ESTRO 2022 - Abstract Book

S1486

Abstract book

ESTRO 2022

Fig. 2: Left) sCT by the 2D approach, Right) sCT by the 3D approach.

Conclusion A 3D approach can outperform a 2D approach by reducing the MAE and the discontinuity error in generated sCT scans. This is because the network utilizes spatial correlation among MR slices during the sCT generation process. Acknowledgement: This work is part of a grant supported by Varian Medical Systems.

PO-1687 Phenomenological parametrizations of the FLASH effect normal tissue sparing as a function of dose

T.T. Böhlen 1 , J. Germond 1 , M. Vozenin 2 , B.S. Sørensen 3 , J. Bourhis 2 , C. Bailat 1 , F. Bochud 4 , R. Moeckli 4

1 Lausanne University Hospital (CHUV), Institute of Radiation Physics (IRA), Lausanne, Switzerland; 2 Lausanne University Hospital (CHUV) and University of Lausanne, Radiation-Oncology, Lausanne, Switzerland; 3 Aarhus University Hospital, Oncology, Aarhus, Denmark; 4 Lausanne University Hospital (CHUV) and University of Lausanne, Institute of Radiation Physics (IRA), Lausanne, Switzerland Purpose or Objective A quantitative understanding of magnitudes of normal tissue protection by the FLASH effect and its basic dependencies on dose delivery parameters is an essential requirement for a successful and optimized clinical translation of FLASH RT. However, to date, the mechanism for the FLASH effect is still under investigation. We gathered therefore available in vivo data of normal tissue sparing of ultra-high dose rate (UHDR) beams and followed a phenomenological data-driven approach that is not related to a mechanistic modelling of the FLASH effect to parameterize the data as a function of dose. Materials and Methods We gathered available experimental data of normal tissue reactions for UHDR versus conventional dose rates (CONV) and expressed them in terms of dose modifying factor (DMF) on a common scale. We then performed linear and non-linear regressions using different functions including logistic and log-logistic functions. An augmented covariance matrix and Monte Carlo sampling was used to evaluate confidence and prediction intervals of the fits. Results The gathered data allows to evaluate general trends and magnitudes of DMF of UHDR radiation as a function of dose. It was found that individual data series can be mostly well described by a bi-linear function when expressed as D/DMF versus D. This is corroborated by R 2 > 0.9 for linear fits to data at doses exhibiting a FLASH effect for most individual data series. Furthermore, the analysis highlights that the DMF of pooled skin reaction data follow a consistent general trend as a function of dose that can be parametrized by relatively simple functions (see Figure 1). Skin reactions data show an onset of the FLASH effect for doses of about 17 Gy and reach DMF of about 1.4 only for high doses >30 Gy, where the DMF starts to saturate. Other normal tissues and endpoints show an earlier onset of the FLASH effect (~7 Gy) and significant sparing factors are already achieved at lower doses.