Thyroid Retention Research Articles

Korean-specific internal dosimetry data for radioiodine bioassay and evaluation of the dosimetric impact

Our earlier works focused on exploring the Korean iodine biokinetic model and calculating Korean-specific S values and dose coefficients. Building on this foundation, this study extended our efforts to establish an extensive dosimetry dataset for Korean-specific iodine bioassay. We calculated bioassay functions (e.g., thyroid retention and urinary excretion functions), dose-per-content-functions, and content for the specified doses based on the Korean biokinetic model and dose coefficients previously developed. Also, this study provides a comprehensive understanding of the practical impact of using Korean-specific dosimetry data. Compared to the ICRP reference data, the thyroid retention functions for Koreans were significantly lower (for I-131, peaks at 18% vs. 27%). The physical half-lives of radioiodine played a significant role in the differences of dose-per-content-function. For long-lived iodine isotopes such as I-125, the dose-per-content-functions were comparable to ICRP data, whereas for short-lived iodine isotopes such as I-131, the higher values were observed in the Korean data, indicating a greater dose estimation for a given iodine activity measured in the thyroid. Although we revealed the differences between Korean and ICRP reference data, the comprehensive understanding of Korean data suggests that using the Korean-specific data should be considered only when high doses are predicted.

Uncertainty quantification of bioassay functions for the internal dosimetry of radioiodine.

Bioassay functions, which are provided by the International Commission on Radiological Protection, are used to estimate the intake activity of radionuclides; however, they include considerable uncertainties in terms of the internal dosimetry for a particular individual. During a practical internal dose assessment, the uncertainty in the bioassay function is generally not introduced because of the difficulty in quantification. Therefore, to clarify the existence of uncertainty in the bioassay function and provide dosimetrists with an insight into this uncertainty, this study attempted to quantify the uncertainty in the thyroid retention function used for radioiodine exposure. The uncertainty was quantified using a probabilistic estimation of the thyroid retention function through the propagation of the distribution of biokinetic parameters by the Monte Carlo simulation technique. The uncertainties in the thyroid retention function, expressed in terms of the scattering factor, were in the ranges of 1.55–1.60 and 1.40–1.50 for within 24 h and after 24 h, respectively. In addition, the thyroid retention function within 24 h was compared with actual measurement data to confirm the uncertainty due to the use of first-order kinetics in the biokinetic model calculation. Significantly higher thyroid uptakes (by a factor of 1.9) were observed in the actual measurements. This study indicates that consideration of the uncertainty in the thyroid retention function can avoid a significant over- and under-estimation of the internal dose, particularly when a high dose is predicted.

Assessment from in vivo measurements of thyroid dose due to iodine-131 inhalation when stable iodine has been administered

Potassium iodide (KI) is a well-known thyroid prophylactic agent that blocks the incorporation of radioactive iodide in the thyroid; it is generally available for oral administration by the population in case of a nuclear release. However, the blockage provided by KI is not 100% effective and therefore activity could still be measured in the thyroid after an intake of radioactive iodine. As a consequence of KI administration the thyroid retention function and the thyroid dose coefficient are modified. To assess the thyroid dose from in vivo measurements these two quantities must be taken into account.In this work we considered the inhalation of 131-iodine by adult, children (1, 5, 10 years-old) and adolescents (15 years). The effect of KI administration was modelled by a time-dependent blood to thyroid transfer rate coefficient. The model was benchmarked against dose coefficient in the absence of KI and against the protective effect curves depending on KI administration time.This KI specific model was used to provide correction factors for dose assessment. These multiplicative correction factors apply to a “classical” dose assessment, i.e. a dose assessment based on the ICRP default model that ignores the KI effect. This solution has been preferred since it provides ready to use values avoiding implementing the KI specific model. The correction factors depend on the measurement time and on the KI administration time. They are relatively independent of age and can be described by simple analytic functions. Working examples are provided in this study.For examples, KI administration 12h before the intake and early in vivo measurements (between 4h and 64h) after the intake give correction factors between 1.2 and 15. For late measurements the correction factors are generally small. If KI has been taken after the intake the correction factors are also generally small, except for very early measurements.

PRACTICAL METHODS FOR INTERNAL DOSE ASSESSMENT FOR RADIOIODINE INTAKE AFTER THYROID BLOCKING: CLASSIFICATION OF DEGREE OF BLOCKAGE AND DETERMINATION OF INSENSITIVE MEASUREMENT POINT.

Iodine thyroid blocking (ITB) suppresses the uptake of iodine to the thyroid and reduces internal doses after radioiodine intake; however, its disturbance of thyroid biokinetics causes considerable uncertainty in the use of dosimetric data intended for assessment of unblocked normal thyroid. To more accurately assess internal dose after ITB, practical dosimetry methods were proposed that consider the ITB effect in a dosimetric manner. A method using the ratio of urine excretion to thyroid retention activity was proposed to retrospectively determine individual-specific ITB levels; bioassay functions and dose coefficients corresponding to ITB levels were calculated separately using the latest biokinetic model and fundamental data. Moreover, insensitive measurement points of time, which led to similar results regardless of ITB level, were determined based on the dose per unit content. Proposed insensitive points for inhalation of vapour forms and particulate forms, respectively, were 1.5 days and 2 days after exposure.

