Energy resolution improvement in simplified alpha spectroscopy for radionuclidic purity tests on 225Ac production floors.

This study investigated the feasibility of a simplified method of alpha spectroscopy for radionuclidic purity tests at 225Ac production sites that eliminates the need for a vacuum chamber. The impact of enhancing the energy resolution using a collimator was evaluated through radiation transport simulations. The results showed that a full width at tenth maximum (FWTM) of <300 keV was achieved for alpha particles from 241Am, for which the main energy peak was 5.5 MeV. Experimental validation using an electrodeposition source containing 237Np, 241Am, and 244Cm confirmed an FWTM of 272 keV for both 241Am and 244Cm. These two peaks, with a difference of ~300 keV, were effectively separated. In response to the growing demand for targeted radioisotope therapy, this simplified alpha spectroscopy method offers the potential to detect 226Ra mixed with 225Ac generated by accelerators, given the alpha energy difference of ~700 keV.

Efficacy of dotinurad in patients with severe renal dysfunction.

BACKGROUND: Although hyperuricemia is associated with the progression of chronic kidney disease (CKD), a reduction in CKD progression by uric acid (UA)-lowering therapy has been controversial. Recently, dotinurad, a uricosuric drug with selective urate reabsorption inhibitory properties, has been developed. However, its efficacy in lowering serum UA levels and its effects on renal function in patients with severe renal dysfunction are unclear. Thus, this study aimed to determine the effects of dotinurad on renal function in patients with severe renal dysfunction. METHODS: Data from 53 outpatients with hyperuricemia who newly received dotinurad between December 2020 and October 2022 were retrospectively analyzed. The mean baseline estimated glomerular filtration rate (eGFR) was 38.7 ± 17.0 mL/min/1.73 m2. The patients were divided into three groups based on their baseline eGFR: eGFR < 30 (n = 17), 30 ≤ eGFR < 45 (n = 17), and eGFR ≥ 45 (n = 19). RESULTS: The mean follow-up period was 9.8 ± 4.5 (range, 3-21) months. Serum UA levels significantly decreased in all groups. Although eGFR did not significantly change in patients with 30 ≤ eGFR < 45 and eGFR ≥ 45 (P = 0.918 and P = 0.535, respectively), it improved significantly in patients with eGFR < 30 (P = 0.032). The proportion of patients with improved eGFR was significantly higher in patients with eGFR < 30 (P = 0.038) than in patients with 30 ≤ eGFR < 45 and eGFR ≥ 45. In the multivariate logistic regression analysis, baseline eGFR < 30 and achieving a serum UA level of ≤ 6.0 mg/dL were significantly associated with improved eGFR (P = 0.033 and P = 0.015, respectively). CONCLUSIONS: Dotinurad may have UA-lowering effects and the potential to improve kidney function in patients with severe renal dysfunction.

Biokinetic model analysis with DTPA administration for a case of accidental inhalation of actinides in Japan.

Accidental inhalation intake of plutonium isotopes and 241Am occurred at a Pu research facility in Japan in 2017, and the five workers involved in this accident were treated by the administration of Ca/Zn-diethylenetriaminepentaacetic acid (DTPA). For the worker who was most internally exposed, the therapy was continued over 1 y after the accident. Urinary samples collected before and after each administration were subject to bioassay to evaluate the efficacy of the dose reduction. This study performed numerical analyses using a biokinetic model dealing with 241Am-DTPA with reference to the European Coordinated Network on Radiation Dosimetry approach, which assumes that the complex of actinides and Ca/Zn-DTPA is generated in the designated compartments in the biokinetic model. The results of the model prediction well captured the trend of the observed urinary excretion in the long-term bioassay and would be useful to evaluate the efficacy of the Ca/Zn-DTPA administration for the worker involved in the accident.

Rapid measurement of actinides in urine by mass and alpha spectrometric methods.

To provide timely information for prompting decision-making in emergency radiation therapy, we developed simple and rapid mass and alpha spectrometric methods for urinary bioassays to determine ultra-trace actinide isotopes. For the mass spectrometric method, after organic matter decomposition, LaF3/CaF2 co-precipitation and chromatographic purification using 2 ml of AG MP-1 M anion exchange resin, U and Pu isotopes were measured in a 20-ml urine sample by inductively coupled plasma-mass spectrometry. In the alpha spectrometric method, after organic matter decomposition, iron hydroxide co-precipitation and chromatographic purification using 2 ml of TEVA and 2 ml of DGA resin cartridges, Pu, U and Am/Cm isotopes were measured in a 500-ml urine sample by alpha spectrometry. These alpha and mass spectrometric methods were then applied for participation in the 2020 intercomparison organized by the Association for the PROmotion of Quality COntrol in RADiotoxicological Analysis (PROCORAD), France, for method validations.

