Absorbed fractions in ellipsoidal volumes for β- radionuclides employed in internal radiotherapy

We developed a Monte Carlo simulation in Geant4 to calculate the absorbed fractions for electrons emitted by Au-199, Lu-177, I-131, Sm-153, Re-186 and Y-90, characterized by average energies ranging from 86 keV to 949 keV, uniformly distributed in ellipsoidal volumes of soft tissue. Code validation results with respect to reference data for doses, ranges and absorbed fractions in spheres are presented. An analytical relationship between the absorbed fraction and a 'generalized radius' is introduced in analogy with the transfer function of a first-order high-pass filter, and the dependence of its parameters rho(0) and s from the average electron energy and range is discussed. A generalization for the estimation of absorbed fractions for other radionuclides is also proposed. Such results can be useful to improve accuracy and easiness of calculation in dosimetry during internal radiotherapy.