ESTRO 2020 Abstract book

S1062 ESTRO 2020

Conclusion These data will contribute to the integration of MRI based NPs quantification into specific radiotherapy dosimetric plans and confirm the theragnostic properties of AGuIX at clinically relevant concentrations.

Conclusion Thresholds for a number of parameters, including dose and dose rate, have been determined for a feasible FLASH effect caused by oxygen depletion, which can be used as a starting point for experimental verification. The development of a full mechanistic understanding of the FLASH effect is essential for its translation into a clinical setting. 1. Favaudon et al. Ultrahigh dose-rate FLASH irradiation increases the differential response between normal and tumor tissue in mice. Sci Transl Med, 2014. 6 (245): 245ra93. PO‐1811 The prognostic value of CD8+ infiltration and PD‐L1 expression in salivary gland cancers N. Kesar 1 , R. Winkelmann 2 , F. Rödel 1 , J. Oppermann 1 , S. Balster 3 , S. Ghanaati 4 , C. Brandts 5 , D. Martin 1 , C. Rödel 1 , P. Balermpas 6 , J. Von der Gruen 1 1 University of Frankfurt, Department of Radiotherapy and Oncology, Frankfurt, Germany ; 2 University of Frankfurt, Dr. Senckenbergisches Institute of Pathology, Frankfurt, Germany ; 3 University of Frankfurt, Department of Otorhinolaryngology, Frankfurt, Germany ; 4 University of Frankfurt, Department of Oral- Maxillofacial and Facial Plastic Surgery, Frankfurt, Germany ; 5 University of Frankfurt, Department of Medicine- Hematology/Oncology, Frankfurt, Germany ; 6 University of Zurich, Department of Radiation Oncology, Zurich, Switzerland Purpose or Objective Salivary gland carcinomas (SGC) account for 1-6% of all head and neck tumors with 20 different histological subtypes. Surgical resection is the standard treatment for SGC, followed by postoperative radiotherapy (PORT) in case of high-risk factors. Exploration of the immunological microenvironment of these tumors could facilitate a more individualized treatment approaches in the future. We examined the prognostic value of CD8+ tumor infiltrating lymphocytes (TIL) and programmed death ligand 1 (PD-L1) expression in a single center patient cohort. Material and Methods We retrospectively analyzed the clinical data of 127 patients treated at the University Hospital of Frankfurt between 1981 and 2017. Patients were treated by surgery alone (n=65), surgery followed by PORT (n=56) or primary radiotherapy (RT, n=6) alone. Furthermore, CD8+ TIL and PD-L1 status was assessed via immunohistochemistry (IHC) for a subset of n=84 patients. IHC data analysis was performed by QuPath Software V0.2.0 (Belfast, Northern Ireland). Clinico-pathological characteristics and immune marker expression were correlated with progression-free and overall survival (PFS, OS). Poster: Radiobiology track: Immuno‐radiobiology

Poster: Radiobiology track: Tumour microenvironment

PO‐1810 Determining the boundaries of the FLASH effect B. Rothwell 1 , N. Kirkby 1,2 , M. Merchant 1,2 , A. Chadwick 1,2 , M. Lowe 1,3 , R. Mackay 1,3 , K. Kirkby 1,2 1 University of Manchester, Division of Cancer Sciences, Manchester, United Kingdom ; 2 The Christie NHS Foundation Trust, Manchester Academic Health Science Centre, Manchester, United Kingdom ; 3 The Christie NHS Foundation Trust, Christie Medical Physics and Engineering, Manchester, United Kingdom Purpose or Objective Ultra-high dose rate, or ‘FLASH’, radiotherapy is an area of worldwide interest, after studies have demonstrated normal-tissue-sparing advantages over conventional irradiation, while apparently retaining tumour control 1 . Currently, there is a distinct lack of understanding of the underlying mechanisms behind this effect. Many early reports have discussed radiation-induced oxygen depletion as a possible explanation for normal-tissue sparing, but this still needs further investigation. There is a wealth of knowledge of the radiobiological effects of hypoxia which can be used to explore this hypothesis further. This work is aimed at determining whether oxygen depletion can feasibly explain the normal-tissue-sparing effect of FLASH. Material and Methods Cellular automata techniques have been employed to solve a model of oxygen diffusion and consumption in cells, and to investigate how these are affected by ultra-high-dose- rate irradiation. Results A biologically-relevant parameter space has been identified in which, to a first approximation, FLASH radiation depletes sufficient oxygen for a transient hypoxic response. An example case for three model parameters is given in Figure 1.