Electron contamination from different materials in high energy photon beams

Area of Science:

• Medical Physics
• Radiation Oncology
• Dosimetry

Background:

• Accurate determination of electron surface dose is critical for radiation therapy.
• Sources of contaminating electrons include beam flattening filters and air, influencing patient dose.
• Understanding electron transport and scattering is essential for dose calculation models.

Purpose of the Study:

• To model and quantify lateral electron surface dose distributions from filters and air in high-energy photon beams.
• To investigate the impact of various parameters on surface dose, including material properties, geometry, and beam characteristics.
• To provide guidance on selecting optimal filters to minimize surface dose.

Main Methods:

• Application of the Fermi-Eyges theory of multiple scattering.
• Inclusion of transmission and angular scattering effects in materials and air.
• Estimation of backscatter from phantom materials.
• Systematic investigation of parameter variations (atomic number, thickness, position, field size, photon energy).

Main Results:

• Calculated data demonstrated good agreement with experimental findings.
• For 60Co gamma rays, medium atomic number filters yielded the lowest surface dose.
• For higher energies, low to medium atomic number materials are recommended, especially with short material-phantom distances and large fields.
• A thin high-Z foil after a low-to-medium Z filter can reduce surface dose by over 30%.

Conclusions:

• The choice of electron filter material and its placement significantly affects surface dose in high-energy photon beams.
• Air is a major electron source for 60Co gamma rays, while beam flattening filters dominate at higher energies.
• Optimizing filter design and configuration is essential for minimizing electron contamination and improving radiation therapy safety.