The use of electron-emitting radionuclides in tumor imaging and therapy has presented some new challenges to conventional radiation dosimetry. The fraction of electron energy absorbed in most source regions has usually been assumed to be unity. In small structures such as localized tumors or isolated regions containing moderate to high energy electron emitters, however, this may not always be the case.
Methods: Using an extension of Berger's scaled absorbed dose distributions for point sources to represent a spherical geometry, absorbed fractions of electron energy for sources uniformly distributed in spheres of various sizes have been calculated.
Results: Beta particle and monoenergetic electron energies studied range from 0.025 to 4.0 MeV and sphere masses range from 0.01 to 1000 g. S values have also been calculated for 90Y, 123I and 131I based on the results of the absorbed fraction calculations.
Conclusion: These calculated absorbed fractions are valuable in estimating electron energy loss from small spherical structures and may be useful in estimating the radiation dose to these small volumes.