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Ionization produced by electron beams beneath curved surfaces.

E R Ritenour, R K Cacak, W R Hendee

    Medical Physics
    |September 1, 1983
    PubMed
    Summary

    High-energy electron beams create different ionization patterns under curved surfaces compared to flat ones. This ionization difference, up to 50%, impacts radiation therapy dose calculations for patient contours.

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    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.

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