ESTRO 2021 Abstract Book

S670

ESTRO 2021

models: in the HPV- tumor model FaDu (TCD50 59.1Gy vs. 39.8Gy; p<0.005), the HPV+ tumor model UM-SCC-47 (TCD50 38.0Gy vs. 27.5Gy; p<0.005) and the HPV+ model UPCI-154 (TCD50 34.9Gy vs. all tumors in all dose groups controlled after RCTx). In contrast, the HPV- model SAS showed significantly better local tumor control after RTx alone, however dose-response in this model was very shallow with pronounced scatter of the data (TCD50 90.0Gy vs. 151.0 Gy; p<0.005). For Cal 33 (HPV-; TCD50 50.0Gy vs. 50.2Gy; p>0.05) and UD-SCC-2 (HPV+; TCD50 56.9Gy vs. 49.7Gy; p=0.19) no difference was observed between the treatment arms (Fig.1). The enhancement ratios for TCD50 after RTx vs. RCTx were 1.52 (FaDu), 0.64 (SAS), 0.96 (Cal33), 1.38 (UM- SCC-47), 1.14 (UD-SCC-2) and 1.3 (UPCI-154). In order to evaluate the overall efficacy of RTx vs RCTx a cumulative analysis was performed taking into account the three HPV- and HPV+ cell lines. In HPV- tumor models RTx alone showed a TCD50 of 63.6 Gy vs TCD50 of 59.6 Gy when treated with RCTx (p=0.452). Whereas a statistically significant improvement was observed in the the HPV+ tumor models receiving Cisplatin: TDC50=40.6 Gy and TDC50= 31.4 Gy (p=0.001) in the arm receiving RTx and RCTx respectively (Fig.1b). Growth delay was similar or prolonged in all six tumor models after RCTx compared to RTx. Statistical significance was reached for FaDu (HPV-; GVD2 p=0.01), UM-SCC-7 (HPV+; GDV2 and 5 p<0.005) and UPCI-154 (HPV+; GDV2 and 5 p<0.005).

Conclusion Treatment response after RCTx vs. RTx is heterogenous both in HPV- and HPV+ HNSCC, with some suggestion that cisplatin enhances local tumor control more consistently in HPV+ than in HPV- tumors. Our observations call for additional biomarkers for personalized combined radiation treatments on the basis of HPV stratification. PD-0835 Bone-tropic circulating tumor cell population in mCRPC patients under ablative radiotherapy D. Klusa 1,2 , F. Lohaus 3 , H. Neubauer 4 , A. Franken 4 , M. Rivandi 4 , B. Polzer 5 , D. Husman 6 , M. Kücken 7 , T. Hölscher 3 , I. Kurth 8 , M. Krause 9,10,2,3,11 , A. Dubrovska 10,2,11 , M. Baumann 12,8 , C. Peitzsch 9,2,11 1 National Center for Tumor Diseases (NCT), Partner Site Dresden, Germany: German Cancer Research Center (DKFZ), Heidelberg, Germany , Faculty of Medicine and University Hospital Carl Gustav Carus, Technische Universität Dresden, Dresden, GERMANY, and; Helmholtz-Zentrum Dresden – Rossendorf (HZDR), Dresden, Germany; 2 OncoRay – National Center for Radiation Research in Oncology, Faculty of Medicine and University Hospital Carl Gustav Carus, Technische Universität Dresden, Helmholtz-Zentrum Dresden - Rossendorf, Dresden, Germany; 3 Faculty of Medicine and University Hospital Carl Gustav Carus, Technische Universität Dresden, Department of Radiotherapy and Radiation Oncology, Dresden, Germany; 4 Heinrich-Heine-University of Düsseldorf, Department of Gynecology and Obstetrics, Düsseldorf, Germany; 5 Fraunhofer-Institute for Toxicology and Experimental Medicine, Division of Personalized Tumor Therapy, Regensburg, Germany; 6 denovoMATRIX , GmbH, Dresden, Germany; 7 Center of Information Services and High-Performance Computing (ZIH), Technische Universität, Dept. for Innovative Methods of Computing, Dresden, Germany; 8 German Cancer Research Center (DKFZ), Division of Radiooncology / Radiobiology, Heidelberg, Germany; 9 National Center for Tumor Diseases (NCT), Partner Site Dresden, Germany: German Cancer Research Center (DKFZ), Heidelberg, Germany, Faculty of Medicine and University Hospital Carl Gustav Carus, Technische Universität Dresden, Dresden, Germany; 10 Helmholtz-Zentrum Dresden -Rossendorf, Institute of Radiooncology – OncoRay, Dresden, Germany; 11 German Cancer Consortium (DKTK), Partner Site Dresden, and German Cancer Research Center (DKFZ), Heidelberg, Germany; 12 OncoRay – National Center for Radiation Research in Oncology, Faculty of Medicine and University Hospital Carl Gustav Carus, Dresden, Germany Purpose or Objective Despite the high efficiency of ablative radiotherapy (aRT) in oligometastatic prostate cancer (PCa), a subset of patients does not respond and the disease becomes incurable when spreading further. To date, there is no biomarker available to identify these patients. We hypothesized that stem-like and bone-tropism-specific markers on circulating tumor cells (CTCs) may have prognostic potential. Furthermore, we functionally analyzed these markers to identify and characterize the functional link between radioresistance (RR) and