its possible influence on MRI-sim image quality due to electromagnetic interferences. In this study, we aim to test the compatibility of this newly-designed patient transfer system with a 1.5T MR-simulator (MR-sim) and assess whether this new design would affect the MR-sim image quality due to electromagnetic interferences. Material and Methods An ACR MRI phantom was scanned 5 times each under two settings: with the new-designed electronics-integrated hose connected to the hover board laid on the patient table of the MR scanner (hose-in), or leaving all the new- designed system outside the MR scan room (hose-out). Sagittal localizer (TE/TR = 20/200ms), axial T1 (TE/TR = 20/500 ms) and T2 scans (TE1/TE2/TR = 20/80/2000ms) were acquired (NEX = 1). Percent-signal ghosting (PG) and image intensity uniformity (PIU) were conducted following ACR guidelines. SNR was calculated using the image of the homogeneous portion of the phantom. The radiofrequency noise images were also acquired 5 times for each setting (by setting a 4-channel and a 18-channel array coils at receive only mode for 6 minutes). The noise image was inspected to look for the noticeable artifacts such as streaks, dots or patterns. A rank-sum test was performed to compare PG, PIU and SNR difference between the two settings. Results Similar PG, PIU and SNR were obtained between the hose-in and hose-out settings (PG: 0.023±0.003 (hose-in), 0.024±0.010 (hose-out), p=0.69; PIU T1: 91.0±0.4 (hose- in), 90.8±1.0 (hose-out), p=0.84; PIU T2=90.8±0.5 (hose- in), 90.7±1.0 (hose-out), p=1; SNR T1: 310±57 (hose-in), 323±44 (hose-out), p=0.42; SNR T2: 361±37 (hose-in), 342±55 (hose-out), p=0.69). No noticeable artifacts were observed for all noise images. Conclusion No degrade in image quality and no RF interference were noted when the newly-designed electronics-integrated hose in the MR scanner room. The newly-designed patient transfer system is compatible with 1.5T MR-simulator, and may smooth the procedure of patient transportation. EP-2177 dosimetric evaluation of carbon-ion beam grid therapy of brain tumors T. Tsubouchi 1 , A. Valdman 2 , A. Siegbahn 3 1 Osaka University Graduate School of Medicine, Department of Radiation Oncology, Osaka, Japan 2 Karolinska University Hospital, Department of Oncology and Pathology, Stockholm, Sweden 3 Stockholm University, Department of Medical Radiation Physics, Stockholm, Sweden Purpose or Objective Radiotherapy with beam grids has been performed on a small scale for more than a century. Research on grid therapy using arrays of minibeams or microbeams have

been carried out during the past two decades. In the pre- clinical trials made for this kind of grid therapy, it has been found that the normal tissue is tolerating irradiations up to remarkably high peak doses if the valley doses in-between the beam grid elements are maintained at low levels. It has been hypothesized that such a micro- and mini-beam grid therapy could be useful for CNS treatments. In this study, we made a dosimetric evaluation with Monte Carlo simulations of different irradiation geometries of potential use for carbon-ion grid therapy of brain tumors. Material and Methods The PHITS Monte Carlo code was used for the simulations. Beam elements of different widths in the interval 0.5-3.0 mm were used to build the grids. A beam element width of 0.5 mm has been considered for minibeam therapy in the past, whereas a width of 3.0 mm could be produced and delivered with more technical ease, while still providing a certain increase in the tolerance doses of the risk organs. The simulated carbon-ion beam elements were unidirectional and of rectangular shape, with a Gaussian spatial distribution and a spread in kinetic energy of 1%. Dose distributions were then calculated in a head phantom for different irradiation setups, i.e . unidirectional-grid setup, interlaced-crossfiring setup, orthogonal-crossfiring setup and a combination of the interlaced- and orthogonal-crossfiring setups. Thereafter, the beam-element separation was selected based on an optimality criterion. Realistic treatment simulations were then performed by simulating the dose distributions produced in the human head by carbon-ion grid irradiations, using CT image sets from real patients. Results The carbon-ion beam elements, constituting the grid, remained narrow down to the depth of the target volume. Highly conformal dose distributions could be produced. With the unidirectional-grid irradiation, it was not possible to deliver a high minimum dose to the target volume while keeping the valley dose low in the normal tissue located close to the target volume. When interlaced crossfiring was employed, the treatment objectives, i.e. a high minimum target dose, combined with low valley doses throughout the normal tissue, could In this study we proposed a new carbon-ion grid therapy method. The specific physical interaction properties of carbon-ion beams were found to provide unique opportunities to explore the high tissue tolerance to radiation-grid irradiations to create new forms of radiotherapy. An irradiation setup based on interlaced- crossfiring was in this study found necessary to produce low valley doses throughout the normal tissue irradiated, which is known to be of importance to maintain the toxicity of grid treatments at low levels. EP-2178 Use of the PTW Starcheck Maxi MR for commissioning a 1.5 Tesla MR Lina J. Berresford 1 , J. Agnew 1 , G. Budgell 1 1 The Christie NHS Foundation Trust, CMPE, Manchester, United Kingdom Purpose or Objective MR Linacs are becoming commercially available but most conventional dosimetry equipment is not suitable for use in their strong magnetic fields. The purpose of this work was to test the new MR conditional PTW Starcheck Maxi MR array to determine its suitability for use in the Elekta Unity 1.5T MR Linac and to assess how to use it for commissioning the MR Linac. Material and Methods Measurements were made with the array both on a conventional linac and the Elekta Unity MR Linac to check be reached. Conclusion