Relations between scatter factor, quality index and attenuation for x-ray beams.

The relations between the attenuation factor, the normalized phantom scatter factor and the quality index are examined using a semiempirical formula for the dose on the central axis of an x-ray beam in water. The study is restricted to depths and field sizes sufficient for electron equilibrium. The results are compared with data in the recent literature. It is concluded that for x-ray beams in the energy range 4-25 MV the normalized scatter factors can be calculated from the dose-weighted average linear attenuation coefficient in water, determined from transmission measurements in a narrow-beam geometry or from the quality index.