ESTRO 2021 Abstract Book

S669

ESTRO 2021

down regulated in HPV positive tumors compared to HPV negative tumors, maybe explaining the more favorable prognosis of the HPV+ tumors. Regarding associations with the TNM staging system, interestingly, repair of lesions handled by Nucleotide Excision Repair and IntraCrossLink Repair significantly differed between T1 and T4 tumors, whereas repair of oxidative damage varied with lymph node tumor staging. Finally tumors from patients with a complete response displayed lower repair capacity toward Cisplatin adducts. Conclusion For the first time, an extensive characterization of HNSCC DNA repair systems could be conducted. Assessing the real DNA repair capacities of the different major repair pathways in parallel, opens new therapeutic perspectives for the patients, allowing to adapt the treatments strategy to the tumor DNA repair properties. This approach is particularly suited for cancers induced by carcinogens and treated by agents targeting the DNA, as HNSCC. PD-0834 Radiotherapy versus radiochemotherapy in HPV positive and negative HNSCC xenograft models C. Valentini 1 , N. Ebert 1 , K. Lydia 2 , S. Loeck 3 , M. Krause 3 , M. Baumann 4 1 Medizinische Fakultät und Universitätsklinikum Carl Gustav Carus Dresden, Technische Universität Dresden, Klinik und Poliklinik für Strahlentherapie und Radioonkologie,, Dresden, Germany; 2 OncoRay, Helmholtz- Zentrum Dresden - Rossendorf, Institut für Radioonkologie, Dresden, Germany; 3 Medizinische Fakultät und Universitätsklinikum Carl Gustav Carus Dresden, Technische Universität Dresden,, Klinik und Poliklinik für Strahlentherapie und Radioonkologie, Dresden, Germany; 4 Deutsches Krebsforschungzentrum , (DKFZ), Heidelberg, Germany Purpose or Objective To compare in-vivo treatment response after fractionated radiotherapy (RTx) with and without simultaneous cisplatin for HPV+ and HPV- head-and-neck (H&N) xenograft tumor models and thus to evaluate the benefit of simultaneous chemotherapy in HPV+ tumors versus RTx. In addition, to evaluate the efficacy of radiochemotherapy (RCTx) with HPV- tumors. Materials and Methods Tumor lines from six HNSCC were used: three HPV- (FaDu, SAS and Cal-33) and three HPV+ (UM-SCC-47, UD- SCC-2 and UPCI-154). Animals, NMRI nu/nu nude mice, were randomized into two treatment groups, RTx or RCTx with weekly cisplatin for each tumor line. To determine the local tumor control dose 50% (TCD50), 30 fractions RTx were given over six weeks with eight different dose groups (30 Gy -120 Gy). In the second project part, median tumor growth delay, defined as the time to reach 2 times (GDV2) or 5 times (GDV5) baseline volume, was assessed using 10 fractions RTx over two weeks with a dose per fraction of 2 Gy. Results RCTx resulted in a significant improvement of tumor control compared to RTx alone in the following tumor