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The polarity effect for commercially available plane-parallel ionization chambers.

B J Gerbi, F M Khan

    Medical Physics
    |March 1, 1987
    PubMed
    Summary
    This summary is machine-generated.

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    The polarity effect in plane-parallel ionization chambers is minimal for electron beams at dmax but significantly increases at greater depths. For photon beams, polarity effects can reach up to 30% in the buildup region.

    Area of Science:

    • Medical Physics
    • Radiation Dosimetry
    • Radiotherapy Physics

    Background:

    • Plane-parallel ionization chambers are crucial for accurate dose measurements in radiotherapy.
    • Understanding the polarity effect is essential for reliable dosimetry, especially with high-energy beams.
    • Previous research has indicated polarity effects, but comprehensive studies across different beam types and depths are needed.

    Purpose of the Study:

    • To investigate the polarity effect in three commercial plane-parallel ionization chambers.
    • To evaluate the polarity effect as a function of depth for various photon and electron beam energies.
    • To assess the influence of field size and material interfaces on the polarity effect.

    Main Methods:

    • Three plane-parallel ionization chambers (Memorial Pipe, Victoreen/Nuclear Associates 30-329, Capintec PS-033) were studied.

    Related Experiment Videos

  • Measurements were performed using 6-, 10-, 18-, and 24-MV x-ray beams and 9- and 22-MeV electron beams.
  • The polarity effect was analyzed at different depths, including the buildup region, and at the polystyrene-aluminum interface, considering various field sizes.

    • Main Results:

    • For electron beams at dmax, the polarity effect was small (1%-2%).
    • At depths greater than dmax for electron beams, the polarity effect increased, reaching up to 4.5% in some cases.
    • For high-energy photon beams in the buildup region, collected charge differences between polarities reached as high as 30%.

    Conclusions:

    • Plane-parallel ionization chambers exhibit a minimal polarity effect for electron beams at dmax.
    • Significant polarity effects are observed for electron beams at depths beyond dmax and are particularly pronounced for photon beams in the buildup region.
    • The findings highlight the importance of considering polarity effects in dosimetry, especially when using these chambers with high-energy photon beams.