Thyroid abnormalities associated with protracted childhood exposure to 131I from atmospheric emissions from the Mayak weapons facility in Russia

Between 1948 and 1960, the Mayak nuclear weapons facility in Ozyorsk, Russia discharged relatively high levels of radionuclides, primarily (131)I, into the atmosphere, resulting in appreciable exposure to the residents of Ozyorsk. To evaluate the association between thyroid diseases and childhood exposure to radioiodines, we screened 894 Ozyorsk residents born between 1952 and 1953. The study population was comprised of 581 exposed individuals living in Ozyorsk during the years of heaviest exposure and 313 nonexposed individuals who moved to Ozyorsk when radiation exposure from Mayak largely had ended. The screening protocol included a patient interview, palpation of the thyroid, cervical lymph nodes and salivary glands, an ultrasound examination, and measurement of fT4, TSH and TPOAb. Twenty-eight percent of the study group was diagnosed with a thyroid abnormality. The prevalence of nodular disease was significantly higher in the exposed group (20.7%) compared with the nonexposed (14.4%) group (relative risk = 1.4, 95% CI = 1.1; 1.9). Risks were larger for solitary nodules and for nodules > or = 10 mm in diameter. Expansion of the study to increase the number of persons screened as well as detailed dose estimation would offer an unique opportunity to evaluate thyroid disease in relation to chronic exposure to radioiodines during childhood.     

Prevalence of hyperthyroidism after exposure during childhood or adolescence to radioiodines from the chornobyl nuclear accident: dose-response results from the Ukrainian-American Cohort Study

Relatively few data are available on the prevalence of hyperthyroidism (TSH concentrations of <0.3 mIU/liter, with normal or elevated concentrations of free T4) in individuals exposed to radioiodines at low levels. The accident at the Chornobyl (Chernobyl) nuclear plant in Ukraine on April 26, 1986 exposed large numbers of residents to radioactive fallout, principally to iodine-131 ((131)I) (mean and median doses = 0.6 Gy and 0.2 Gy). We investigated the relationship between (131)I and prevalent hyperthyroidism among 11,853 individuals exposed as children or adolescents in Ukraine who underwent an in-depth, standardized thyroid gland screening examination 12-14 years later. Radioactivity measurements taken shortly after the accident were available for all subjects and were used to estimate individual thyroid doses. We identified 76 cases of hyperthyroidism (11 overt, 65 subclinical). Using logistic regression, we tested a variety of continuous risk models and conducted categorical analyses for all subjects combined and for females (53 cases, n = 5,767) and males (23 cases, n = 6,086) separately but found no convincing evidence of a dose-response relationship between (131)I and hyperthyroidism. There was some suggestion of elevated risk among females in an analysis based on a dichotomous dose model with a threshold of 0.5 Gy chosen empirically (OR = 1.86, P = 0.06), but the statistical significance level was reduced (P = 0.13) in a formal analysis with an estimated threshold. In summary, after a thorough exploration of the data, we found no statistically significant dose-response relationship between individual (131)I thyroid doses and prevalent hyperthyroidism.     

Karyopathological traits of thyrocytes and exposure to radioiodines in Belarusian children and adolescents following the accident at the Chernobyl nuclear power plant

The Belarus-American (BelAm) thyroid study cohort consists of persons who were 0-18 years of age at the time of exposure to radioactive iodine fallout from the 1986 Chernobyl nuclear power plant accident and who have undergone serial thyroid screenings with referral for fine-needle aspiration biopsy (FNAB) using standardized criteria. We investigated thyrocyte nuclear abnormalities in cytological samples from FNABs in 75 BelAm subjects with single and multiple thyroid nodules and 47 nodular goiter patients from Leningrad, Russia, unexposed to Chernobyl fallout. Nuclear abnormalities examined included internuclear chromosome bridges and derivative nuclei with broken bridges (i.e., "tailed" nuclei), which are formed from dicentric and ring chromosomes and thus may be cellular markers of radiation exposure. Among subjects with single-nodular goiter, thyrocytes with bridges were present in 86.8% of the exposed BelAm cohort compared with 27.0% of unexposed controls. The average frequency of thyrocytes with bridges and with tailed nuclei was also significantly higher in the BelAm subjects than in controls. Among subjects with multinodular goiters, thyrocytes with bridges were present in 75.7% of exposed BelAm patients compared with 16.7% of unexposed controls; thyrocytes with tailed nuclei were observed in all of the BelAm subjects but in only 40% of controls, and the mean frequencies of bridges and tailed nuclei were significantly higher in the exposed group. Unusually, long bridges were detected in 29% of BelAm patients with single-nodular goiters and 35% of those with multinodular goiters, while no such abnormalities were observed among patients from the Leningrad region. In the exposed subjects from BelAm, we also found positive correlations between their estimated dose of Iodine-131 from Chernobyl fallout and the frequency of tailed nuclei (p = 0.008) and bridges (p = 0.09). Further study is needed to confirm that these phenomena represent consequences of radiation exposure in the human organism.     

