Estimates of beta absorbed fractions in small tissue volumes for selected radionuclides.

Energy deposition patterns are dependent upon the size and geometry of the source region, distribution of radioactive material, types of radiations and energies emitted by the radionuclide, as well as interfaces between different materials which may exist within the region. Commonly, in absorbed dose calculations for internally deposited beta-emitting radionuclides, it is assumed that the absorbed fraction of energy for the mean beta energy is a sufficient representation of the beta spectrum. The accuracy of this assumption was tested by comparing absorbed fractions calculated using actual beta spectral energies with those obtained using the mean beta energy for several radionuclides commonly used in nuclear medicine. A sphere composed of tissue was chosen as the preferred geometry. Spheres of 0.1, 0.5, 1.0, and 2.0 cm radius were used, and absorbed fractions were calculated as a function of surface-to-volume ratios. This allows the assessment of absorbed fraction in spheres where there is a uniform distribution of a radionuclide.