Internal dose calculations for electrons and beta particles

Abstract

A computer code was developed for die transport of photon and electrons in an improved mathematical reference man. The code generated absorbed fractions of energy for monoenergetic electrons for selected source organs. The organs or regions considered were those with high surface to volume ratio, organs with walls, and organs that have subdivisions. The calculated absorbed fractions for electrons supersede the values specified by the Medical Internal Radiation Dose (MIRD) methodology. New estimates of S-values for eighty radionuclides were calculated using the new electron absorbed fractions for 21 combinations of source-target regions. Revised S-values were compared with those presented by MIRD Pamphlet No. 5 Revised. An assumption used in the calculation of S-values was the use of the average energy of the 8 spectra rather than using the full spectra. A study was carried out to assess the limitations of using the average energy of the beta spectra. This assumption was studied by generating electron absorbed fraction profiles for different beta decaying radionuclides in small sphere volumes. Absorbed fractions were calculated using their beta average energy and their full spectra. The use of average beta energies overestimates the absorbed fraction in source region over small volumes. The overestimation is dependent on the beta spectra of the radionuclide. A relationship was found between the absorbed fraction (AF) in source region and the surface-to-volume ratio of the region (S/V). The relation is independent of the geometry of the source region; consequently, there was a unique relation between absorbed fractions of energy in source region and (S/V) ratio tor every radionuclide studied. The EGS4 code was used to calculate absorbed doses to the circulatory system. Absorbed doses per transformation per cm3 delivered to the blood and blood vessel walls for specific radionuclides were calculated for different blood vessel sizes. Absorbed doses to both the blood and blood vessel can be calculated for any radionuclide given the blood vessel radius. This model can be applied to any organ vascular system to assess the dose to tissue and blood.