ESTRO 2021 Abstract Book

S335

ESTRO 2021

Conclusion Daily CBCT is essential to mitigate set-up error and allows safe margin reduction. Although PTV5mm covers the majority of patients, using PTV7mm provides a more cautious approach. As this was retrospective analysis, enhanced bladder and rectal preparation could have further improved PTV coverage.

Poster highlights: Poster highlights 16: Radiobiology

PH-0435 ADAM17-dependent paracrine and intercellular communication in response to irradiation F. Tschanz 1 , V. Waller 1 , I. Telarovic 2 , M. Guckenberger 3 , M. Pruschy 4 1 Laboratory for Applied Radiobiology, Deptartment of Radiation Oncology, University Hospital Zurich, Zurich, Switzerland; 2 Laboratory for Applied Radiobiology, Department of Radiation Oncology, University Hospital Zurich, Zurich, Switzerland; 3 Radiation Oncology, Department of Radiation Oncology, University Hospital Zurich, Zurich, Switzerland; 4 Laboratory for Applied Radiobiology, Department of Radiation Oncology, Zurich, Switzerland Purpose or Objective The cellular response to ionizing radiation (IR) depends on tumor cell intrinsic and microenvironmental factors resulting in differential radiosensitivities among different cell types and tumors. Based on an IR-induced secretome analysis we recently demonstrated that, among other factors, the activity of the matrix metalloproteinase ADAM17 is upregulated following IR in a time and dose dependent way, correlating with a more aggressive cancer phenotype. Through its sheddase activity, several substrates are released from the cell surface serving e.g. as receptor ligands in an auto- and paracrine way. Here, we investigated the role of ADAM17 and its shed factors for the intercellular communication between tumor cells and cells of the tumor microenvironment in response to IR alone and in combination with an ADAM17-directed monoclonal antibody. Materials and Methods Using trans-well migration assays the migration of endothelial cells (ECs) towards the secretome derived from non-small cell lung cancer (NSCLC) cells was investigated in response to IR and in dependence of ADAM17 activity. ADAM17 was inhibited in attracting cancer cells using an inducible ADAM17-directed shRNA construct or an ADAM17-directed inhibitory antibody. Tumor-secreted factors relevant for EC migration towards attracting cancer cells were investigated by ELISA and immunocytochemical analysis. A subcutaneous and an orthotopic NSCLC-derived xenograft tumor model was employed to study efficacy- and vasculature-related endpoints including tumor microvessel size and density after IR in combination with ADAM17-inhibition. In vivo irradiation was performed using a small animal image-guided radiotherapy platform. Results Ionizing radiation-induced migration of ECs was abrogated through inhibition of ADAM17 in the attracting tumor cells. VEGF was identified as the major factor responsible for tumor cell-directed, IR-induced endothelial cell migration and was cleaved from tumor cells in an ADAM17-dependent way. Tumor vasculature- related in vivo endpoints showed that microvessel size and density was strongly decreased in response to the combined treatment modality of IR and ADAM17-inhibition but not by either treatment modality alone. Additionally, the combination treatment resulted in significant tumor growth reduction in the subcutaneous tumor model and long term tumor growth control in the orthotopic tumor model, monitored by sequential bioluminescence measurements. Conclusion Our data demonstrate that IR-induced, tumor cell-associated ADAM17-activity releases VEGF and thereby coordinates the communication between the tumor cell compartment and the tumor vasculature. The in vivo