ESTRO 2023 - Abstract Book

S585

Monday 15 May 2023

ESTRO 2023

Conclusion A significant deleterious interobserver variability appears for GTV delineations, which can be explained by the intrinsic limitation of CT images, differences in interpretation of the clinical information provided by the endoscopy, level of experience, or working practice proper to each center. The comparison with histology is a powerful tool for understanding systematic errors and improving the training of radiation therapists toward the definition of improved guidelines. Further prospective randomized trials are warranted to assess this approach's practical feasibility and quantify the therapeutic gain in decreased toxicities for an identical tumor cure. MO-0715 Tumor volume and cancer stem cells as prognostic markers for loco-regional control in HNSCC M.H. Kristensen 1 , J. Alsner 1 , B.S. Sørensen 1,2 , C.R. Hansen 2,3,4 , R. Zukauskaite 5 , E. Samsøe 6 , C. Maare 7 , J. Johansen 5 , H. Primdahl 8 , Å. Bratland 9 , C.A. Kristensen 10 , M. Andersen 11 , J.K. Lilja-Fischer 1 , T. Tramm 12 , J. Overgaard 1 , J.G. Eriksen 1 1 Aarhus University Hospital, Department of Experimental Clinical Oncology, Aarhus, Denmark; 2 Aarhus University Hospital, Danish Centre for Particle Therapy, Aarhus, Denmark; 3 Odense University Hospital, Laboratory of Radiation Physics, Odense, Denmark; 4 University of Southern Denmark, Department of Clinical Research, Odense, Denmark; 5 Odense University Hospital, Department of Oncology, Odense, Denmark; 6 Zealand University Hospital, Department of Oncology, Næstved, Denmark; 7 Herlev Hospital, Department of Oncology, Herlev, Denmark; 8 Aarhus University Hospital, Department of Oncology, Aarhus, Denmark; 9 Oslo University Hospital, Department of Oncology, Oslo, Norway; 10 Rigshospitalet, Department of Oncology, Copenhagen, Denmark; 11 Aalborg University Hospital, Department of Oncology, Aalborg, Denmark; 12 Aarhus University Hospital, Department of Pathology, Aarhus, Denmark Purpose or Objective Radioresistance is assumed to be the main reason for failure after primary curative (chemo-)radiotherapy ((C-)RT) for HNSCC. Characterization of radioresistant tumors is debated, but has been suggested to be linked to the absolute number of cancer stem cells (CSC). We aimed to identify if putative CSC markers and tumor volume could identify patients (pts) with different outcome following primary (C-)RT. Materials and Methods The DAHANCA 19 cohort from 2007-12 was included. Treatment was primary curative IMRT-based (C-)RT (66-68Gy/33-34fx 6 fx/wk (+/- cisplatin 40mg/m2 weekly)) and nimorazole. Primary (GTV-T) and nodal (GTV-N) volume was extracted from planning-CTs. RNA qPCR from formalin-fixed paraffin embedded (FFPE) tumor tissue was used to analyse gene expression of putative CSC markers (SLC3A2 and MET). p16-status was determined by immunohistochemistry (cut-off: 70%). The absolute number of CSC was estimated as the sum of the products of tumor volume (V) and expression (Exp) of CSCs: CSC(total) = V(tumor) × Exp(SLC3A2) + V(tumor) × Exp(MET) Analyses were conducted separately on groups of pts with p16-positive oropharyngeal (OPSCCp16+) and non-HPV-driven tumors. Variables were categorized group-vice in tertiles and evaluated by Kaplan-Meier-method and Cox regression analyses. Endpoints were 3-year T-site failure in analyses where primary tumor volume (GTV-T) was a factor, N-site failure with nodal volume (GTV-N) and loco-regional failure (LRF) with total tumor volume (GTV-Tot).

Results