8.3.1 - Primary And Secondary Radiation

Dose distribution is dependent on the primary radiation, originating from the target or source, and the secondary scattered radiation caused by attenuation of the primary radiation.
The dose from primary radiation falls linearly as the beam is attenuated and is similar between most fields.
Dose due to scattered radiation accounts for many of the variations seen with altered field sizes. Scattered radiation varies considerably with beam quality, as secondary electrons set in motion by high energy photons may travel further than those from low megavoltage beams.
Treatment planning systems frequently use superposition/convolution methods to account for primary and scattered radiation. The primary beam attenuation and dose are calculated initially. For every voxel that is irradiated, a kernel that represents the scatter from that point is applied. This allows the TPS to account for scattered radiation in determining dose.
In the central parts of a beam, dose from scattered radiation is distributed to neighbouring areas. At the same time, the scattered radiation from those areas is distributed to the central part of the beam. If the amount of scattered radiation leaving a point is equivalent to that entering the point, electronic equilibrium – an equal distribution of dose within the region. Electronic equilibrium may be affected by beam energy, field size, inhomogeneities or source-surface distance.