ESTRO 2020 Abstract Book

S237 ESTRO 2020

were heat cured to allow for crosslinking of the elastomer. Silicone oil/elastomer combinations, elastomer, and neat silicone oils were measured using an inversion recovery pulse sequence on a 3T Siemens MRI and 0.5T benchtop relaxometer. Results Silicone sensor format can be highly customized based on the desired application (Figure 1A). MRI acquisition parameters were selected such that the signal from the elastomer is substantially suppressed (Figure 1B). This allows the relaxation time of the sensor to be processed with a single exponential fit equation. Silicone oil relaxation time is a function of chain length (Figure 1C). In addition to sensor components, the relaxation time and hence oxygen sensitivity are also a function of the relative ratios between silicone oil and elastomer (Figure 1D). This ratio also impacts the mechanical properties of the sensor.

contrast that obstruct the use of stereotactic body radiation therapy (SBRT). Hybrid MR linac systems, like the Unity MR-linac (Elekta AB, Stockholm, Sweden) open up the possibility for MR-guided radiation therapy (MRgRT) where tumour and surrounding organs are visible. Metallic implants, however, induce large geometric distortions that possibly impair treatment quality. We propose a clinical workflow to determine whether or not a metallic hip prosthesis patient is eligible for SBRT on the MR-linac by measuring the severity of the field distortions on the planning MRI. Material and Methods Magnetic field maps convey information on the expected geometric distortions in sequences with known bandwidth. To this end, magnetic field maps were created on a 3T Ingenia MR (Philips Healthcare, Best, Netherlands), which had a range of ±1.70ppm (±217Hz on 3T). As distortions outside this range result in phase wraps, field maps were visually checked to contain no phase wraps inside the prostate. The bandwidth of the anatomical sequence on the MR-linac was 4.87ppm/mm. The prostate contouring and matching of MR and CT images was performed in Mirada (Mirada Medical, Oxford, England). Using Mirada, statistics on field inhomogeneities in the prostate were converted to expected geometric distortions of the sequence used on the MR-linac. Geometric distortions ≤1mm inside the prostate, sourced by field inhomogeneities, were deemed acceptable for plan adaptation on the MR-linac to treat with SBRT. After informed consent, a patient with a metallic hip prosthesis was included in the study. Results were qualitatively compared with field maps (range ±1.71ppm) acquired on the MR-linac prior to treatment. Results A T2w image and field map acquired on the MR simulation scanner are shown together with the planning CT in Figure 1. The magnetic field in the prostate varied from -0.79— 0.63ppm with a median of -0.07ppm and standard deviation of 0.20ppm, indicating a maximum shift of 0.29mm inside the prostate due to field inhomogeneities. In Figure 2, the T2w image and field map acquired on the MR-linac prior to the first treatment fraction are shown at the position of maximum field distortions. Phase wrapping in the prostate did not occur for any of the field maps acquired prior to treatment.

Conclusion Oxygen sensitive silicones are highly customizable, and formulations offer a trade-off between sensitivity, mechanical properties, and data acquisition time. The ideal material formulation for a given application should be selected based on the required sensitivity, whether the sensor is load bearing, and the allowed acquisition time. We present a comprehensive design and analysis of silicone sensors for clinical oxygen sensing applications. Acknowledgements: Work is supported by a Bridge Project Expansion Grant (The Koch Institute for Integrative Cancer Research/Dana Farber Cancer Center) and Frontier Research Grant (The Koch Institute for Integrative Cancer Research ). J. Tokuda is supported by the Image Guided Therapy Center (NIH P41EB015898). G. Ekchian is supported by the Koch Institute Quinquennial Cancer Research Fellowship and the Kavanaugh Translational Innovation Fellowship. 1. Liu, V. H. et al., Proc. Natl. Acad. Sci. (2014). 2. Kodibagkar, V. D. et al., Magn. Reson. Med. (2006). PD-0433 Clinical workflow for treating patients with a metallic hip prosthesis on the MR-linac R. Keesman 1 , E. Van der Bijl 1 , T. Janssen 1 , T. Vijlbrief 1 , F. Pos 1 , U. Van der Heide 1 1 Netherlands Cancer Institute, Radiotherapy, Amsterdam, The Netherlands Purpose or Objective Pelvic CBCT-guided radiation therapy of patients with metallic hip prosthesis suffer from artefacts and low

Figure 1: prostate on CT (A), T2w TSE (B), and field map (C) on MRsim.

Figure 2: Anatomical image (A) on MR-linac with field map (B). Conclusion We proposed a workflow to decide whether or not a patient with metallic hip implant is eligible for SBRT on the MR-linac. For the patient included in this study, field variations inside the prostate were low enough (lower than the water-fat shift at 3.4ppm) that treatment proceeded. We showed that the decision of whether or not to treat on the MR-linac can be made by a medical physicist based on