Post-Chernobyl thyroid cancers in Ukraine. Report 2: risk analysis

On April 26, 1986, the worst nuclear reactor accident to date occurred at the Chornobyl (Chernobyl) power plant in Ukraine. Millions of people in Ukraine, Belarus and Russia were exposed to radioactive nuclides, especially (131)I. Since then, research has been conducted on various subgroups of the exposed population, and it has been demonstrated that the large increase in thyroid cancer is related to the (131)I exposure. However, because of study limitations, quantified risk estimates are limited, and there remains a need for additional information. We conducted an ecological study to investigate the relationship between (131)I thyroid dose and the diagnosis of thyroid cancer in three highly contaminated oblasts in Northern Ukraine. The study population is comprised of 301,907 persons who were between the ages of 1 and 18 at the time of the Chornobyl accident and were living in 1,293 rural settlements in the three study oblasts. Twenty-four percent of the study population had individual thyroid dose estimates and the other 76% had "individualized" estimates of thyroid dose based on direct thyroid measurements taken from a person of the same age and gender living in the same or nearby settlement. Cases include 232 thyroid cancers diagnosed from January 1990 through December 2001, and all were confirmed histologically. Dose-response analyses took into account differences in the rate of ultrasound examinations conducted in the three study oblasts. The estimated excess relative risk per gray was 8.0 (95% CI = 4.6-15) and the excess absolute risk per 10,000 person-year gray was estimated to be 1.5 (95% CI = 1.2-1.9). In broad terms, these estimates are compatible with results of other studies from the contaminated areas, as well as studies of external radiation exposure.     

Mutagenicity of diagnostic and therapeutical doses of radiopharmaceutical iodine-131 in Wistar rats

Iodine-131 (¹³¹I) is a radioisotope used for the diagnosis and treatment of thyroidal disorders such as hyperthyroidism and cancer. During its decay, ¹³¹I emits beta particles and gamma rays; its physical half-life is 8 days, and it is accumulated preferentially in the thyroid tissue. This study aimed to evaluate the cytotoxicity and mutagenicity of diagnostic and therapeutic doses of ¹³¹I using bone marrow cells of rats treated in vivo in a test system with a single dose by gavage. Concentrations of 5, 25, 50 and 250 μCi in 1 ml of water were used, and after 24 h, the animals were killed. Also, a concentration of 25 μCi/ml of water was used, and the animals were killed after 5 days. The results showed that no concentration of ¹³¹I was cytotoxic and that all concentrations were mutagenic. As a result, there was no statistically significant difference detected by the χ² test in the induction of chromosomal aberrations between the different doses. Thus, the present study demonstrated a significant increase in chromosomal aberration in bone marrow cells exposed to ¹³¹I regardless of the dose or the treatment time.     

