The separate contributions of all source regions to organ doses from 131I and 123I administered to the body were calculated using voxelized reference phantoms. The photon and electron components of organ doses were also evaluated for each source region. The MCNPX Monte Carlo particle transport code was utilized for dose calculations. All organs and tissues of male and female phantoms were taken into account as source regions with their corresponding cumulated activities. The results showed that cumulated activities assigned to source regions and inter-organ distances were two factors that strongly affected the contribution of each source to the organ dose. The major contribution of the dose to the main source regions arose from self-irradiation of electrons, while for nearby organs it was due to photons emitted by the main source organs. In addition, self-irradiation plays an important role in the dose delivered to most target organs for lower thyroid uptakes.

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