An alternative beam quality index for medium-energy x-ray dosimetry.

To determine absorbed dose to water it is essential that the radiation quality of the x-ray beam is known accurately. The unique method of defining the beam quality is to acquire a detailed knowledge of the photon spectrum at the point of measurement in water, but this is impractical. The aim of this work was to investigate a quality index that uniquely defines the ratio of mass-energy absorption coefficients of water to air at 2 cm deep in water ([(mu en/rho)w/a]z = 2,phi). Three parameters, HVL, mean energy at 2 cm deep in water and the ratio of the doses at 2 and 5 cm deep in water (D2/D5) were investigated as functions of [(mu en/rho)w/a]z = 2,phi. The quality index that best defines [(mu en/rho)w/a]z = 2,phi is the ratio of doses at 2 and 5 cm depth in water.