A prospective study of medical diagnostic radiography and risk of thyroid cancer.

Although diagnostic x-ray procedures provide important medical benefits, cancer risks associated with their exposure are also possible, but not well characterized. The US Radiologic Technologists Study (1983-2006) is a nationwide, prospective cohort study with extensive questionnaire data on history of personal diagnostic imaging procedures collected prior to cancer diagnosis. We used Cox proportional hazard regressions to estimate thyroid cancer risks related to the number and type of selected procedures. We assessed potential modifying effects of age and calendar year of the first x-ray procedure in each category of procedures. Incident thyroid cancers (n = 251) were diagnosed among 75,494 technologists (1.3 million person-years; mean follow-up = 17 years). Overall, there was no clear evidence of thyroid cancer risk associated with diagnostic x-rays except for dental x-rays. We observed a 13% increase in thyroid cancer risk for every 10 reported dental radiographs (hazard ratio = 1.13, 95% confidence interval: 1.01, 1.26), which was driven by dental x-rays first received before 1970, but we found no evidence that the relationship between dental x-rays and thyroid cancer was associated with childhood or adolescent exposures as would have been anticipated. The lack of association of thyroid cancer with x-ray procedures that expose the thyroid to higher radiation doses than do dental x-rays underscores the need to conduct a detailed radiation exposure assessment to enable quantitative evaluation of risk.

EPR TOOTH DOSIMETRY OF SNTS AREA INHABITANTS.

The determination of external dose to teeth of inhabitants of settlements near the Semipalatinsk Nuclear Test Site (SNTS) was conducted using the EPR dosimetry technique to assess radiation doses associated with exposure to radioactive fallout from the test site. In this study, tooth doses have been reconstructed for 103 persons with all studied teeth having been formed before the first nuclear test in 1949. Doses above those received from natural background radiation, termed "accident doses", were found to lie in the range from zero to approximately 2 Gy, with one exception, a dose for one person from Semipalatinsk city was approximately 9 Gy. The variability of reconstructed doses within each of the settlements demonstrated heterogeneity of the deposited fallout as well as variations in lifestyle. The village mean external gamma doses for residents of nine[ settlements were in the range from a few tens of mGy to approximately 100 mGy.

Thyroid Radiation Dose to Patients from Diagnostic Radiology Procedures over Eight Decades: 1930-2010.

This study summarizes and compares estimates of radiation absorbed dose to the thyroid gland for typical patients who underwent diagnostic radiology examinations in the years from 1930 to 2010. The authors estimated the thyroid dose for common examinations, including radiography, mammography, dental radiography, fluoroscopy, nuclear medicine, and computed tomography (CT). For the most part, a clear downward trend in thyroid dose over time for each procedure was observed. Historically, the highest thyroid doses came from the nuclear medicine thyroid scans in the 1960s (630 mGy), full-mouth series dental radiography (390 mGy) in the early years of the use of x rays in dentistry (1930s), and the barium swallow (esophagram) fluoroscopic exam also in the 1930s (140 mGy). Thyroid uptake nuclear medicine examinations and pancreatic scans also gave relatively high doses to the thyroid (64 mGy and 21 mGy, respectively, in the 1960s). In the 21st century, the highest thyroid doses still result from nuclear medicine thyroid scans (130 mGy), but high thyroid doses are also associated with chest/abdomen/pelvis CT scans (18 and 19 mGy for males and females, respectively). Thyroid doses from CT scans did not exhibit the same downward trend as observed for other examinations. The largest thyroid doses from conventional radiography came from cervical spine and skull examinations. Thyroid doses from mammography (which began in the 1960s) were generally a fraction of 1 mGy. The highest average doses to the thyroid from mammography were about 0.42 mGy, with modestly larger doses associated with imaging of breasts with large compressed thicknesses. Thyroid doses from dental radiographic procedures have decreased markedly throughout the decades, from an average of 390 mGy for a full-mouth series in the 1930s to an average of 0.31 mGy today. Upper GI series fluoroscopy examinations resulted in up to two orders of magnitude lower thyroid doses than the barium swallow. There are considerable uncertainties associated with the presented doses, particularly for characterizing exposures of individual identified patients. Nonetheless, the tabulations provide the only comprehensive report on the estimation of typical radiation doses to the thyroid gland from medical diagnostic procedures over eight decades (1930-2010). These data can serve as a resource for epidemiologic studies that evaluate the late health effects of radiation exposure associated with diagnostic radiologic examinations.