食盐加碘后甲状腺摄131I 率的变化及其与尿碘的关系

目的：了解食盐加碘后健康人及甲亢患者甲状腺摄131I 率的变化及其与24 小时尿碘含量的相关性,探讨甲状腺摄131I 率与 碘营养状况的关系。方法：对比食盐加碘前后健康体检者及甲亢患者甲状腺摄131I 率的变化,分析健康体检者甲状腺摄131I 率、晨 尿碘浓度及经肌酐校正的尿碘含量与24小时尿碘含量的相关关系。结果：健康人及甲亢患者食盐加碘后3、6 及24 小时甲状腺 摄131I 率均显著降低;健康体检者甲状腺摄131I 率与24 小时尿碘含量呈负相关（r=－0.7651, P<0.001）,晨尿碘浓度与24 小时尿碘 含量呈正相关（r=0.8231, P<0.001）,经肌酐校正的尿碘含量与24 小时尿碘含量呈正相关（r=0.9054, P<0.001）。结论：食盐加碘对甲 状腺摄131I 率有显著影响,应重新确立甲状腺摄131I 率的正常范围及甲亢的诊断标准;经肌酐校正的尿碘含量较晨尿碘浓度能更 准确地反映碘营养状况;甲状腺摄131I率可作为评估个体碘营养状况的指标,可以稳定地反映近期的碘营养状况。     

Thyroid cancer risk 40+ years after irradiation for an enlarged thymus: an update of the Hempelmann cohort

Although ionizing radiation is a known carcinogen, the long-term risk from relatively higher-dose diagnostic procedures during childhood is less well known. We evaluated this risk indirectly by assessing thyroid cancer incidence in a cohort treated with "lower-dose" chest radiotherapy more than 55 years ago. Between 2004 and 2008, we re-surveyed a population-based cohort of subjects treated with radiation for an enlarged thymus during infancy between 1926 and 1957 and their unexposed siblings. Thyroid cancer occurred in 50 irradiated subjects (mean thyroid dose, 1.29 Gy) and in 13 nonirradiated siblings during 334,347 person-years of follow-up. After adjusting for attained age, Jewish religion, sex and history of goiter, the rate ratio for thyroid cancer was 5.6 (95% CI: 3.1-10.8). The adjusted excess relative risk per gray was 3.2 (95% CI: 1.5-6.6). The adjusted excess absolute risk per gray was 2.2 cases (95% CI: 1.4-3.2) per 10,000 person-years. Cumulative thyroid cancer incidence remains elevated in this cohort after a median 57.5 years of follow-up and is dose-dependent. Although the incidence appeared to decrease after 40 years, increased risk remains a lifelong concern in those exposed to lower doses of medical radiation during early childhood.     

食盐加碘后甲状腺摄 ~（131）I率的变化及其与尿碘的关系

目的：了解食盐加碘后健康人及甲亢患者甲状腺摄131I率的变化及其与24小时尿碘含量的相关性,探讨甲状腺摄131I率与碘营养状况的关系。方法：对比食盐加碘前后健康体检者及甲亢患者甲状腺摄131I率的变化,分析健康体检者甲状腺摄131I率、晨尿碘浓度及经肌酐校正的尿碘含量与24小时尿碘含量的相关关系。结果：健康人及甲亢患者食盐加碘后3、6及24小时甲状腺摄131I率均显著降低;健康体检者甲状腺摄131I率与24小时尿碘含量呈负相关（r=-0.7651,P〈0.001）,晨尿碘浓度与24小时尿碘含量呈正相关（r=0.8231,P〈0.001）,经肌酐校正的尿碘含量与24小时尿碘含量呈正相关（r=0.9054,P〈0.001）。结论：食盐加碘对甲状腺摄131I率有显著影响,应重新确立甲状腺摄131I率的正常范围及甲亢的诊断标准;经肌酐校正的尿碘含量较晨尿碘浓度能更准确地反映碘营养状况;甲状腺摄131I率可作为评估个体碘营养状况的指标,可以稳定地反映近期的碘营养状况。     

Exposure to the Chinese Famine in Early Life and the Thyroid Function and Nodules in Adulthood

