Electron contamination in 8 and 18 MV photon beams.

The contribution from contaminant electrons in the buildup region of a photon beam must be separated when calculating the dose using a photon convolution kernel. Their contribution can be extrapolated from fractional depth dose (FDD) data using the fractional depth kerma (or the "equilibrium dose") derived from measured quantities such as beam attenuation with depth, phantom scatter factor as a function of field size and depth, and inverse-square law for the incident photon beam. Good agreement is observed between the extrapolated and the EGS4 Monte Carlo simulated, primary dose-to-kerma ratios in the surface region for the photon beams, excluding electron contamination. The FDD was measured using a Scanditronix photon diode and was normalized to a reference depth far beyond maximum range of contaminant electrons. An analysis for the 8 and 18 MV photon beams from a Varian 2100CD indicates that at a source-to-surface distance (SSD) of 100 cm, the maximum electron contaminant dose (relative to its maximum FDD) varies from 1% to 33% for 8 MV and 2% to 44% for 18 MV, for square collimator settings ranging from 5 to 40 cm (defined at 100 cm from the source). This value at a depth of maximum dose (2 cm for 8 MV and 3.5 cm for 18 MV) can reach 1% for 8 MV and 2.3% for 18 MV. This contaminant electron dose is almost independent of SSD for 8 MV and starts to fall off for 18 MV at SSDs larger than 120 cm. Compared with the open beam, the contaminant electron dose increases when a solid tray is used, and the magnitude of increase increases with field size, reaching 19% and 16% for a 40 x 40 cm2 field for 8 and 18 MV photons, respectively. The contaminant electron dose increases slightly for a blocked beam compared with an open beam of the same field size if a tray is used in both cases. The contaminant electron dose for the wedged field is less than that for an open field. However, the reduction is less significant at larger collimator settings (c = 20 cm) and may increase slightly for 8 MV photons.