Off-axis beam quality change in linear accelerator x-ray beams

Abstract

The effective energy of the X-ray beam from radiotherapy linear accelerators changes as a function of the position in the beam due to nonuniform filtration by the flattening filter. The transmittance through a water column was measured in good geometry and the beam quality characterized in units of HVL [half-value layer] in water. Measurements were made on a variety of linear accelerators from 4-10 MV. The beam energy decreased with increasing distance from the central ray for all accelerators measured.