Retrospective dose assessment for the population living in areas of local fallout from the Semipalatinsk nuclear test site Part I: External exposure.

A short analysis of all 111 atmospheric events conducted at the Semipalatinsk Test Site (STS) in 1949-1962 with regard to significant off-site exposure (more than 5 mSv of the effective dose during the first year after the explosion) has been made. The analytical method used to assess external exposure to the residents living in settlements near the STS is described. This method makes use of the archival data on the radiological conditions, including the measurements of exposure rate. Special attention was given to the residents of Dolon and Kanonerka villages exposed mainly as a result of the first test, detonated on August 29, 1949. For the residents of those settlements born in 1935, the dose estimates calculated according to the analytical method, are compared to those derived from the thermoluminescence measurements in bricks and electron paramagnetic resonance measurements in teeth. The methods described in this paper were used for external dose assessment for the cohort members at an initial stage of an ongoing epidemiological study conducted by the U.S. National Cancer Institute in the Republic of Kazakhstan. Recently revised methods and estimates of external exposure for that cohort are given in another paper (Simon et al.) in this conference.

External dose estimates for Dolon village: application of the U.S./Russian joint methodology.

Methods to estimate external dose from radioactive fallout from nuclear tests have for many years depended on two types of data: measurements of exposure rate in air and an empirically derived power function to describe the change in exposure rate with time, Over the last four years, a working group with American and Russian participation has developed a bi-national joint methodology that offers an improved capability for estimating external dose. In this method, external dose is estimated using exposure rate functions derived from data from American nuclear tests similar in construction to SNTS (Semipalatinsk Nuclear Test Site) devices. For example, in this paper, we derive doses for test #1 (August 29, 1949) at the SNTS using an exposure rate function for the U.S. TRINITY test. For the case of test #1, the average external dose for a person in Dolon is estimated to have been about 0.5 Gy compared to 1 to 2 Gy estimated in other work. This prediction agrees better with reported EPR measurements in teeth from village residents and with measurements of TL signals in bricks from Dolon buildings. This report presents the basic elements of the joint methodology model for estimation of external dose received from SNTS fallout.

Around Semipalatinsk nuclear test site: progress of dose estimations relevant to the consequences of nuclear tests (a summary of 3rd Dosimetry Workshop on the Semipalatinsk nuclear test site area, RIRBM, Hiroshima University, Hiroshima, 9-11 of March, 2005).

The paper is an analytical overview of the main results presented at the 3rd Dosimetry Workshop in Hiroshima(9-11 of March 2005), where different aspects of the dose reconstruction around the Semipalatinsk nuclear test site(SNTS) were discussed and summarized. The results of the international intercomparison of the retrospective luminescence dosimetry(RLD) method for Dolon' village(Kazakhstan) were presented at the Workshop and good concurrence between dose estimations by different laboratories from 6 countries (Japan, Russia, USA, Germany, Finland and UK) was pointed out. The accumulated dose values in brick for a common depth of 10mm depth obtained independently by all participating laboratories were in good agreement for all four brick samples from Dolon' village, Kazakhstan, with the average value of the local gamma dose due to fallout (near the sampling locations) being about 220 mGy(background dose has been subtracted).Furthermore, using a conversion factor of about 2 to obtain the free-in-air dose, a value of local dose approximately 440 mGy is obtained, which supports the results of external dose calculations for Dolon': recently published soil contamination data, archive information and new models were used for refining dose calculations and the external dose in air for Dolon village was estimated to be about 500 mGy. The results of electron spin resonance(ESR) dosimetry with tooth enamel have demonstrated the notable progress in application of ESR dosimetry to the problems of dose reconstruction around the Semipalatinsk nuclear test site. At the present moment, dose estimates by the ESR method have become more consistent with calculated values and with retrospective luminescence dosimetry data, but differences between ESR dose estimates and RLD/calculation data were noted. For example mean ESR dose for eligible tooth samples from Dolon' village was estimated to be about 140 mGy(above background dose), which is less than dose values obtained by RLD and calculations. A possible explanation of the differences between ESR and RLD/calculations doses is the following: for interpretation of ESR data the "shielding and behaviour" factors for investigated persons should be taken into account. The "upper level" of the combination of "shielding and behaviour" factors of dose reduction for inhabitants of Dolon' village of about 0.28 was obtained by comparing the individual ESR tooth enamel dose estimates with the calculated mean dose for this settlement. The biological dosimetry data related to the settlements near SNTS were presented at the Workshop. A higher incidence of unstable chromosome aberrations, micronucleus in lymphocytes, nuclear abnormalities of thyroid follicular cells, T-cell receptor mutations in peripheral blood were found for exposed areas (Dolon', Sarjal) in comparison with unexposed ones(Kokpekty). The significant greater frequency of stable translocations (results of analyses of chromosome aberrations in lymphocytes by the FISH technique) was demonstrated for Dolon' village in comparison with Chekoman(unexposed village). The elevated level of stable translocations in Dolon' corresponds to a dose of about 180 mSv, which is close to the results of ESR dosimetry for this village. The importance of investigating specific morphological types of thyroid nodules for thyroid dosimetry studies was pointed out. In general the 3rd Dosimetry Workshop has demonstrated remarkable progress in developing an international level of common approaches for retrospective dose estimations around the SNTS and in understanding the tasks for the future joint work in this direction. In the framework of a special session the problems of developing a database and registry in order to support epidemiological studies around SNTS were discussed. The results of investigation of psychological consequences of nuclear tests, which are expressed in the form of verbal behaviour, were presented at this session as well.

