Electron disequilibrium in high-energy x-ray beams

The ratio between scatter dose and scatter kerma for points on the central axis of 15-MV x-ray beams has been examined by Monte Carlo calculations. This ratio, beta(S)', behaves differently from that between the primary dose and kerma, beta(p)'. Both the primary and scatter components of beta' undergo an initial rapid buildup, however, beta(S)' begins from a much higher surface value. In addition, the depth required for longitudinal electron equilibrium is larger for beta(S)', than for beta(p)'. The variation of beta(S)', with field size and depth is attributed to the spatial variation of scatter kerma in the photon beam. The approximation beta(S)' = 1 is accurate enough for clinical dose-calculation purposes, leading to less than 0.5% error in total dose, while the assumption beta(S)' = beta(p)' may cause up to 2.5% error, relative to the maximum dose, near the surface at 15 MV. (C) 1996 American Association of Physicists in Medicine.