Re-evaluation of absorbed fractions for photons and electrons in spheres of various sizes.

UNLABELLED: Absorbed fractions for unit density spheres in an infinite unit density medium, previously calculated for photon emitters and electron emitters, were reevaluated with the Monte Carlo codes EGS4 and MCNP4B.
METHODS: Activity was assumed to be distributed uniformly throughout the spheres, and absorbed fractions for self-irradiation were calculated at discrete photon and electron energies.
RESULTS: For electrons, the codes were in very good agreement with each other (+/-5%) and with published values, except at higher energies in the very smallest spheres, where some differences exceeded 10%. For photons, the codes were again in good agreement with each other but produced results that varied considerably from published MIRD values. For energies <1 MeV and sphere sizes <50 g, the absorbed fractions determined using the Monte Carlo codes were typically 20%-40% higher than values in MIRD 3 and 8. For energies >1 MeV, the Monte Carlo values were sometimes lower than those in the MIRD documents. Recommended values, generally the average results from the 2 Monte Carlo codes, are given for all sphere sizes and energies for both electrons and photons.
CONCLUSION: The absorbed fractions calculated using the Monte Carlo codes should replace the older values and are helpful in evaluating tumor doses, doses to small organs, and other situations in which a uniform distribution of activity throughout a spherical structure of unit density can be assumed.