ESTRO 2022 - Abstract Book

S736

Abstract book

ESTRO 2022

Figure 1: Microdosimetric spectra obtained in the plateau and distal dose fall-off of a 62.4 MeV proton beam (solid lines) and their corresponding MC simulated spectra (dashed lines). Conclusion A workflow for experimental microdosimetry with a TEPC was successfully established for our institute’s proton beam. Further MC simulations need to be performed to improve our understanding of the deviations with respect to measurements. In particular, the influence of a more detailed detector design and different physics lists on the simulations will be investigated. Other theoretical concepts, e.g. different extrapolations to the keV/ µ m region below the noise level, and the establishment of detailed uncertainty budgets, will be investigated in the future. D. Sanchez-Parcerisa 1 , I. Sanz-Garcia 2 , P. Ibañez 1 , S. España 1 , A. Espinosa 1 , C. Gutierrez-Neira 3 , G. García 3 , A. López 4 , J.A. Vera 5 , A. Mazal 5 , L.M. Fraile 1 , J.M. Udias 1 1 UCM & Instituto de Investigación Sanitaria San Carlos, Grupo de Fisica Nuclear & IPARCOS, Madrid, Spain; 2 UCM, Grupo de Fisica Nuclear, Madrid, Spain; 3 UAM, CMAM, Madrid, Spain; 4 Hospital de Fuenlabrada, Departamento de radiofísica y protección radiológica, Fuenlabrada, Spain; 5 Hospital Quironsalud, Centro de Protonterapia, Pozuelo, Spain Purpose or Objective Radiochromic films, widely used in dosimetry, suffer from a decrease in relative efficiency (RE) when exposed to high-LET radiation. This effect is particularly relevant for beams of heavy ions and low-energy protons and must therefore be accounted and corrected for. RE calibrations published in the literature are not entirely consistent with each other, mainly due to the different methods used for estimating the LET of the beams. We propose a common framework for integrating available data and back it up with experimental measures using a 10-MeV proton beam. Materials and Methods Literature data on relative efficiency for films of the EBT family was extracted from 10 different sources. Reported LET values were recalculated using Monte Carlo to extract dose-averaged, in-water average beam LET values in the active layer of the film (Figure 1, left panel). Additionally, 60 experimental data points for proton energies in the 1-10 MeV energy range were measured for each of the three film types under study: EBT2, EBT3 and unlaminated EBT3 (Figure 1, right panel). Results Measured values were consistent with recalculated literature values. Combined literature and measured data in the LET range of 5-80 keV/ µ m was fitted to a two-parameter model with expression RE(LET) = 1 – a · LET^b . The results of the fit support the hypothesis of a linear decrease of RE with LET, with no remarkable differences between the three types of films analyzed. PD-0816 LET Quenching of EBT3 radiochromic films with low-energy protons

Fig 1. RE vs LET in active layer from different published datasets (left) and measured values (right). Marker color indicates type of film of the set: purple=EBT1, red=EBT2, blue=EBT3 and green=Unlaminated EBT3. Confidence bands represent 1- sigma confidence interval derived from the fit parameters.

Conclusion