A preliminary report on retrospective dose assessment by FISH translocation assay in FDNPP Nuclear Emergency Worker Study (NEWS).

In Japan, a national project of longitudinal health care and epidemiological research (NEWS) was developed in 2014 to analyse the effects of radiation on human health for workers who responded to the Fukushima Dai-ichi nuclear emergency in 2011. In 2018, peripheral blood for chromosome translocation analysis was collected from 62 workers. Retrospective dose assessment was performed with fluorescence in situ hybridisation translocation (FISH-Tr) assay. The range of estimated doses by FISH-Tr assay was 0-635 mGy, in which 22 workers had estimated doses of more than 189 mGy. Biological dose estimates were five times higher in workers with physically measured total exposure recordings above 70 mGy. It is likely that smoking and medical exposure caused the discrepancy between estimated biological and physical total exposure doses. Thus, there is a possibility that retrospective biodosimetry assessment might over-estimate occupational exposures to workers exposed to chronic radiation during nuclear emergency work.

No evidence of thyroid consequences in seven nuclear workers at the Tokyo Electric Power Company Fukushima Daiichi Nuclear Power Plant accident: 10-year follow-up results of thyroid status.

Seven emergency nuclear workers, who had internal exposure due to an intake of radionuclides, mainly I-131, during the emergency response operation in March 2011, after the accident at the Tokyo Electric Power Company (TEPCO) Fukushima Daiichi Nuclear Power Plant (FDNPP), visited the National Institute of Radiological Sciences (NIRS) outpatient clinic for medical evaluation. They were followed up after their first visit for 10 years. The estimated committed equivalent doses to the thyroid were distributed between 3.2 to 1.2 × 10 Sv. This group thought to be received highest exposure at the accident. None of the workers had symptoms related to abnormal thyroid function. The examinations, including thyroid function tests and ultrasound, detected no abnormalities related to radiation exposure. However, there is a need for continuous monitoring of their thyroid status for longer periods in the future.

Sex Differences in Culprit Plaque Characteristics Among Different Age Groups in Patients With Acute Coronary Syndromes.

BACKGROUND: Despite the significant decline in cardiovascular mortality in women over the past several decades, sex differences in the underlying pathology of acute coronary syndromes remain poorly understood. Previous postmortem studies have demonstrated sex differences in coronary plaque morphology with a higher prevalence of plaque erosion in young women and more plaque rupture in older women after menopause, whereas men showed no increase in prevalence of plaque rupture with age. However, in vivo data are limited. METHODS: This study included patients who presented with acute coronary syndrome and underwent preintervention optical coherence tomography imaging of the culprit lesion. The culprit plaque was categorized as plaque rupture, plaque erosion or culprit plaque with calcification, and stratified by age. Features of plaque vulnerability at culprit lesion were also analyzed. RESULTS: In 1368 patients (women=286), women and men had a similar distribution of culprit plaque morphology (plaque rupture versus plaque erosion). However, significant sex differences were found in the underlying mechanisms of acute coronary syndrome among different age groups: women showed a significant ascending trend with age in plaque rupture (P<0.001) and the features of plaque vulnerability such as lipid plaque (P<0.001), thin-cap fibroatheroma (P=0.005), and microstructures including macrophages, cholesterol crystals, and calcification (P=0.026). No trend was observed in men. CONCLUSIONS: Age related sex differences in culprit plaque morphology and vulnerability were identified in patients with acute coronary syndrome: prevalence of plaque rupture and vulnerability increased with age in women but not in men. REGISTRATION: URL: https://www. CLINICALTRIALS: gov; Unique identifier: NCT01110538 and NCT03479723.

Age and Phenotype of Patients With Plaque Erosion.

