Reducing electron contamination for photon beam-quality specification.

The percentage depth dose at 10 cm in a 10 x 10-cm2 photon beam at an SSD of 100 cm, %dd(10), is a better beam-quality specifier for radiotherapy beams than the commonly used values of TPR10(20) or nominal accelerating potential. The presence of electron contamination affects the measurement of %dd(10) but can be removed by the use of a 0.1-cm lead filter, which reduces surface dose from contaminant electrons from the accelerator by more than 95% for radiotherapy beams with energies from 60Co to 50 MV. The filter performs best when it is placed immediately below the head. An electron-contamination correction factor is introduced to correct for electron contamination from the filter and air. It converts the %dd(10) which includes the electron contamination with the filter in place [hereafter %dd(10)m], into %dd(10) for just the photons in the filtered beam. The correction factor is a linear function of %dd(10)m for all filtered beams with %dd(10)m > 70%. A small correction for the photon filtering effect converts the pure photon %dd(10) for the filtered beam into that for the unfiltered beam, which can be used to determine stopping-power ratio. Calculations show that the values of water-to-air stopping power ratio in the unfiltered beam are related to the values of %dd(10)m in the filtered beam by a cubic function. The uncertainty of stopping-power ratios in unfiltered beams for the same value of the %dd(10)m is within 0.2% for all beams.