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Displacement corrections used in absorbed dose determination

J R Cunningham, M R Sontag

    Medical Physics
    |November 1, 1980
    PubMed
    Summary

    Accurate absorbed dose determination requires accounting for phantom material displaced by ionization chambers. New calculations and measurements suggest revised displacement correction factors for improved accuracy in photon beam dosimetry.

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    Area of Science:

    • Medical Physics
    • Radiation Dosimetry
    • Radiotherapy Physics

    Background:

    • Ionization chambers are crucial for absorbed dose determination in photon beams.
    • A correction factor for phantom material displaced by the chamber is necessary.
    • Current correction factors may not be accurate across all energies.

    Purpose of the Study:

    • To investigate and propose updated displacement correction factors for ionization chambers.
    • To assess the impact of chamber size, shape, and radiation energy on these factors.
    • To provide a comprehensive set of factors for various photon beam energies.

    Main Methods:

    • Performed "first scatter" calculations for a range of photon beam energies and spectra.
    • Conducted measurements using a Farmer-type ionization chamber in water phantoms.
    • Validated calculations against experimental data from Cobalt-60 and 25 MV photon beams.

    Main Results:

    • Identified a potentially more accurate displacement correction factor of 0.975 for Farmer-type chambers in Cobalt-60 beams, compared to the commonly used 0.985.
    • Calculated displacement correction factors for a wider range of photon beam energies.
    • Experimental measurements supported the theoretical calculations.

    Conclusions:

    • The magnitude of the displacement correction factor is dependent on chamber characteristics and radiation beam energy.
    • Updated correction factors are essential for precise absorbed dose calculations, especially when comparing "in phantom" and "in air" measurements.
    • The study presents a set of proposed displacement correction factors for improved dosimetry accuracy.

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