Objective: Previous studies found that exposure to famine was associated with a higher risk of metabolic syndrome and fatty liver in adult women. In the present study, we investigated the relationship between exposure to the Chinese famine in early life and thyroid function and nodules in adulthood.Methods: We retrospectively analyzed the data of subjects who underwent routine physical check-up in the Public Health Center of our hospital in 2017. Subjects were divided into 3 groups: post-, pre-, and nonexposed groups. The basal metabolic rate (BMR) was estimated by the revised Harris-Benedict formulation. The serum levels of thyroid hormones were detected. Thyroid ultrasonography was performed by experienced technicians. The diagnosis of thyroid nodules was according to the Thyroid Imaging Reporting and Data System (TI-RADS).Results: Compared to nonexposed subjects, the postnatally exposed subjects had a significantly lower level of thyroxine, and statistically higher ultrasensitive thyroid stimulating hormone (P<.05). There was no significant difference in thyroid autoimmune antibodies between groups exposed to the famine and the nonexposed group (P>.05). There were no statistical differences in heart rate and BMR among these groups (P>.05). Exposure to the famine did not affect either the numbers of thyroid nodules, the TI-RADS score of thyroid nodules, or the maximal diameters of thyroid nodules.Conclusion: Our results indicate a significant association between famine exposure in early life and down-regulated thyroid function in adulthood. Postnatal famine exposure may be more vulnerable to nutrient deficiency and lead to restricted thyroid development in later life.Abbreviations: BMR = basal metabolic rate; FT3 = free triiodothyronine; FT4 = free thyroxine; IDD = iodine deficiency disorder; T3 = triiodothyronine; T4 = thyroxine; TG-Ab = thyroglobulin antibody; TI-RADS = Thyroid Imaging Reporting and Data System; uTSH = ultrasensitive thyroid stimulating hormone     

甲状腺癌术后短期甲状腺功能低下对骨代谢的影响

目的:观察分化型甲状腺癌(DTC)患者手术后、碘131(131I)清甲治疗前停用甲状腺素造成的短期甲减对骨代谢的影响。方法:选择DTC术后患者53例作为试验组,50例健康人作为对照组,试验组于停服甲状腺素第二天空腹采血行游离甲状腺素(FT4)、游离三碘甲状腺原氨酸(FT3)、促甲状腺激素(TSH)、血钙(Ca)、血磷(P)、血清碱性磷酸酶(ALP)、血清骨钙素(BGP),I型前胶原羧基末端前肽(PICP)、I型胶原交联羧基末端肽(ICTP)等各项检查,停服甲状腺素4周后于131I治疗前再行上述检查,对照组于体检当日采空腹血测相同项目。结果:试验组患者停药前血清FT3、FT4水平明显高于对照组,差异有统计学意义(P0.05)。停服甲状腺素后,试验组患者血清TSH升高,T3、T4水平降低,血BGP、血Ca、PICP、ICTP水平降低,与停药前及对照组的检验结果相比均有明显差异(P0.05)。停药后,骨密度与停药前比较差异无统计学意义(P0.05)。停服甲状腺素前后血磷水平无明显变化,与对照组相比也没有明显差异(P0.05)。结论:甲状腺癌术后造成甲状腺功能低下可明显影响患者骨代谢,应于131I后及时给予甲状腺激素,及时纠正甲低状态,同时也可适量补充钙剂。     

Discussion on radioactive iodine treatment of hyperthyroidism and the risk of induced cancer

Iodine-131 (¹³¹I, radioiodine) has been used for over eight decades for the treatment of Graves’ disease, either as initial therapy or following failure of thionamides, as well as for the treatment of autonomous thyroid nodules. ¹³¹I treatment is simple to administer, effective, and relatively inexpensive. Recently, there has been some turmoil after a study published in JAMA Internal Medicine reported an increased risk of cancer from ¹³¹I treatment. The impact was of short duration however, as the paper received severe criticisms from many nuclear medicine physicians as well as from endocrinologists. Here we explain why that paper's conclusions are doubtful. We also review the major data on the topic of ¹³¹I therapy of hyperthyroidism and the risk of cancer.     

Thyroid neoplasia following low-dose radiation in childhood

The thyroid gland is highly sensitive to the carcinogenic effects of ionizing radiation. Previously, we reported a significant increase of thyroid cancer and adenomas among 10,834 persons in Israel who received radiotherapy to the scalp for ringworm. These findings have now been extended with further follow-up and revised dosimetry. Overall, 98 thyroid tumors were identified among the exposed and 57 among 10,834 nonexposed matched population and 5392 sibling comparison subjects. An estimated thyroid dose of 9 cGy was linked to a fourfold (95% Cl = 2.3-7.9) increase of malignant tumors and a twofold (95% Cl = 1.3-3.0) increase of benign tumors. The dose-response relationship was consistent with linearity. Age was an important modifier of risk with those exposed under 5 years being significantly more prone to develop thyroid tumors than older children. The pattern of radiation risk over time could be described on the basis of a constant multiplication of the background rate, and an absolute risk model was not compatible with the observed data. Overall, the excess relative risk per cGy for thyroid cancer development after childhood exposure is estimated as 0.3, and the absolute excess risk as 13 per 10(6) PY-cGy. For benign tumors the estimated excess relative risk was 0.1 per cGy and the absolute risk was 15 per 10(6) PY-cGy.     

