KAHIBARO
34.3. Electron Beams
Table of Contents
Electron transport
In external beam radiotherapy, electron beams are often used for treating superficial targets, such as skin lesions or shallow tumors. In GATE, electron beams are handled with the same overall workflow as other beams, but their transport in matter has some specific features that are important for realistic dose calculations.
Electron transport in a Monte Carlo simulation describes how electrons lose energy, scatter, and eventually stop as they travel through materials. GATE relies on Geant4 physics models to describe these processes. You typically select an appropriate physics list that includes detailed electromagnetic interactions for electrons, for example one of the standard Geant4 “EM” options that are recommended for medical applications.
Electrons in the megavoltage energy range used for therapy, typically from about 4 MeV to 20 MeV, undergo many small interactions in a short distance. Instead of simulating every single collision, Geant4 uses multiple scattering and continuous slowing down approaches. The code groups many small deflections into an effective angular spread and many small energy losses into a continuous loss along the step. This is why transport parameters and production cuts, covered in separate chapters, are especially important for electron beams.
As electrons penetrate tissue or a water phantom, they first build up lateral and angular spread, then deposit most of their energy over a relatively shallow depth before their dose falls off. In a simple homogeneous water phantom, this creates a depth dose curve with a rapid rise to a plateau and then a relatively sharp drop, but much shallower than a proton Bragg peak. The detailed shape of this curve, including the surface dose and the falloff, depends strongly on how electron transport is modeled.
In GATE, you define your electron beam source, for example a parallel or Gaussian beam at the phantom surface, and configure the beam energy spectrum and direction. The physics list then controls which electron processes are active: ionization for energy loss, bremsstrahlung production, multiple scattering, and possibly more advanced models for low energy electrons if needed. Electrons also produce secondary photons through bremsstrahlung and, depending on the cuts, secondary electrons as well. These secondaries can contribute to out-of-field dose and to deeper dose than you would expect from electrons alone.
For accurate electron beam simulations, you must:
- Use an electromagnetic physics list suitable for medical applications.
- Choose production cuts and transport parameters that are fine enough in the patient or phantom region to capture the surface dose and depth dose shape.
- Ensure that the material definitions and densities, especially in heterogeneities such as bone or lung, are correct, because electron transport is very sensitive to material properties.
Electron beams are highly affected by heterogeneities, such as air cavities, bone, or lung. Electrons scatter strongly and may be deflected or stopped more rapidly in high density materials. In a water phantom, their lateral spread produces broad penumbras and significant out-of-field dose compared with photons. GATE will model these effects automatically once geometry, materials, and physics are properly defined, but you should be aware that small geometric or material errors can lead to noticeable changes in the computed dose distributions.
In a typical electron beam radiotherapy simulation, you combine a realistic beam model with a dose actor that scores dose in voxels inside a phantom or patient CT. The electron transport handled by the physics list, together with your chosen cuts and step limits, determines the accuracy of the surface dose and the dose falloff. In later chapters about dose calculation and validation, you will see how to extract depth dose curves and profiles for electron beams, and how to compare them with measurements to validate that your electron transport settings in GATE are appropriate for clinical use.
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