ESTRO 2023 - Abstract Book

S1561

Digital Posters

ESTRO 2023

Conclusion A complete framework for calculating patient-specific dose to lymphocytes was created. This framework allows for personalization in an anatomical and dosimetric way due to IS2aR and Periphocal 3D. By using the model proposed by Sung et., al it was also possible to obtain lymphocyte survival. The model agrees with measurements for all patients except one, which can be explained by a higher lymphocyte radiosensitivity or a slower lymphocyte regeneration. The differences in the late measurements indicate that a remodelling of the long-term repletion of lymphocytes is needed, probably, also considering the damage to hematopoietic structures and CL. Acknowledgments: Fondecyt N1181133

PO-1830 Choosing the regularization parameter during the reconstruction of the energy spectrum for IOERT

T. Piotrowski 1 , A. Ryczkowski 2 , P. Adrich 3

1 Greater Poland Cancer Centre, Medical Physics, Pozna ń , Poland; 2 Poznan University of Medical Sciences, Electroradiology, Poznan, Poland; 3 National Centre for Nuclear Research, Nuclear Equipment Division, Otwock, Poland Purpose or Objective The shape of the energy spectrum is an essential component of any electron beam Monte Carlo model. Due to specialized equipment and the long measurement time for the direct methods for determining the energy spectrum, attractive alternatives are backward spectrum reconstructions from the measured data. One such approach is solving the first-degree Fredholm integral equation with appropriate regularization. Thanks to this, it is possible to calculate the depth distribution as the sum of the distributions from monoenergetic beams. This study aims to determine the optimal value of the regularization parameter for the problem of determining the spectrum of the electron beam produced by a mobile accelerator used during intraoperative radiotherapy. Materials and Methods The study was carried out with the use of MC simulation methods. The Geant4 package was used to generate the distributions of deep doses for monoenergetic beams for two models with different degrees of complexity, i.e. simple (theoretical) and full (for the mobile accelerator) (Figure 1). The dose distributions for four different shapes of energy spectrum (for each model) were obtained similarly. They were established as the reference data for further calculations. The Dual Annealing optimization method was used to obtain the reconstructed spectrum. The multiple optimizations that differ by the regularization parameter (ranging from 0 to 1) were performed. For each reconstruction, similarity indicators of the energy spectrum and the dose distribution to the referenced data were calculated to determine the optimal regularization parameters.