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ESTRO 37

planning. It will certainly take many years until all these issues have been addressed and solved adequately. The existing systems will therefore have to start with limited functionality like e.g. using gating instead of MLC tracking and have to rely on a limited number of MR images. With an increasing number of systems installed, the technology will be continuously developed further and has the potential to gradually shift the paradigms for adaptive RT. § 1. Lagendijk JJW and Bakker CJG 2000 MRI guided radiotherapy: a MRI based linear accelerator Proc. ESTRO Istanbul 19th Annual Meeting (Brussels: European Society for Theraputic Radiology and Oncology) vol 56, suppl. 1, pp S1–S255 § 2. Mutic S and Dempsey JF 2014 The ViewRay system: magnetic resonance-guided and controlled radiotherapy Semin. Radiat. Oncol. 24 196–9 § 3. BW Raaymakers, IM Jürgenliemk-Schulz, GH Bol et al., First patients treated with a 1.5 T MRI-Linac: clinical proof of concept of a high-precision, high-field MRI guided radiotherapy treatment. Phys. Med. Biol. 62 (2017) L41–L50. 4. S Yaghoobpour, TK Wachowicz, BG Fallone: A non-axial superconducting magnet design for optimized patient access and minimal SAD for use in a Linac-MR hybrid: proof of concept Phys. Med. Biol. 62 (2017) N147–N160. SP-0547 Multiparametric MR-PET for differentiation of residual disease vs treatment-induced inflammatory changes M. Becker 1 1 Hôpitaux Universitaires de Genève, Division of Radiology- Department of Imaging and Medical Informatics, Geneva, Switzerland Abstract text The purpose of this lecture is to provide a simplified, systematic approach on how to detect tumor recurrence on MR/PET examinations of patients treated for head and neck squamous cell carcinoma. First, the relevant imaging findings of post-therapeutic expected tissue alterations with a special focus on their temporal relationship are discussed. Then a brief discussion of common complications affecting the soft tissues, vasculature and bony structures will follow. A systematic review will include key MRI, diffusion-weighted imaging (DWI) and PET features of osteoradionecrosis, soft tissue necrosis, neck fibrosis and scar tissue mimicking tumor recurrence. Typical radiologic findings of tumor recurrence will be discussed with an emphasis on the early detection of lesions, their appearance on MR-PET with DWI and the added value and complementarity of multiparametric information extracted from hybrid imaging techniques. The potential pitfalls of post- therapeutic image interpretation and how to avoid them taking into consideration multiparametric data will be addressed. Results of combined MR-PET evaluations for the correct staging of recurrent tumors will be presented based on histologically verified data including additional long-term follow-up. Major emphasis will be put recently published research results and on quantitative information that can be easily extracted in clinical routine. The complementarity of apparent diffusion coefficient (ADC) values and of FDG - based standardized uptake values (SUV) will be discussed, as well as the newly proposed multiparametric approach for the assessment of post-radiotherapy necks based on concordant and discordant evaluations with MRI, DWI and FDG-PET. Pitfalls of image interpretation will be addressed and how to avoid them based on complementary multiparametric information. New quantification tools, including histogram based analysis and textural features will be briefly addressed.

Symposium: Biomarker driven hypoxic modification

SP-0548 Genomic biomarkers for selection of patients to hypoxia modification H. Lyng 1 1 Lyng Heidi, Department of Radiation Biology, Oslo, Norway Abstract text Recent advances in the cancer genomic field and understanding of the molecular response to hypoxia, have led to promising biopsy based biomarkers to select patients who are most likely to benefit from hypoxic modifications. A considerable focus has been on gene expression signatures as biomarkers, which measure the transcriptional response of a tumors to its hypoxic environment. In this talk, I will review recent advances in the discovery and validation of such biomarkers. I will further discuss how to translate the biomarkers into clinical practice and face challenges related to tissue sampling and intratumor heterogeneity. SP-0549 Multi-parametric functional imaging for patient stratification and hypoxia modification D. Thorwarth 1 1 University Hospital Tübingen, Section for Biomedical Physics- Department for Radiation Oncology, Tübingen, Germany Abstract text Tumor hypoxia is known to be a major cause for radiation resistance. Several functional imaging methods such as positron emission tomography (PET), magnetic resonance imaging (MRI) or combined PET/MRI have the potential to visualize functional and biological properties of tumor tissue. To discuss the ability of different imaging techniques like [18F]-FMISO, [18F]-FAZA and [18F]-FDG PET, diffusion weighted (DW) or dynamic contrast enhanced (DCE) MRI for patient stratification, methodological aspects as well as results of recent studies will be summarized. Once the prognostic value of an imaging biomarker is proven, it may be used as a basis for hypoxia modification during radiotherapy either by hypoxic sensitizers or by modifying the radiation dose instructed by imaging. As an example for the latter scenario, results of a randomized trial investigating the effectiveness of hypoxia dose painting in head-and-neck cancer based on [18F]-FMISO PET will be presented. Recent studies have shown that multi-parametric functional imaging in terms of hypoxia PET and DW or DCE MRI allow the stratification of patients according to RT outcome. Thus, these imaging modalities allow the measurement of three-dimensional resistance maps, which may in the future serve as a basis for hypoxia modifying RT SP-0550 Clinical trials J. Alsner 1 1 Aarhus University Hospital, Department of Experimental Clinical Oncology, Aarhus C, Denmark Abstract text A number of different clinical trials have investigated the possibility of improving outcome of radiotherapy by hypoxic modification. The methods have include increasing oxygen availability, using hypoxic cell radiation sensitizers (such as nimorazole and hyperthermia), using hypoxic cell cytotoxins (such as tirapazamine, targeting tumour vasculature, and increasing radiation dose to resistant tumours or sub- populations within a tumour. Hypoxia displays a large