ESTRO 36 Abstract Book

S856 ESTRO 36 2017 _______________________________________________________________________________________________

the competing events of cell mutation and inactivation. The RBE variation for the different end-points was included by introducing and varying the ratio (k) between cell mutation and inactivation for C-ions. The median and range of the patient-specific RRs were calculated from the physical dose distributions and the published input model parameters. Results The dose distributions (Fig 1) illustrated the sharper lateral penumbra of C-ions, which resulted in lower lung doses compared to protons, while the C-ion fragmentation tail contributed to higher doses to the thyroid than from protons. The SC risk estimates strongly depended on the ratio k, and the RR decreased for increasing k for both organs (Fig 2). For the thyroid, the RR was higher from the C-ion plans for the entire scanned range of k. Despite a better sparing of the lungs with C-ions, the carcinogenic potential of C-ions was not consistently lower than for protons: Not including a difference in end-point resulted in RRs in favour of C-ions, while increasing the ratio k gave higher risks for C-ions compared to protons. For the lungs, the median RR turned in favour of IMPT at a threshold value k=1.1.

Conclusion Varying the RBE depending on end-point may strongly influence results when estimating carcinogenic risks from C-ion therapy and should be included in modelling risk of radiation-induced SC from C-ion therapy. EP-1608 Deriving HPV status from standard CT imaging: a radiomic approach with independent validation R. Leijenaar 1 , M. Nesteruk 2 , G. Feliciani 1 , F. Hoebers 1 , J. Van Timmeren 1 , W. Van Elmpt 1 , S. Walsh 1 , A. Jochems 1 , S. Huang 3 , B. Chan 3 , J. Waldron 3 , B. O'Sullivan 3 , D. Rietveld 4 , C. Leemans 5 , O. Riesterer 2 , K. Ikenberg 6 , P. Lambin 1 1 MAASTRO Clinic, Department of Radiation Oncology- GROW- School for Oncology and Developmental Biology- Maastricht University Medical Centre, Maastricht, The Netherlands 2 University Hospital Zurich and University of Zurich, Department of Radiation Oncology, Zurich, Switzerland