Abstract
BackgroundRadioiodine is routinely used or proposed for diagnostic and therapeutic purposes: 123I, 125I and 131I for diagnostics and 125I and 131I for therapy. When radioiodine-labelled pharmaceuticals are administered to the body, radioiodide might be released into the circulation and taken up by the thyroid gland, which may then be an organ at risk. The aim of this study was to compare dosimetric properties for 123I, 125I and 131I in previously developed thyroid models for man, rat and mouse.MethodsDosimetric calculations were performed using the Monte Carlo code MCNPX 2.6.0 and nuclear decay data from ICRP 107. Only the non-radiative transitions in the decays were considered. The S value was determined for the cell nuclei in species-specific thyroid follicle models for mouse, rat and man for different spatial distributions of radioiodine.ResultsFor the species-specific single follicle models with radioiodine homogeneously within the follicle lumen, the highest S value came from 131I, with the largest contribution from the β particles. When radioiodine was homogeneously distributed within the follicle cells or the follicle cell nucleus, the highest contribution originated from 125I, about two times higher than 123I, with the largest contribution from the Auger electrons. The mean absorbed dose calculated for our human thyroid multiple follicle model, assuming homogenous distribution of for 123I, 125I, or 131I within the follicle lumens and follicle cells, was 9%, 18% and 4% higher, respectively, compared with the mean absorbed dose according to Medical Internal Radiation Dose (MIRD) formalism and nuclear decay data. When radioiodine was homogeneously distributed in the follicle lumens, our calculations gave up to 90% lower mean absorbed dose for 125I compared to MIRD (20% lower for 123I, and 2% lower for 131I).ConclusionsThis study clearly demonstrates the importance of using more detailed dosimetric methods and models than MIRD formalism for radioiodine, especially 123I and 125I, in the thyroid. For radioiodine homogeneously distributed in the follicle lumens our calculations for the human multiple follicle models gave up to 90% lower mean absorbed dose compared with MIRD formalism.
Highlights
Radioiodine is routinely used or proposed for diagnostic and therapeutic purposes: 123I, 125I and 131I for diagnostics and 125I and 131I for therapy
This study clearly demonstrates the importance of using more detailed dosimetric methods and models than Medical Internal Radiation Dose (MIRD) formalism for radioiodine within the thyroid
For radioiodine homogeneously distributed in the follicle cells and lumens, calculations for our human multiple follicle model gave up to 18% higher mean absorbed dose
Summary
Radioiodine is routinely used or proposed for diagnostic and therapeutic purposes: 123I, 125I and 131I for diagnostics and 125I and 131I for therapy. When radioiodine-labelled pharmaceuticals are administered to the body, radioiodide might be released into the circulation and taken up by the thyroid gland, which may be an organ at risk. 131I, as iodide, is routinely used for diagnosis and treatment of various thyroid disorders such as for treatment of various neuroendocrine tumours [6,7], 123I-receptor ligands for brain scintigraphy [8], 125I methylene blue for sentinel node localization [9] and 131I-labelled monoclonal antibodies for treatment of lymphoma [10]. Radioiodine labelled pharmaceuticals are administered to the body, radioiodide might be released into the circulation, e.g. by enzymatic reactions by dehalogenases in tissues, and taken up by the thyroid gland [11]. After the Chernobyl accident in 1986, contamination with 131I (and other short-lived isotopes such as 132I and 133I) led to an increased incidence of differentiated thyroid cancers in children but not in adults, with a higher incidence with lower age [12,13,14,15]
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