Background A recent study reported that the outcome of patients with plaque erosion treated with stenting is poor when the underlying plaque is lipid rich. However, the detailed phenotype of patients with plaque erosion, particularly as related to different age groups, has not been systematically studied. Methods and Results Patients with acute coronary syndromes caused by plaque erosion were selected from 2 data sets. Demographic, clinical, angiographic, and optical coherence tomography findings of the culprit lesion were compared between 5 age groups. Among 579 erosion patients, male sex and current smoking were less frequent, and hypertension, diabetes, and chronic kidney disease were more frequent in older patients. ST-segment-elevation myocardial infarction was more frequent in younger patients. Percentage of diameter stenosis on angiogram was greater in older patients. The prevalence of lipid-rich plaque (27.3% in age <45 years and 49.4% in age ≥75 years, P<0.001), cholesterol crystal (3.9% in age <45 years and 21.8% in age ≥75 years, P=0.027), and calcification (5.5% in age <45 years and 54.0% in age ≥75 years, P<0.001) increased with age. After adjusting risk factors, younger patients were associated with the presence of thrombus, and older patients were associated with greater percentage of diameter stenosis and the presence of lipid-rich plaque and calcification. Conclusions The demographic, clinical, angiographic, and plaque phenotypes of patients with plaque erosion distinctly vary depending on age. This may affect the clinical outcome in these patients. Registration URL: https://www.clinicaltrials.gov. Unique identifiers: NCT03479723, NCT02041650.

Rapid analysis of 237Np and Pu isotopes in small volume urine by SF-ICP-MS and ICP-MS/MS.

Internal contamination with alpha-particle emitting actinides, such as 237Np, 239Pu, 240Pu, is likely to bring a large amount of dose to the tissues of persons even if the intake amount is small. To provide timely information for prompt decision-making in radiation emergency therapy, we developed a simple and rapid method for urinary bioassay to determine ultra-trace 237Np and Pu isotopes using SF-ICP-MS and ICP-MS/MS. To avoid polyatomic interferences and tailing effects from U, 237Np and Pu isotopes were collected after removing U effectively using a simple single chromatographic column packed with 2 mL AG MP-1M anion exchange resin, exhibiting a high decontamination factor of 108 for 238U. The overall chemical fractionation between 237Np and 242Pu for the whole analytical procedure was 0.974 ± 0.064 (k = 2), allowing us to measure 237Np and Pu isotopes using 242Pu as a yield tracer with yields of 76 ± 5%. Using ICP-MS/MS with low background provided the method detection limits for 237Np, 239Pu, 240Pu, and 241Pu of 0.025, 0.025, 0.015, and 0.020 fg mL-1, respectively, for 20 mL of urine sample. Those were comparable to detection limits of SF-ICP-MS with high sensitivity. Subsequently, three urine reference materials with Pu spike, provided by the Association for the PROmotion of Quality COntrol in RADiotoxicological Analysis (PROCORAD), France, were analyzed by the developed method and the conventional alpha spectrometry technique for validation. Finally, the developed method was successfully employed to measure the contamination level of 237Np, 239Pu, 240Pu, and 241Pu in urine samples collected during decorporation therapy using DTPA, after a Pu inhalation exposure accident in Japan. The high throughput (9 h for 12 samples), simplicity, low cost, and high sensitivity of the method will allow greater numbers of related laboratories to be involved in screening activities for unexpected actinide exposure, such as in the case of a large scale radiological disaster.

Lessons from past radiation accidents: Critical review of methods addressed to individual dose assessment of potentially exposed people and integration with medical assessment.

The experiences of the Chernobyl and Fukushima nuclear accidents showed that dosimetry was the essential tool in the emergency situation for decision making processes, such as evacuation and application of protective measures. However, at the consequent post-accidental phases, it was crucial also for medical health surveillance and in further adaptation to changed conditions with regards to radiation protection of the affected populations. This review provides an analysis of the experiences related to the role of dosimetry (dose measurements, assessment and reconstruction) regarding health preventive measures in the post-accidental periods on the examples of the major past nuclear accidents such as Chernobyl and Fukushima. Recommendations derived from the review are called to improve individual dose assessment in case of a radiological accident/incident and should be considered in advance as guidelines to follow for having better information. They are given as conclusions.

Degree of luminal narrowing and composition of thrombus in plaque erosion.