A summary of evidence on radiation exposures received near to the Semipalatinsk nuclear weapons test site in Kazakhstan.

The presently available evidence about the magnitude of doses received by members of the public living in villages in the vicinity of Semipalatinsk nuclear test in Kazakhstan, particularly with respect to external radiation, while preliminary, is conflicting. The village of Dolon, in particular, has been identified for many years as the most highly exposed location in the vicinity of the test site. Previous publications cited external doses of more than 2 Gy to residents of Dolon while an expert group assembled by the WHO in 1997 estimated that external doses were likely to have been less than 0.5 Gy. In 2001, a larger expert group workshop was held in Helsinki jointly by the WHO, the National Cancer Institute of the United States, and the Radiation and Nuclear Safety Authority of Finland, with the expressed purpose to acquire data to evaluate the state of knowledge concerning doses received in Kazakhstan. This paper summarizes evidence presented at that workshop. External dose estimates from calculations based on sparse physical measurements and bio-dosimetric estimates based on chromosome abnormalities and electron paramagnetic resonance from a relatively small sample of teeth do not agree well. The physical dose estimates are generally higher than the biodosimetric estimates (1 Gy or more compared to 0.5 Gy or less). When viewed in its entirety, the present body of evidence does not appear to support external doses greater than 0.5 Gy; however, research is continuing to try and resolve the difference in dose estimates from the different methods. Thyroid doses from internal irradiation, which can only be estimated via calculation, are expected to have been several times greater than the doses from external irradiation, especially where received by small children.

Current use and future needs of biodosimetry in studies of long-term health risk following radiation exposure.

Biodosimetry measurements can potentially be an important and integral part of the dosimetric methods used in long-term studies of health risk following radiation exposure. Such studies rely on accurate estimation of doses to the whole body or to specific organs of individuals in order to derive reliable estimates of cancer risk. However, dose estimates based on analytical dose reconstruction (i.e., models) or personnel monitoring measurements (i.e., film badges) can have substantial uncertainty. Biodosimetry can potentially reduce uncertainty in health risk studies by corroboration of model-based dose estimates or by using them to assess bias in dose models. While biodosimetry has begun to play a more significant role in long-term health risk studies, its use is still generally limited in that context due to one or more factors including inadequate limits of detection, large inter-individual variability of the signal measured, high per-sample cost, and invasiveness. Presently, the most suitable biodosimetry methods for epidemiologic studies are chromosome aberration frequencies from fluorescence in situ hybridization (FISH) of peripheral blood lymphocytes and electron paramagnetic resonance (EPR) measurements made on tooth enamel. Both types of measurements, however, are usually invasive and require biological samples that can be difficult to obtain. Moreover, doses derived from these methods are not always directly relevant to the tissues of interest. To increase the value of biodosimetry to epidemiologic studies, a number of issues need to be considered, including limits of detection, effects of inhomogenous exposure of the body, how to extrapolate from the tissue sampled to the tissues of interest, and how to adjust dosimetry models applied to large populations based on sparse biodosimetry measurements. The requirements of health risk studies suggest a set of characteristics that, if satisfied by new biodosimetry methods, would increase the overall usefulness of biodosimetry in determining radiation health risks.