Ionization produced by electron beams beneath curved surfaces

Area of Science:

• Medical Physics
• Radiation Oncology
• Dosimetry

Background:

• Accurate dose calculation is crucial in radiation therapy, especially when dealing with complex patient geometries.
• Electron beam therapy often encounters curved surfaces, such as patient contours, which can alter dose distribution.
• Existing models may not fully account for the perturbations caused by these curved surfaces.

Purpose of the Study:

• To quantify the ionization differences beneath cylindrical surfaces compared to flat surfaces for high-energy electrons.
• To investigate the impact of varying radii of curvature, depths, off-axis distances, and electron energies on ionization ratios.
• To assess the adequacy of current methods for compensating dose perturbations in curved geometries.

Main Methods:

• Ionization measurements were performed using phantoms with cylindrical entrance surfaces (radii 6-15 cm) simulating patient contours.
• Measurements were conducted for high-energy electrons ranging from 6 to 18 MeV.
• Ionization ratios were calculated by comparing measurements under curved surfaces to those under a corresponding flat surface across various parameters.

Main Results:

• Ionization ratios significantly deviated from unity, reaching up to a 50% difference under cylindrical surfaces.
• These deviations were observed across a range of depths, off-axis distances, radii of curvature, and electron energies.
• The study highlights substantial perturbations in electron dose distribution due to curved surfaces.

Conclusions:

• Curved surfaces significantly alter electron beam ionization and dose distribution compared to flat surfaces.
• Simple adjustments like shifting isodose curves are likely insufficient to accurately compensate for these perturbations.
• Further research and improved models are needed for precise electron dosimetry in complex patient geometries.