Usefulness of SPECT/CT in Parathyroid Lesion Detection in Patients with Thyroid Parenchymal 99mTc-Sestamibi Retention

Parathyroid adenoma detection with dual-phase 99mTc-sestamibi (MIBI) scintigraphy depends on differential MIBI washout from thyroid. However, autoimmune thyroid disease (AITD) may cause MIBI to be retained in the thyroid gland and reduce parathyroid detection. We evaluated the impact of AITD on MIBI thyroid retention and additional benefit of SPECT/CT in these patients. Dual phase planar MIBI and SPECT/CT was performed on 82 patients. SPECT/CT was performed immediately after delayed planar scan. Thyroid density (Hounsfield unit, CT-HU) and size were measured on CT component of SPECT/CT. MIBI uptake in early scans and retention in delayed scans were visually graded and correlated with clinical factors and CT findings. Finally, planar and SPECT/CT findings were compared for parathyroid lesion visualization according to thyroid MIBI retention. In early scan, multivariate analysis showed only thyroid size predicted early uptake. In delayed scan, multivariate analysis showed higher visual grade in early scan, lower CT-HU or AITD were significant predictors for delayed thyroid parenchymal retention. Overall, ten more parathyroid lesions were visualized on SPECT/CT compared to planar scans (57 vs. 47, p = 0.002). SPECT/CT was especially more useful in patients with thyroidal MIBI retention, as eight out of the ten additional lesions detected were found in patients with thyroid MIBI retention. AITD is an important factor for MIBI thyroid parenchymal retention on delayed scans, and may impede parathyroid lesion detection. Patients with MIBI retention in the thyroid parenchyma on delayed scans are likely to benefit from an additional SPECT/CT.

Thyroid Doses and Skin Contaminations of Radioiodine

Radioiodine treatment, using iodine I-131, is one of the oldest clinical radionuclide therapy methods, and it is widely spread and currently used in the treatment of both thyreotoxicosis and thyroid cancer. The use of iodine may cause skin contaminations which may be a radiobiologic hazard and diagnostic problem. Reported hazards in the radiopharmaceutical production of I-131 for therapy purposes are rare, but they might cause therapeutic approaches due to radioiodine uptake in the thyroid and destruction of thyroid tissue (mean beta range, 0.8 mm). In this work a skin contamination causing a radioiodine thyroid uptake has been reported, together with thyroid cancer patients of receiving therapeutic doses of I-131 having skin contaminations from exudates. These skin contaminations were quantificated, dose calculations were performed based on serial gamma detector and quantitative whole body gamma imaging, including SPECT/CT. For serial gamma imaging, thyroid retention and whole clearance after oral administration, rhTSH stimulation and thyroxin withdrawal give different results and therefore serial uptake estimates and time-activity curves are essential parameters for dose evaluation. Similarly, accidental subcutaneous administration for calculating thyroid uptake requires serial measurement as well. In these patients, the skin contaminations did not cause any potential hazard to the environment nor to the hospital even though potential risks have been reported in the early days. Skin doses due to contaminations varied from 0.64 to 2.7 Gy. In the radiopharmaceutical production accidental skin contamination did not cause any harmful thyroid dose (80 mGy) due to rapid cleaning actions. Quantitative gamma imaging together with activity calibrated detectors is the prerequisite for appropriate dose calculations when routes of administration differ from clinical routine.

An alternative method for the release criteria and calculation of the total dose equivalent to another individual from a patient treated with a therapeutic dose of 131I.

The Nuclear Regulatory Commission's Regulatory Guide 8.39, "Release of Patients Administered Radioactive Materials," permits licensees to authorize the release of patients treated with 131I [(T 1/2)p = 8.04 d] if the total dose equivalent to another individual from the released individual is not likely to exceed 5 millisieverts (0.5 rem). This paper guides the reader through the derivation of an easy-to-use formula that demonstrates how to calculate the total dose equivalent, using patient-specific parameters that an individual may receive from a patient who has been administered a therapeutic dose of 131I, as compared to the NRC's Reg. Guide 8.39 (see Appendix A). Patient-specific parameters include the individual thyroidal uptake, the patient's physical dimensions, attenuation of radiation by body tissues, buildup of secondary photons, whole body and thyroid retention functions as well as the distance and time the patient is around others. This paper also demonstrates how to calculate an inpatient's discharge time, so that the patient will not expose other individuals above the desired total dose equivalent set by the Radiation Safety Officer (RSO).

