A theoretical study of the radiological properties and water equivalence of Fricke and polymer gels used for radiation dosimetry.

A method is described which determines the radiological equivalence of different materials by comparing their macroscopic photon and electron interaction parameters over the energy range of interest. This method has been applied to Fricke and polymer gels used for radiation dosimetry. Photon and electron dose distributions were calculated using a Monte Carlo technique. 6 MV photon gel depth dose curves were all within 1% of those in water over the depth range studied. 6 MeV electron beam depth dose curves were all within 1 mm of those in water. The results show that for the gels investigated, at megavoltage energies the dominant property determining water equivalence is the electron density of the gel, and therefore future research into gel manufacture for external beam radiotherapy should aim to have the electron density (electrons per unit volume) as close as possible to water. However, care needs to be taken when using gels for dosimetry of photons with incident energy < 100 keV.