As the degree of luminal narrowing increases, shear stress increases, and high shear stress is known to activate platelets. However, the relationship between the degree of luminal narrowing and the composition of thrombus in patients with plaque erosion has not been studied. A total of 148 patients with plaque erosion and thrombus detected by optical coherence tomography were divided into tertiles based on the minimum lumen area (MLA) at the culprit lesion. Thrombus was categorized as platelet-rich or fibrin-rich. Among 148 patients, 50 (34%) were in the mild stenosis group, 49 (33%) were in the moderate stenosis group, and 49 (33%) were in the severe stenosis group. The composition of thrombus was significantly different among the 3 groups (prevalence of platelet-rich thrombus was 60% in the mild stenosis group; 78% in the moderate stenosis group; and 84% in the severe stenosis group; P = 0.021). The pattern of fibrin-rich thrombus showed the opposite: 40%, 22%, and 16%, respectively. In the multivariate analysis, current smoking was independently associated with fibrin-rich thrombus (odds ratio [OR] 2.364 [95% CI 1.004-5.567], P = 0.049). This study demonstrated that platelet-rich thrombus was the predominant type of thrombus in plaque erosion. The prevalence of fibrin-rich thrombus was highest in the mild stenosis group.

Optical Coherence Tomography Predictors for a Favorable Vascular Response to Statin Therapy.

Background Specific plaque phenotypes that predict a favorable response to statin therapy have not been systematically studied. This study aimed to identify optical coherence tomography predictors for a favorable vascular response to statin therapy. Methods and Results Patients who had serial optical coherence tomography imaging at baseline and at 6 months were included. Thin-cap area (defined as an area with fibrous cap thickness <200 µm) was measured using a 3-dimensional computer-aided algorithm, and changes in the thin-cap area at 6 months were calculated. A favorable vascular response was defined as the highest tertile in the degree of reduction of the thin-cap area. Macrophage index was defined as the product of the average macrophage arc and length of the lesion with macrophage infiltration. Layered plaque was defined as a plaque with 1 or more layers of different optical density. In 84 patients, 140 nonculprit lipid plaques were identified. In multivariable analysis, baseline thin-cap area (odds ratio [OR] 1.442; 95% CI, 1.024-2.031, P=0.036), macrophage index (OR, 1.031; 95% CI, 1.002-1.061, P=0.036), and layered plaque (OR, 2.767; 95% CI, 1.024-7.479, P=0.045) were identified as the significant predictors for a favorable vascular response. Favorable vascular response was associated with a decrease in the macrophage index. Conclusions Three optical coherence tomography predictors for a favorable vascular response to statin therapy have been identified: large thin-cap area, high macrophage index, and layered plaque. Favorable vascular response to statin was correlated with signs of decreased inflammation. Registration URL: https://www.clinicaltrials.gov; Unique identifier: NCT01110538.

Relation of Low-Density Lipoprotein Cholesterol Level to Plaque Rupture.

Statin therapy reduces low-density lipoprotein cholesterol (LDL-C), inflammation, and atherosclerotic cardiovascular disease. We investigated the association between LDL-C and statin therapy on the prevalence of plaque rupture (PR). Patients with acute coronary syndromes who underwent optical coherence tomography imaging of the culprit lesion were divided into 4 groups based on LDL-C level and statin use (Group 1: LDL-C ≤ 100 without statin; Group 2; LDL-C ≤ 100 with statin; Group 3: LDL-C > 100 with statin; Group 4: LDL-C > 100 without statin), and the prevalence of PR was compared between the groups. Among 896 patients, PR was diagnosed in 444 (49.6%) patients. The prevalence of PR was significantly different among the 4 groups (p = 0.007): it was highest in the high LDL-C without statin group and lowest in the low LDL-C without statin group (53.9% and 39.2%, respectively). Compared with the high LDL-C without statin group, the low LDL-C without statin and low LDL-C with statin groups had a significantly lower prevalence of PR (p = 0.001, p = 0.040, respectively), and the low LDL-C with statin group had a significantly higher prevalence of calcification (p = 0.037). The patients with naturally low LDL-C have the lowest risk of PR. The patients with low LDL-C achieved by statin therapy had a higher prevalence of calcification. When LDL-C level is elevated, early and aggressive treatment with statin may help to prevent PR by stabilizing plaques through calcification.

Healed Plaques in Patients With Stable Angina Pectoris.

