ESTRO 2021 Abstract Book

S26

ESTRO 2021

Results Table 1 reports the number of patients in whom a particular combination of LNLs was involved to highlighted selected findings: 1) Ipsilateral level IV was involved in 27% of patients in whom level II and III were involved, but only in 2% of patients in whom level II but not III is involved. 2) Prevalence of involvement of ipsilateral levels II, III, IV, V was 79%, 34%, 7%, 3% for early T-stage patients (T1/T2) and 85%, 50%, 17%, 9% for late T- stage (T3/T4), quantifying increasing involvement with T-stage especially for the more downstream levels. 3) Contralateral levels II, III, IV were involved in 41%, 19%, 4% for tumors with midline extension and 12%, 3%, 2% for tumors without midline extension. 4) The dataset provided no indication that LNL involvement is different for HPV positive vs negative tumors. 5) Ipsilateral level VII was involved in 10% of all patients and in 14% and 6% of patients with primary tumors in the tonsil and the base of tongue, respectively.

Conclusion Detailed reporting and quantification of LNL involvement in HNSCC depending on involvement of upstream LNLs, T-stage, primary tumor subsite, midline extension, and potential other risk factors may allow for further personalization of CTV-N definition and reduced toxicity in the future. An example is identification of patients with minimal risk of level IV involvement to allow save exclusion of level IV from the CTV-N. PH-0051 Outcomes after definitive (C)RT in HPV+ OPC: Largescale comparison of two population-based cohorts P. Lassen 1 , S.H. Huang 2 , J. Su 3 , B. O'Sullivan 2 , J. Waldron 4 , M. Andersen 5 , H. Primdahl 6 , J. Johansen 7 , C. Andrup Kristensen 8 , E. Andersen 9 , J. Eriksen 10 , C. Rønn Hansen 11 , J. Alsner 12 , J. Lilja-Fisher 12 , S. V. Bratman 2 , J. Ringash 2 , J. Kim 2 , A. Hope 2 , A. Spreafico 13 , J. de Almeida 14 , W. Xu 15 , J. Overgaard 12 1 Aarhus University Hospital, Department of Experimental Clinical Oncology and Department of Oncology, Aarhus, Denmark; 2 Princess Margaret Cancer Centre, Department of Radiation Oncology, Toronto, Canada; 3 Princess Margaret Cancer centre, Department of Biostatistics, Toronto, Canada; 4 Princess Margaret Cancer centre, Department of Radiation Oncology, Toronto, Canada; 5 Aalborg University Hospital, Department of Oncology, Aalborg, Denmark; 6 Aarhus University Hospital, Department of Oncology, Aarhus, Denmark; 7 Odense University Hospital, Department of Oncology, Odense, Denmark; 8 Copenhagen University Hospital, Department of Oncology, Copenhagen, Denmark; 9 Herlev Hospital, Copenhagen, Department of Oncology,