照射野および深さによるエネルギースペクトルの変化に関する研究

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  • A study on field size and depth dependence of energy spectrum.

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Stereotactic irradiation using a linear accelerator has been widely adopted in many hospitals recently. For this radiotherapy technique, accurate dosimetry is important for defining the position and dimensions of the irradiation field, and determining the absorbed dose to target volume.<BR>Nevertheless, there are some uncertainties caused by a smaller field than conventional techniques. A major cause for the uncertainties is a variation in the photon and electron energy spectrum depends on field size and depth.<BR>In this study, the energy spectra of photons and electrons in water were calculated using a Monte Carlo simulation. It was evident that photon and electron energy spectra changed as a function of field size and depth in water. To estimate the influence of the variation in energy spectrum on an ionization chamber, the ratios of the mean restricted mass collision stopping power of water to air were determined using the electron energy spectra. It was evident that uncertainty of the stopping power ratio caused by a variation in field size and depth was less than 1% for an energy range below 10 MV. Nevertheless, the uncertainty exceeded 1% at points deeper than 15cm for fields smaller than 1cm in diameter of 15 and 24 MV.

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