ESTRO 2022 - Abstract Book

S697

Abstract book

ESTRO 2022

1 University Hospital Heidelberg, Department of Radiation Oncology, Heidelberg, Germany; 2 National Center for Radiation Research in Oncology (NCRO), Heidelberg Institute for Radiation Oncology (HIRO), Heidelberg, Germany; 3 German Cancer Research Center (DKFZ), Department of Medical Physics in Radiation Oncology, Heidelberg, Germany; 4 Heidelberg Ion- Beam Therapy Center, HIT, Heidelberg, Germany; 5 Heidelberg Ion-Beam Therapy Center , HIT, Heidelberg, Germany; 6 Clinical Cooperation Unit Radiation Oncology, German Cancer Research Center (DKFZ), Heidelberg, Germany; 7 core center Heidelberg, German Cancer Consortium (DKTK), Heidelberg, Germany Purpose or Objective MR guided particle radiotherapy holds the potential to increase therapy precision by daily monitoring of tumour and organ at risk location, size and shape. The ability to gate the beam to breathing motion has been shown to be beneficial in MR guided photon therapy (1,2) and is even more important for particle radiotherapy to ensure precise location of the Bragg peak. Hitherto, no commercial systems are available, though it has been reported on experimental set-ups (3). In this work we report on the installation of an open MR scanner in front of an ion beamline and the analysis of simultaneous operation of MR imaging and particle irradiation. Materials and Methods An MR system consisting of a 0.25 T orthopaedic device featuring a permanent magnet with a vertical B 0 field (Esaote, Italy) within a radiofrequency (RF)-shielded cabin is positioned in front of a horizontal ion beamline (Fig 1a). The geometrical relationship of MR and beam isocentre is verified using an external laser system. 12 C ions were used due to their higher mass in comparison to protons. Two sets of experiments were performed: (i) homogeneous spherical phantom; 3D GRE (TE/TR 14/28ms, spatial res. (0.8x0.8x0.7)mm 3 ); irradiation of a central spot with beam energies from 89MeV to 430MeV; calculation of the centre of gravity for all slices, SNR analysis; (ii) geometrical phantom; 2D GRE (TE/TR 10/500ms, spatial res. (0.4x0.4x5)mm 3 ); irradiation of a scanned 2D grid with step size of 5mm; line profile analysis. Results Fig. 1c,d illustrate the range of the x- and y-coordinate of the centre of gravity for all slices for 5 sequential baseline measurements and all applied beam energies. The value range of the measurements with simultaneous spot irradiation is comparable to that of the baseline and the overall range is in the sub-pixel regime, meaning that there is no shift. The results of the SNR analysis in Fig. 1b show that there is no change in signal levels when the phantom is irradiated during image acquisition. The line profiles in Figure 2b covering small structures do not show any offset or shift when comparing the baseline to the acquisition with active 2D beam scanning.

Figure 1: (a) photograph of the 0.25 T MR scanner in front of the beam line. (b) SNR values. (c) x-coordinates of the centre of gravity. (d) y-coordinates.

Figure 2: Line profile along the green line with elements in the sub-millimetre range.

Conclusion Our experiments revealed that the presented setup of an MR scanner in front of a horizontal ion beam line enables high quality MR imaging while simultaneously irradiating a target using active beam scanning. One limitation of this study is the