OBJECTIVE: Healed plaques, signs of previous plaque destabilization, are frequently found in the coronary arteries. Healed plaques can now be diagnosed in living patients. We investigated the prevalence, angiographic, and optical coherence tomography features of healed plaques in patients with stable angina pectoris. Approach and Results: Patients with stable angina pectoris who had undergone optical coherence tomography imaging were included. Healed plaques were defined as plaques with one or more signal-rich layers of different optical density. Patients were divided into 2 groups based on layered or nonlayered phenotype at the culprit lesion. Among 163 patients, 87 (53.4%) had layered culprit plaque. Patients with layered culprit plaque had more multivessel disease (62.1% versus 44.7%, P=0.027) and more angiographically complex culprit lesions (64.4% versus 35.5%, P<0.001). Layered culprit plaques had higher prevalence of lipid plaque (83.9% versus 64.5%, P=0.004), macrophage infiltration (58.6% versus 35.5%, P=0.003), calcifications (78.2% versus 63.2%, P=0.035), and thrombus (28.7% versus 14.5%, P=0.029). Lipid index (P=0.001) and percent area stenosis (P=0.015) were greater in the layered group. The number of nonculprit plaques, evaluated using coronary angiograms, tended to be greater in patients with layered culprit plaque (4.2±2.5 versus 3.5±2.1, P=0.053). Nonculprit plaques in patients with layered culprit lesion had higher prevalence of layered pattern (P=0.002) and lipid phenotype (P=0.005). Lipid index (P=0.013) and percent area stenosis (P=0.002) were also greater in this group. CONCLUSIONS: In patients with stable angina pectoris, healed culprit plaques are common and have more features of vulnerability and advanced atherosclerosis both at culprit and nonculprit lesions.

Reassessment of Internal Thyroid Doses to 1,080 Children Examined in a Screening Survey after the 2011 Fukushima Nuclear Disaster.

The dose reconstruction of populations potentially affected by the accident at the Fukushima Daiichi nuclear power plant in March 2011 is of great importance. However, it has been difficult to assess internal thyroid doses to Fukushima residents (mainly from their intake of I) due to the lack of direct measurements. For the residents, only about 1,300 data points related to I are available, and 1,080 of the data points were obtained from the screening campaign that was conducted by the Nuclear Emergency Response Local Headquarters at the end of March 2011 in Kawamata Town, Iwaki City, and Iitate Village. Here, we reassessed thyroid doses to 1,080 subjects aged ≤15 y old using new age-specific conversion factors to determine I thyroid contents from net signals of the devices used, with consideration for the possible uncertainty related to the measurements. The results demonstrated that thyroid equivalent doses to the subjects were <30 mSv (excluding outliers). We also demonstrate dose distributions of each age group from the above three municipalities and those of subjects from Minamisoma City and Fukushima City. One of the findings was that the I intake was similar among different age groups in each of the three municipalities. This was consistent with the assumption that ingestion was a dominant route of intake rather than inhalation. The range of thyroid doses to Iitate Village residents was similar to that to Iwaki City residents even though the I concentration in tap water was much higher in Iitate Village than Iwaki City. The range of thyroid doses to Minamisoma City residents was similar to that to Iitate Village and Iwaki City residents, and the range for Fukushima City residents was smallest among the five municipalities. Since the major route of intake has remained unclear, this paper presents the plausible upper and lower thyroid doses, between which the actual doses are thought to mostly exist, based on two intake scenarios: single inhalation and repeated ingestion. Further research is thus necessary to extract useful evidence from the individual evacuation behaviors for improving the present internal thyroid dose assessment.

Monitoring and Dose Assessment for Children Following a Radiation Emergency-Part II: Calibration Factors for Thyroid Monitoring.

Past radiological and nuclear accidents have demonstrated that monitoring a large number of children following a radiological and nuclear emergency can be challenging, in accommodating their needs as well as adapting monitoring protocols and applying age-specific biokinetics to account for various ages and body sizes. This paper presents the derived calibration factors for thyroid monitoring of children of all ages recommended by the International Commission on Radiological Protection using four selected detectors at given times following a short-term (acute) intake of I by inhalation. These calibration factors were derived by Monte Carlo simulations using the models of various detectors and pediatric voxel phantoms. A collection of lookup tables is presented in this paper which may be directly used as a quick reference by emergency response personnel or technical experts performing thyroid monitoring and assessment without doing time-consuming calculations.

AN ACCIDENT OF INTERNAL CONTAMINATION WITH PLUTONIUM AND AMERICIUM AT A NUCLEAR FACILITY IN JAPAN: A PRELIMINARY REPORT AND THE POSSIBILITY OF DTPA ADMINISTRATION ADDING TO THE DIAGNOSIS.