Childhood thyroid cancer, radiation dose from Chernobyl, and dose uncertainties in Bryansk Oblast, Russia: a population-based case-control study

A population-based case-control study was conducted to estimate the radiation-related risk of thyroid cancer in persons who were exposed in childhood to (131)I from the Chernobyl accident of April 26, 1986 and to investigate the impact of uncertainties in individual dose estimates. Included were all 66 confirmed cases of primary thyroid cancer diagnosed from April 26, 1986 through September 1998 in residents of Bryansk Oblast, Russia, who were 0-19 years old at the time of the accident, along with two individually matched controls for each case. Thyroid radiation doses, estimated using a semi-empirical model based on environmental contamination data and individual characteristics, ranged from 0.00014 Gy to 2.73 Gy and had large uncertainties (median geometric standard deviation 2.2). The estimated excess relative risk (ERR) associated with radiation exposure, 48.7/Gy, was significantly greater than 0 (P = 0.00013) but had an extremely wide 95% confidence interval (4.8 to 1151/Gy). Adjusting for dose uncertainty nearly tripled the ERR to 138/Gy, although this was likely an overestimate due to limitations in the modeling of dose uncertainties. The radiation-related excess risk observed in this study is quite large, especially if the uncertainty of dose estimation is taken into account, but is not inconsistent with estimates previously reported for risk after (131)I exposure or acute irradiation from external sources.     

Long-Term Follow-Up of a Patient With Papillary Thyroid Carcinoma,Elevated Thyroglobulin Levels,and Negative Imaging Studies

ObjectiveTo describe a patient with papillary thyroid carcinoma who had measurable thyroglobulin (Tg) levels for 20 years without clinical or imaging evidence of a malignant lesion.MethodsWe reviewed the clinical course, pathologic findings, Tg measurements, and results of various imaging studies in our patient and reviewed the literature regarding Tg-positive, diagnostic total-body radioiodine scan-negative patients with thyroid cancer.ResultsFour months after a 3.5- by 3.5-cm follicular thyroid cancer was removed from the anterior neck area of a 5-year-old girl, a bilateral subtotal thyroidectomy was performed. At age 12 years, she presented with a 2-cm mass on the right side of the neck. After a completion thyroidectomy, recurrent mixed papillary-follicular thyroid cancer was found scattered throughout the remaining thyroid parenchyma. Although a postoperative diagnostic total-body radioiodine scan did not reveal uptake of ¹³¹I, the Tg level was 58 ng/mL. Despite Tg levels as high as 2,528 ng/mL, the patient had no clinical evidence of thyroid cancer during a 20-year period of follow-up. Moreover, numerous imaging studies, including total-body scanning after the administration of 150 mCi of ¹³¹I and [¹⁸F]fluorodeoxyglucose positron emission tomography, were negative. Review of pathologic specimens from both operations with use of updated diagnostic criteria indicated that the tumor was a papillary thyroid carcinoma.ConclusionOur observations and the observations of other investigators indicate that some thyroid cancers produce Tg so efficiently that high levels of Tg may be associated with tumors that remain too small to be detected by imaging studies. The Tg levels may remain stable, decline, or even disappear over time without treatment. (Endocr Pract. 2005;11:43-48)     

Estimation of I131 doses absorbed by the thyroid in the period from May to June 1986

In a randomly selected group of subjects the uptake of iodine radioisotopes was measured over the thyroid in the period May 6-23, 1986. From the obtained results the doses were estimated which were absorbed from April 28, 1986 until the time of complete disappearance of I131 from the thyroid. The highest dose of radiation absorbed from I131 by the thyroid was found in the youngest group (children aged 5 to 12 years) and it was 227.8 mGy (about 23 rads).     

Ultrasonographic thyroid volume as a reliable prognostic index of radioiodine-131 treatment outcome in Graves' disease hyperthyroidism.