Radiological protection guidance for radioactive patients — new data for therapeutic 131 I

Patients leaving hospital after 131I treatment for thyrotoxicosis face restrictions on their contact with other members of the public. These restrictions depend on the level of residual body radioactivity which for practical purposes can be taken to be almost entirely in the thyroid gland. This study provides an appropriate data base from which to draw advice to patients consistent with current radiological protection requirements in terms of the duration of these restrictions. Thyroidal retention of 131I was measured in 77 thyrotoxic patients over a period of 1-50 days after a first therapeutic administration of the radionuclide. Mean 131I activity in the gland (+/- S.D.) at 1 day was 56.1 +/- 11.1% of the administered dose activity and thereafter retention followed a single exponential decay pattern with a mean effective half-life (+/- S.E.M.) of 6.35 +/- 0.14 days. In patients who required further 131I therapy, there was evidence that retention could be markedly reduced if there was virtual ablation of thyroid tissue. It is proposed that these retention data can be used to determine body radioactivity at any interval after the administration of 131I for treatment of thyrotoxicosis, thus obviating the need for serial measurements in every individual patient.

High thyroid radiation dose associated with131I-19-iodocholesterol adrenal scanning

We have examined two patients undergoing 131I-19-iodocholesterol adrenal scans in order to assess thyroidal radiation dose. Thyroid uptakes of 3--5% of the administered 2 mCi dose were found despite prior administration of Lugol's iodine. Analysis of serum samples over a ten day period after iodocholesterol injection indicated that less than 10% of the radioactivity was free iodide. Kinetic results showed a high value for the thyroid/plasma 131I ratio indicating thyroid retention of 131I. This was substantiated by thyroid scintigrams. The thyroid radiation dose was estimated to be 200--250 rad for the two patients. The associated carcinogenic risk is of sufficient magnitude, in our opinion, to warrant regular follow-up of all patients who have undergone 131I-19-iodocholesterol adrenal scanning.

A Biphasic Response of Rats to Cobalt

The study was designed to confirm a previous, unexpected observation of a strong growth depressing effect of 1 microgram cobalt/g in rats fed lactalbumin based diets. The addition of 0.25, 0.5, and 1.0 microgram of cobalt/g to the basal diet containing 0.056 microgram/g depressed growth rates of rats progressively with increasing doses. This depression was overcome by increasing the cobalt supplement to 2 microgram/g, and additional weight gain was observed with 3 microgram/g. Higher concentrations were progressively toxic. Hemoglobin concentrations, hematocrits, and thyroid retention of intravenously injected sodium iodide all were lowest in rats fed the diet containing 1 microgram cobalt/g and increased with lower and higher concentrations of cobalt. The opposite was true for fasting serum glucose levels, which were elevated in rats fed the 1 microgram/g diet and low in rats fed the 3 microgram/g diet or control diet. This biphasic response to cobalt is consistent with the hypothesis that cobalt in low concentrations may have an essential function in the rat. However, an alternative explanation, an interaction of cobalt with a toxic constituent of the diet, has not yet been ruled out.

Comparative Thyroid Uptake Studies with131I and99mTcO4−

Using a scintillation camera equipped with a data processor and image-enhancement system, we compared 20-min 131I and 99mTcO4− uptakes in euthyroid and hyperthyroid subjects. 131I/99mTcO4− uptake ratios were significantly lower in hyperthyroid patients (1.6 ± 0.5 vs 3.5 ± 0.9) indicating a greater avidity for pertechnetate by the hyperthyroid gland. Following administration of perchlorate, hyperthyroid subjects exhibited significant thyroidal retention of 99mTcO4− (2.1–24.8% of thyroid technetium at 1 hr, mode = 7.7%) whereas euthyroid subjects exhibited none, suggesting thyroidal “binding” of technetium in hyperthyroidism. These findings were not reproduced by prior TSH administration to euthyroid controls. Euthyroid subjects with single autonomous hyperfunctiong thyroid nodules exhibited greater relative avidity for 99mTcO4− compared to euthyroid controls; nodular retention of 99mTcO4− after perchlorate was similar to that seen in the hyperplastic thyroid gland of Graves' disease.

Thyroid Function in Chickens and Rats Equilibration of Injected Iodide with Existing Thyroidal Iodine in White Leghorn Cockerels

The kinetics of equilibration of newly concentrated iodide (I131) with existing thyroidal iodine (I127) stores were studied in White Leghorn cockerels. A long thyroidal retention of I131 was observed. The relative distribution of I131 in the thyroidal iodoamino acids became identical with that of I127 48 hours after injection of radioiodide and remained that way during the rest of the experiment. The results are interpreted to signify that intrathyroidal iodination and deiodination reactions occur continually and lead to randomization of thyroidal iodine.

Thyroidal Iodine Metabolism in Inbred and F1 Hybrid Mice

Thyroidal iodine metabolism was studied in four inbred strains of mice and two groups of F1 hybrids by use of radioiodine. Significant strain differences were found in both the 48-hour thyroidal retention of I131 and its output rate constant. Three closely related groups studied were C57BL/6, C57BR/cd and BBF1, their F1 hybrid. The other three groups included A/Jax, BALB/c and CAF1, their F1 hybrid. C57BL/6 mice had a significantly faster output rate and lower 48-hour I131 retention than any other group. The first three groups listed had a more rapid output rate and lower 48-hour retention than the last three. In intergroup comparisons and inverse relationship between I131 output rate and 48-hour retention was clearly shown. The results indicate that in mice pituitary TSH output, as indicated by thyroidal I131 output rate and thyroidal iodine pool size, is controlled by separate genetic factors.