Absorbed fractions for photons in ellipsoidal volumes.

We studied through Monte Carlo simulation in Geant4 the absorbed fractions for photons, characterized by energies ranging from 10 keV to 1000 keV, which can be emitted by gamma radionuclides uniformly distributed in ellipsoidal volumes of soft tissue. The same analytical relationship between absorbed fraction and the 'generalized radius' as introduced in a previous paper was found, and the dependence of its parameters rho(0) and s on photon energy is discussed and fitted by suitably chosen parametric functions. As a consequence, the absorbed fraction for photons in the 10-1000 keV energy range can be calculated for all volumes and for every ellipsoidal shape of practical interest. Such results can be a useful complement for the dosimetry of beta- and gamma-emitting radionuclides during internal radiotherapy or gamma emitters employed in diagnostic nuclear medicine.