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Electron interaction cross sections of tissue components

23.12.2021

Accurate dose calculation is an essential prerequisite for successful radiation therapy treatment. Monte Carlo (MC) simulations are currently deemed to be the most accurate method for determining the dose distribution under complex radiation conditions. Especially in areas with high tissue inhomogeneity, for example in the head, neck and lung areas, the dose can only be accurately calculated by means of MC simulations.

In addition to statistical uncertainty, the accuracy of MC simulations is primarily limited by the inaccuracy of the implemented cross section data sets and of the modeling of the particle transport processes. Ionizing radiation mainly deposits energy to the tissue via secondary electrons. For this reason, having accurate data on double‑differential electron interaction cross sections of tissue components, and correlating this data to the electron energy and emission or scattering angles, is essential for precisely calculating the local dose in radiation therapy.

Human tissue basically consists of water, proteins and lipids. The interaction cross sections of this tissue can be represented in good approximation as a sum of the functional groups to be found in the tissue. First, the double‑differential electron interaction cross sections of ethanol, which contains a functional group of protein, were measured as a function of energy E and the emission angle using a crossed‑beam arrangement. In this arrangement, an effusive molecular beam is perpendicularly crossed by an electron beam. The primary electron current was 7 μA. The gas density in the molecular beam was approximately 5∙1014/cm3. The electron spectrometer consisted of a hemispheric analyzer with a mean diameter of 65 mm and three channeltron detectors.

Figure 1 shows an example of the double‑differential energy spectrum d2 N/dEdΩ of electrons that are emitted by primary electrons with an energy of T=1 keV in ethanol at 90°. It should be noted that the spectrum range above (T-I)/2 represents the energy loss spectrum of primary electrons while the low‑energy part represents the energy spectrum of secondary electrons. The quantity I stands for the ionization potential of the molecule. In Figure 1, the elastic peak at 1 keV and the Auger line of carbon at approximately 270 eV can be clearly seen.

double-differential energy spectrum

Figure 1:  Double‑differential energy spectrum d2 N/dEdΩ of electrons that are generated by 1 keV primary electrons in ethanol. The emission angle was 90°. The range below approximately 495 eV is the energy spectrum of secondary electrons.

Contact person:

Opens local program for sending emailW. Y. Baek, Department 6.3, Working Group 6.36