This work presents internal dosimetry estimates for diagnostic procedures performed for thyroid disorders by relevant radiopharmaceuticals. The organ doses for Iodine-131, Iodine-123, and Tc-99m incorporated in the body were calculated for the International Commission on Radiological Protection (ICRP) reference voxel phantoms by using the Monte Carlo transport method. A comparison between different thyroid uptakes of iodine in the range of 0% to 55% and various ways of administration of Tc-99m was made. To investigate the necessity of calculation of organ dose due to all source regions, the major source organ and its contribution to total dose was specified for each target organ. Moreover, we compared effective dose from ICRP voxel phantoms to stylized phantoms. In our method, we directly calculated the organ dose without using the S values or SAFs as commonly done. Hence, a distribution of absorbed dose to entire tissues was obtained. The chord length distributions (CLDs) were also computed for the selected source–target pairs to make comparison across the genders. The results showed that the S values for radionuclides in the thyroid are not sufficient to calculate the organ doses especially for I-123 and Tc-99m. Thyroid and its neighboring organs receive greater dose as thyroid uptake increases. The comparisons also showed an underestimation of organ doses reported for the stylized phantoms compared to the values based on the ICRP voxel phantoms in the uptake range of 5% to 55% and an overestimation of absorbed dose up to 2-fold for thyroid blocked and Tc-99m incorporation.

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