ESTRO 37 Abstract book

S928

ESTRO 37

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 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 Beam profile measurements of FOX gel sheets using the onboard MRI of the MR-Linac were comparable to microDiamond, EBT-3 film, and FOX gel sheets read out optically. Future work will include improving the fabrication of the gel sheets to achieve uniform sheet thickness and acquiring MR images with more averaging. The FOX gel sheets demonstrated their usefulness as an alternative to film and other conventional detector measurements for beam profile analysis using onboard MR imaging in MR-IGRT systems. EP-1734 Photo- and proto- acoustic dose monitoring in radiosurgery and proton beams D. Sanchez-Parcerisa 1 , O. Giza 1 , J. Camacho 2 , V. Sanchez- Tembleque 1 , S. Avery 3 , J. Udias 1 1 Universidad Complutense de Madrid, Departamento de Fisica Atomica- Molecular y Nuclear, Madrid, Spain 2 CSIC, ITEFI, Madrid, Spain 3 University of Pennsylvania, Department of Radiation Oncology, Philadelphia, USA Purpose or Objective This work describes the development of an acoustic sensitive device that can be used for range monitoring and dosimetry of clinical beams. It comprises the optimization, performance and analysis of a set of experimental measurements in radiosurgery and proton beams. Material and Methods The experimental setup consists of several ultrasonic transducers of different resonance frequencies and bandwidths, a combination of preamplifiers and differential amplifiers with filtered outputs and a digital oscilloscope. Simulations of the experimental setup were