This article introduces the first accident of internal contamination with plutonium (Pu) or americium (Am) in Japan for which treatment was carried out. An accident of internal contamination with Pu and Am occurred at a Pu research facility at Oarai-town of Ibaraki prefecture in Japan. A plastic bag containing these radionuclides ruptured when five workers were inspecting a storage container in a hood. As a consequence, these workers were internally contaminated with Pu and Am. Although contamination on the body surface was observed in all five workers, a positive nasal swab was detected in only three of them. A chelating agent, calcium diethylenetriaminepenta-acetate (CaDTPA), was administered to all of them including the two workers without a positive nasal swab. However, bioassay detected a significant amount of Pu and Am in urine after administration of DTPA in these two workers, whereas the levels of these nuclides were below minimum detectable levels in urine before the administration. Since the prevalence of adverse reactions in DTPAs is low, the present results suggest that administration of DTPA can be used for the diagnosis of internal contamination even when a nasal swab is negative or contamination around body orifices is not detected.

Impact of Diabetic Retinopathy on Vulnerability of Atherosclerotic Coronary Plaque and Incidence of Acute Coronary Syndrome.

Although an association has been reported between the microvascular complications of diabetic patients and their poor prognosis after cardiovascular events related to advanced atherosclerosis, it is not clear whether there is a relation between diabetic retinopathy (DR) and the severity of plaque vulnerability. Fifty-seven diabetic patients with coronary artery disease, classified as non-DR (n = 42) or DR (n = 15), underwent angioscopic observation of at least 1 entire coronary artery. The number of yellow plaques (NYP) through the observed coronary artery was counted and their color grades, defined as 1 (light yellow), 2 (yellow), or 3 (intense yellow), were evaluated. The NYP per vessel and the maximum yellow grade were determined. The association between the presence of DR and incidences of acute coronary syndrome (ACS) was analyzed during the follow-up period (mean 7.1 ± 3.3 years; range, 0.83 to 11.75 years). Mean NYP per vessel and maximum yellow grade were significantly greater in DR than in non-DR patients (2.08 ± 1.01 vs 1.26 ± 0.77, p = 0.002, and 2.40 ± 0.74 vs 1.90 ± 0.82, p = 0.044, respectively). The cumulative incidences of ACS were higher in the DR group (p = 0.004), and the age-adjusted hazard ratio for ACS was 6.943 (95% CI 1.267 to 38.054; p = 0.026) for DR compared with non-DR patients. Our findings indicate that coronary atherosclerosis and plaque vulnerability are more severe in patients with DR. DR as a microvascular complication may be directly linked with macrovascular plaque vulnerability and fatal cardiovascular events such as ACS.

Managing Internal Radiation Contamination Following an Emergency: Identification of Gaps and Priorities.

Following a radiological or nuclear emergency, first responders and the public may become internally contaminated with radioactive materials, as demonstrated during the Goiânia, Chernobyl and Fukushima accidents. Timely monitoring of the affected populations for potential internal contamination, assessment of radiation dose and the provision of necessary medical treatment are required to minimize the health risks from the contamination. This paper summarizes the guidelines and tools that have been developed, and identifies the gaps and priorities for future projects.

NUMERICAL SIMULATION OF DIRECT MEASUREMENT TO DETERMINE THYROID 131I CONTENT OF TWO TEPCO WORKERS CONSIDERING INDIVIDUAL TISSUE THICKNESS.

After the Fukushima Daiichi Nuclear Power Station accident, the National Institute of Radiological Sciences examined seven heavily exposed emergency workers and performed internal dose estimations. The largest dose contributor was found to be (131)I, which was detected by thyroid monitor with an HPGe detector. Different energy peaks from (131)I were simultaneously identified in the pulse-height spectra of the two subjects with the highest doses regardless of late measurements. A closer look at the spectra indicated that the count ratio of the two peak areas at 80.2 and 365 keV differed somewhat between the individual workers, suggesting a difference in attenuation in the overlaying soft tissue and in the thyroid itself. In this study, the relationship between the count ratio (80.2/365 keV) and the thickness of soft tissue overlying the thyroid was investigated by means of numerical simulations performed using the Japanese Male (JM) phantom varying the thickness of the overlaying tissue. From the measured count ratios, it was possible to estimate that the overlaying tissue was thinner for Worker 1 (difference from the JM phantom: -0.34±1.29 cm) and thicker for Worker 2 (diff.: 2.5±1.2 cm). The thyroid (131)I contents evaluated taking into account the individual thicknesses were 4.3 kBq for Worker 1 and 8.4 kBq for Worker 2, resulting in a significant increase for Worker 2 compared with the content based on the default counting efficiency at 365 keV of the original JM phantom. However, the results have large uncertainty factors of 1.4 for Worker 1 and 1.3 for Worker 2 and should be carefully considered together with other factors influencing the attenuation.