ESTRO 36 Abstract Book

S131 ESTRO 36 _______________________________________________________________________________________________

Purpose or Objective Currently the first MR-Linac systems (Elekta AB, Sweden) are being installed at several clinical institutions worldwide. In order to introduce this new technology safely into clinic it is imperative that the imaging component is rigorously tested. In this work we present a comprehensive set of tests for clinical acceptance and commissioning of a high field (1.5 Tesla) hybrid MR-Linac (MRL) system. Guidelines as well as initial results are presented. Material and Methods The complete test protocol consists of a series of general MRI hardware tests, radiotherapy specific test, and hybrid tests. General MRI hardware and radiotherapy specific tests include established tests to allow comparison with diagnostic and radiotherapy planning 1.5T MRI systems. The hybrid tests are unique to the design and application of the MRL and were developed in on a non-clinical MRL prototype (U1) and performed on the clinical prototype (U2) after installation in September 2016. Hybrid tests include: Spurious Noise check, MR-MV alignment, Gantry influence on B0 homogeneity, and radiation effects. Finally, sequence specific tests are included to ensure geometric fidelity of the MRI protocols that will be performed during clinical use. Results Table 1 lists the tests included in the commissioning protocol, subdivided into six sections. The first four sections contain quality control (QC) tests, which test the individual components of the system. The final two sections include quality assurance (QA) measurements, which probe overall image quality, and thus test a combination of several components. The QC measurements serve as a characterisation of the system, whereas the QA tests serve as a null measurement before the system is introduced into clinic. Fig. 1 shows the results from two independent geometric fidelity measurements, with a) the vendor provided geometric fidelity phantom, and b) a third party phantom [Modus Medical Devices Inc, London, ON, Canada]. Geometric distortions were found to be 0.94mm, 1.82mm, and 2.35mm for diameter spherical volume (DSV) of 300mm, 350mm, and 400mm, respectively. The maximum geometric distortion occurred at the edge of the DSV. Further tests revealed that the influence of the gantry on magnetic field homogeniety was negligible (<0.1 µT) and RF flip angle accuracy was within spec and comparable to our MRI-RT 1.5T Philips Ingenia scanner. Finally, the ACR resolution, geometry, and low contrast detectability tests all passed the ACR criteria using diagnostic imaging protocols (i.e., without additional averaging of the data). No spurious noise was observed during operation of the Gantry and Linac, suggesting good decoupling of the two systems. Conclusion A comprehensive acceptance and commissioning protocol was developed and performed for clinical acceptance of the first 1.5T MR-Linac within the Atlantic consortium. Overall system performance is extremely similar to our diagnostic 1.5T MRI scanner, used for radiotherapy planning. Hybrid tests showed good decoupling of the two systems.

OC-0258 Investigation of magnetic field effects on 3D dosimeters for MR-IGRT applications H.J. Lee 1,2 , Y. Roed 1,3 , S. Venkataraman 1 , M. Carroll 1,2 , G. Ibbott 1 1 The University of Texas MD Anderson Cancer Center, Radiation Physics, Houston, USA 2 University of Texas at Houston, Graduate School of Biomedical Sciences, Houston, USA 3 University of Houston, Physics, Houston, USA Purpose or Objective Conventional QA tools lack the ability to report changes in volumetric dose distributions and discrepancies out of the plane of measurement. The strong magnetic field in the integrated pre-clinical 1.5T MRI – 7MV linear accelerator system (MR-linac, Elekta AB, Stockholm, Sweden) influences secondary electrons resulting in changes in dose deposition in three dimensions. The purpose of this study was to investigate strong magnetic field effects on 3D dosimeters for magnetic resonance image-guided radiation therapy (MR-IGRT) applications. Material and Methods There are currently three main types of 3D dosimeters: radiochromic plastic, radiochromic gel, and polymer gel. For this study, the following three dosimeters were used: PRESAGE®, FOX (Fricke-type), and BANG TM (MGS Research Inc). For the purposes of batch consistency, an electromagnet was used for same-day irradiations with