OBJECTIVE: We studied the relationship between thyroid volume, thyroid function and immunological markers of Graves' disease (GD) to determine prognostic factors of treatment response to low-dose radioiodine-131 (131I). MATERIAL AND METHODS: A prospective study of 40 patients with GD hyperthyroidism treated with 131I (141 +/- 85MBq) and 10 GD patients who went spontaneously into remission (controls). Free T4, total T3 and basal TSH levels, TSH-receptor antibodies (TRAb) and anti-thyroid peroxidase antibodies (TPOAb) were studied. Thyroid volume was determined by ultrasonography. Logistic regression models were used to predict the probability of final thyroid status. Receiver-operating characteristics (ROC) curves and Hosmer Lemeshow tests were used to evaluate the final statistical models. RESULTS: Of 40 patients treated with 131I, 16 became euthyroid, 12 hyperthyroid and 12 hypothyroid at 12 months. Median thyroid volume was reduced from 24.8 ml before to 8.5 ml at 12 months (p<0.001). In 10 control patients, the median reduction was from 16.6 ml to 11.3 ml (p=0.029). Thyroid volume reduction was lower in the hyperthyroid than in the euthyroid group, but higher in the hypothyroid group. Thyroid volume at baseline and at 3 months predicted hyperthyroidism outcome with a cut-off of 45 ml and 24.4 ml, respectively (odds ratio 1.074, p=0.003, ROC curve 0.78 and odds ratio 1.182, p=0.012, ROC curve 0.86 respectively). Thyroid volume at 6 months differentiated the hyperthyroid group with a cut-off of 17 ml. Thyroid volume at 3 and 6 months with a cut-off of 8.5 ml and 9.3 ml respectively, predicts permanent hypothyroidism outcome (odds ratio 0.768 and 0.685, p=0.012 and p=0.008, ROC curve 0.89 and 0.88, respectively). Changes in thyroid echogenicity and TRAb and TPOAb levels did not show any predictive value in the follow-up after 131I therapeutic outcome. CONCLUSION: The study shows that the ultrasonographic thyroid volume at 3 and 6 months after low-dose 131I treatment for GD hyperthyroidism could be a reliable prognostic factor of thyroid function outcome in the first year after treatment, and also reveals that the changes in the thyroid echogenicity and in the immunological markers of GD have no prognostic value.     

Long-Term Outcomes of Radioiodine Therapy for Juvenile Graves Disease with Emphasis on Subsequently Detected Thyroid Nodules: A Single Institution Experience from Japan

Objective: To investigate the long-term outcomes of radioiodine therapy (RIT) for juvenile Graves disease (GD) and the ultrasonographic changes of the thyroid gland.Methods: All of 117 juvenile patients (25 males and 92 females, aged 10 to 18 &lsqb;median 16] years) who had undergone RIT for GD at our clinic between 1999 and 2018 were retrospectively reviewed. Each RIT session was delivered on an outpatient basis. The maximum ¹³¹I dose per treatment was 13.0 mCi, and the total ¹³¹I dose per patient was 3.6 to 29.8 mCi (median, 13.0 mCi). ¹³¹I administration was performed once in 89 patients, twice in 26, and three times in 2 patients. Ultrasonography of the thyroid gland was regularly performed after RIT. The duration of follow-up after the initial RIT ranged from 4 to 226 (median 95) months.Results: At the latest follow-up more than 12 months after RIT (n = 111), the patients' thyroid functions were overt hypothyroidism (91%), subclinical hypothyroidism (2%), normal (5%), or subclinical hyperthyroidism (2%). New thyroid nodules were detected in 9 patients, 4 to 17 years after initial RIT. Patients with newly detected thyroid nodules underwent RIT with lower doses of ¹³¹I and had larger residual thyroid volumes than those without nodules. None of the patients were diagnosed with thyroid cancer or other malignancies during the follow-up period.Conclusion: Over a median follow-up period of 95 months (range, 4 to 226 months), RIT was found to be effective and safe in juvenile GD. However, cumulative evidence from further studies is required to confirm the long-term safety of RIT for juvenile GD.Abbreviations: ATD = antithyroid drug; GD = Graves disease; KI = potassium iodide; LT4 = levothyroxine; MMI = methimazole; PTU = propylthiouracil; RAIU = radio-active iodine uptake; RIT = radioiodine therapy; ^99mTc = technetium-99m; TSH = thyrotropin     

