Some Relationships between Dosimetric Quantities in Photon and Electron Beams

Theoretical considerations of photon and electron dosimetry with general cavity are attempted on the basis of partitioning the kerma into its collisional part and its radiative part. A necessary and sufficient condition for the existence of charged-particle equilibrium in a material irradiated by photons is the equality of the collisional kerma with the absorbed dose. The exposure can be determined indirectly without restriction on the photon energies, if a quantity such as the W-value or G-value of the cavity material of the dosimeter is known. The absorbed dose can also be measured by using a dosimeter calibrated in terms of exposure, air kerma, or absorbed dose in the medium. During exposure calibration, air-kerma calibration, or exposure measurement, a material equivalent to the cavity wall must be used as the build-up cap material.