ESTRO 2023 - Abstract Book

S2017

Digital Posters

ESTRO 2023

Conclusion The tumor model expressing endogenous levels of cGAS fluorescent protein provides an unbiased tool in investigating the role of cGAS localization in cGAS/STING activation in the tumor microenvironment. These studies will yield new data on the early events following the level of cGAS re-localization and immune cell recruitment and activation and help to understand the impact of different RT treatments schedules on the immune system.

PO-2242 HDR-brachytherapy increases PD-L-1 immune expression on prostate cancer (NCT04247217)

B. Vanneste 1,2 , E. Van Limbergen 3 , L. Dubois 4 , L. Wieten 5 , M. Aarts 6 , J. Van Roermund 7 , T. Marcelissen 8 , X. Li 9 , I. Samarasks 10 , D. De Ruysscher 1 1 Maastro, Department of Radiation Oncology, Maastricht, The Netherlands; 2 Ghent University Hospital, Department of Human Structure and Repair; Department of Radiation Oncology, Ghent, Belgium; 3 MAASTRO, Radiation Oncology , Maastricht, The Netherlands; 4 MUMC, M-Lab, Department of Precision Medicine, GROW , Maastricht, The Netherlands; 5 MUMC, Department of Transplantation Immunology, Tissue Typing Laboratory, GROW School for Oncology and Developmental Biology, Maastricht, The Netherlands; 6 MUMC, Department of Medical Oncology, GROW - School for Oncology and Developmental Biology, Maastricht, The Netherlands; 7 MUMC, Department of Urolog, Maastricht, The Netherlands; 8 MUMC, Department of Urology, Maastricht, The Netherlands; 9 MUMC, Department of Pathology, , Maastricht, The Netherlands; 10 MUMC, Department of Pathology, Maastricht, The Netherlands Purpose or Objective Programmed cell death protein 1 (PD-1) is an inhibitory receptor that is expressed by all T cells during activation. The expression of the PD-1 receptor and its ligand, PD-(L)-1, is also observed in several solid cancers [1]. However, the expression levels as well as clustered T-cell infiltrates in prostate cancer are very low suggesting prostate cancer as an immune ‘’cold’’ tumor [2]. It has been suggested that radiotherapy might enhance PD-1 and PD-L-1 expression [3,4]. Our hypothesis is that an increased PD-L-1 is induced by high-dose-rate (HDR)-brachytherapy. Materials and Methods Prospective study of repetitive biopsies from 10 patients with local recurrence in previously irradiated prostate which are selected for salvage HDR brachytherapy consisting of 3 x 10 Gy HDR. Biopsies were taken at 4 different time points (before (T0) and after the 1st fraction (T1); before the 2nd (T2) and 3rd fraction (T3) of the salvage treatment). PD-1/PD-L-1 expression was detected by immunohistochemical (IHC) analysis independently by 2 pathologists (XL, IS). The slides were scored semi-quantitatively for PD-1 : score 0: <1% tumor cells; 1: 1-5%; 2: >5%. PD-L-1 0: <1%; 11: 5%; 2: >5%. PD-L-1 status inflammatory cells: score 0: no staining; 1: <5%; 2: 5-50%; 3: >50%. Immunophenotyping of tumor infiltration lymphocytes and stromal inflammatory cells was investigated. Results None of the patients showed PD-1 expression on the tumor cells or benign prostatic tissue at T0; however, PD-1-positive inflammatory cells were identified in at least four patients at each time point. At T0, there was no PD-L1 expression in tumor cells or inflammatory cells. After the first treatment (T1), PD-L-1 expression was identified on cancer cells in 3/10 patients. At T2-3, PD-L-1 positivity of tumor cells was observed in 9/10 patients. Inflammatory cells showed variable expression of PD-L-1 regardless of the specific time points. There was no association between PD-1/PD-L-1 and Gleason score.