Risk of thyroid cancer among Chernobyl emergency workers of Russia

The presented paper deals with the thyroid cancer incidence in selected cohorts of emergency workers of Russia. In 1986-2003, a total of 87 cases of thyroid cancer were observed. Based on these data, a statistically significant increase in thyroid cancer incidence was found above the reference level for the male population of Russia, corresponding to a standardized incidence rate (SIR) of SIR = 3.47 [95% confidence interval (CI): 2.80; 4.25]. The highest incidence rate (SIR = 6.62, 95% CI: 4.63; 9.09) was shown for those emergency workers who took part in the early recovery operations in April-July 1986. The estimated SIR value increases to 7.97 (95% CI: 5.24; 11.52) after allowing for a 10 years latent period of Chernobyl-related thyroid cancers. These results indicate that the exposure to incorporated (131)I was the major risk factor of thyroid cancer among Chernobyl emergency workers. No statistically significant relationship was found for the thyroid cancer incidence and external radiation dose.     

Thyroid cancer after exposure to external radiation: a pooled analysis of seven studies

The thyroid gland of children is especially vulnerable to the carcinogenic action of ionizing radiation. To provide insights into various modifying influences on risk, seven major studies with organ doses to individual subjects were evaluated. Five cohort studies (atomic bomb survivors, children treated for tinea capitis, two studies of children irradiated for enlarged tonsils, and infants irradiated for an enlarged thymus gland) and two case-control studies (patients with cervical cancer and childhood cancer) were studied. The combined studies include almost 120,000 people (approximately 58,000 exposed to a wide range of doses and 61,000 nonexposed subjects), nearly 700 thyroid cancers and 3,000,000 person years of follow-up. For persons exposed to radiation before age 15 years, linearity best described the dose response, even down to 0.10 Gy. At the highest doses (>10 Gy), associated with cancer therapy, there appeared to be a decrease or leveling of risk. For childhood exposures, the pooled excess relative risk per Gy (ERR/Gy) was 7.7 (95% CI = 2.1, 28.7) and the excess absolute risk per 10(4) PY Gy (EAR/10(4) PY Gy) was 4.4 (95% CI = 1.9, 10.1). The attributable risk percent (AR%) at 1 Gy was 88%. However, these summary estimates were affected strongly by age at exposure even within this limited age range. The ERR was greater (P = 0.07) for females than males, but the findings from the individual studies were not consistent. The EAR was higher among women, reflecting their higher rate of naturally occurring thyroid cancer. The distribution of ERR over time followed neither a simple multiplicative nor an additive pattern in relation to background occurrence. Only two cases were seen within 5 years of exposure. The ERR began to decline about 30 years after exposure but was still elevated at 40 years. Risk also decreased significantly with increasing age at exposure, with little risk apparent after age 20 years. Based on limited data, there was a suggestion that spreading dose over time (from a few days to >1 year) may lower risk, possibly due to the opportunity for cellular repair mechanisms to operate. The thyroid gland in children has one of the highest risk coefficients of any organ and is the only tissue with convincing evidence for risk at about 0.10 Gy.     

A cohort study of thyroid cancer and other thyroid diseases after the Chornobyl accident: objectives, design and methods

The thyroid gland in children is one of the organs that is most sensitive to external exposure to X and gamma rays. However, data on the risk of thyroid cancer in children after exposure to radioactive iodines are sparse. The Chornobyl accident in Ukraine in 1986 led to the exposure of large populations to radioactive iodines, particularly (131)I. This paper describes an ongoing cohort study being conducted in Belarus and Ukraine that includes 25,161 subjects under the age of 18 years in 1986 who are being screened for thyroid diseases every 2 years. Individual thyroid doses are being estimated for all study subjects based on measurement of the radioactivity of the thyroid gland made in 1986 together with a radioecological model and interview data. Approximately 100 histologically confirmed thyroid cancers were detected as a consequence of the first round of screening. The data will enable fitting appropriate dose-response models, which are important in both radiation epidemiology and public health for prediction of risks from exposure to radioactive iodines from medical sources and any future nuclear accidents. Plans are to continue to follow-up the cohort for at least three screening cycles, which will lead to more precise estimates of risk.