Abstract Book

S927

ESTRO 37

Furthermore, the H 2 O was enriched with a total of >90% 18 –Oxygen (18-O) causing a nuclear reaction upon proton irradiation. As a result, the β+ emitter 18-Fluor (18-F) is created which can be measured by Positron-Emission- Tomography (PET). The gel was filled inside a PMMA cube (60x60x60mm³) and irradiated 24h after production. A pencil beam with a total dose of 50Gy with a mean range of 34.5mm was planned (syngo RT Optimize Treatment Plan, Siemens). The proton irradiation was performed at the Heidelberg Ion-Beam Therapy center (HIT) and the phantom was measured 3h post irradiation on a hybrid 3T MR-PET Scanner (Siemens Biograph mMR). One major advantage is that both PET and MR images are automatically co-registered by the scanner. A multi-spin echo sequence with 32 equidistant echoes (TE=22.5ms – 720ms) and a resolution of 1x1x1mm³ was used for the quantitative T 2 measurement. For registration purposes with the planned dose, a high-resolution MR image (0.5x0.5x0.5mm³) was also acquired. For the PET acquisition, a total of 2x10 6 counts were measured and a CT-based attenuation map of the phantom was additionally acquired to retrospectively calculate the 18- F distribution within the phantom. This is necessary as the MR-based attenuation map was not able to correctly recognize the PMMA cube. Results The phantom was able to produce a significant amount of 18-F within the gel and the PG showed a sharp dose gradient. Signal profiles of one slice along the particle track of TPS, MR- and PET data are shown in Fig 1. The range of the dose profile, the falloff position of the PG and the maximum of the PET signal were respectively: 34.5mm, 35.2mm and 26.5mm. The PG presents a direct correlation with the dose deposition, whereas the F-18 signal has to be indirectly correlated to the TPS.

onboard MRI of irradiated FOX gels in comparison with conventional techniques for magnetic resonance image- guided radiation therapy (MR-IGRT) applications. Material and Methods Square fields of 2x2, 3x3, 5x5, and 10x10 cm 2 were delivered to each detector with an integrated pre-clinical 1.5 T MRI – 7 MV linear accelerator system (MR-Linac, Elekta AB, Stockholm, Sweden). Cross-plane profiles were acquired with a PTW60019 microDiamond detector at 5 cm depth inside a water tank. Gafchromic EBT-3 films were irradiated at 5 cm depth inside a solid water phantom and inside a water tank. FOX gel sheets were manufactured in-house to 5-mm thickness and irradiated at 5 cm depth inside a solid water phantom. Gafchromic EBT-3 film and FOX gel sheets were optically scanned with an Epson 10000XL flatbed scanner pre- and post- irradiation at 24 hours and 1 hour, respectively. FOX gel sheets were MR imaged pre- and immediately post- irradiation in the MR-Linac with TR/TE = 500/20 ms and 0.31x0.31x3.00 mm 3 reconstructed voxels. Results The 2x2 cm 2 field sizes measured at the 50/50 penumbra were 0.0%, 0.1%, 13.3%, and -1.0% different from microDiamond measurements for EBT-3 film in solid water, film in water, FOX read out optically, and FOX read out with MRI, respectively. The 3x3 cm 2 field sizes were -0.2%, -0.7%, 6.6%, and -0.5% different, respectively. The 5x5 cm 2 field sizes were -0.2%, -0.4%, 4.3%, and 1.4% different, respectively. The 10x10 cm 2 field sizes were -2.8%, -1.2%, 1.5%, and -1.1% different, respectively. The overall shapes of the cross-plane profiles measured with FOX gel sheets optically and with MRI conformed with both microDiamond and film with some MR signal variations and optical artifacts due to some imperfections in the gel sheets resulting in non- uniform thickness. The FOX gel sheets measured with MRI agreed more consistently with microDiamond and film measurements compared to optical measured approximately 1 hour post-irradiation, which could be attributed to diffusion of the signal contributing to the beam profile.

Conclusion As our first results showed, the polymer-gel-based and 18-O enriched phantom has the potential of a 3D proton range verification system. It combines two processes that can independently be measured by means of MR- and PET-imaging. Ongoing work is focusing on Monte Carlo simulations for both dose-and 18-F distribution that will allow for a better comparison with the measured data. EP-1733 Beam profile measurements using semiconductor, MRI, and optical techniques for MR- IGRT systems H. Lee 1 , Y. Roed 1,2 , D. O'Brien 1 , G. Ibbott 1 1 The University of Texas MD Anderson Cancer Center, Radiation Physics, Houston, USA 2 University of Houston, Physics, Houston, USA Purpose or Objective Conventional methods of measuring beam profiles using microDiamond and Gafchromic film were compared to measurements using a Fricke-type radiochromic gel (FOX). FOX gel sheets were manufactured to allow for both MRI and optical read-out. The purpose of this study was to investigate the accuracy of beam profile measurements, focusing on cross-plane profiles which have the greatest magnetic field influenced shift, using