Estimating Historical Radiation Doses to a Cohort of U.S. Radiologic Technologists

Ionising radiation and cancer risks: what have we learned from epidemiology?

PURPOSE

Epidemiologic studies of persons exposed to ionising radiation offer a wealth of information on cancer risks in humans. The Life Span Study cohort of Japanese A-bomb survivors, a large cohort that includes all ages and both sexes with a wide range of well-characterised doses, is the primary resource for estimating carcinogenic risks from low linear energy transfer external exposure. Extensive data on persons exposed for therapeutic or diagnostic medical reasons offer the opportunity to study fractionated exposure, risks at high therapeutic doses, and risks of site-specific cancers in non-Japanese populations. Studies of persons exposed for occupational and environmental reasons allow a direct evaluation of exposure at low doses and dose rates, and also provide information on different types of radiation such as radon and iodine-131. This article summarises the findings from these studies with emphasis on studies with well-characterised doses.

CONCLUSIONS

Epidemiologic studies provide the necessary data for quantifying cancer risks as a function of dose and for setting radiation protection standards. Leukaemia and most solid cancers have been linked with radiation. Most solid cancer data are reasonably well described by linear-dose response functions although there may be a downturn in risks at very high doses. Persons exposed early in life have especially high relative risks for many cancers, and radiation-related risk of solid cancers appears to persist throughout life.

Polymorphisms in estrogen biosynthesis and metabolism-related genes, ionizing radiation exposure, and risk of breast cancer among US radiologic technologists

Ionizing radiation-associated breast cancer risk appears to be modified by timing of reproductive events such as age at radiation exposure, parity, age at first live birth, and age at menopause. However, potential breast cancer risk modification of low to moderate radiation dose by polymorphic estrogen metabolism-related gene variants has not been routinely investigated. We assessed breast cancer risk of 12 candidate variants in 12 genes involved in steroid metabolism, catabolism, binding, or receptor functions in a study of 859 cases and 1,083 controls within the US radiologic technologists (USRT) cohort. Using cumulative breast dose estimates from a detailed assessment of occupational and personal diagnostic ionizing radiation exposure, we investigated the joint effects of genotype on the risk of breast cancer. In multivariate analyses, we observed a significantly decreased risk of breast cancer associated with the CYP3A4 M445T minor allele (rs4986910, OR = 0.3; 95% CI 0.1-0.9). We found a borderline increased breast cancer risk with having both minor alleles of CYP1B1 V432L (rs1056836, CC vs. GG, OR = 1.2; 95% CI 0.9-1.6). Assuming a recessive model, the minor allele of CYP1B1 V432L significantly increased the dose-response relationship between personal diagnostic X-ray exposure and breast cancer risk, adjusted for cumulative occupational radiation dose (p (interaction) = 0.03) and had a similar joint effect for cumulative occupational radiation dose adjusted for personal diagnostic X-ray exposure (p (interaction) = 0.06). We found suggestive evidence that common variants in selected estrogen metabolizing genes may modify the association between ionizing radiation exposure and breast cancer risk.

Breast cancer risk polymorphisms and interaction with ionizing radiation among U.S. radiologic technologists

Genome-wide association studies are discovering relationships between single-nucleotide polymorphisms and breast cancer, but the functions of these single-nucleotide polymorphisms are unknown and environmental exposures are likely to be important. We assessed whether breast cancer risk single-nucleotide polymorphisms interacted with ionizing radiation, a known breast carcinogen, among 859 cases and 1,083 controls nested in the U.S. Radiologic Technologists cohort. Among 11 Breast Cancer Association Consortium risk single-nucleotide polymorphisms, we found that the genotype-associated breast cancer risk varied significantly by radiation dose for rs2107425 in the H19 gene (P(interaction) = 0.001). H19 is a maternally expressed imprinted mRNA that is closely involved in regulating the IGF2 gene and could exert its influence by this or by some other radiation-related pathway.

Routine diagnostic X-ray examinations and increased frequency of chromosome translocations among U.S. radiologic technologists

The U.S. population has nearly one radiographic examination per person per year, and concern about cancer risks associated with medical radiation has increased. Radiologic technologists were surveyed to determine whether their personal cumulative exposure to diagnostic X-rays was associated with increased frequencies of chromosome translocations, an established radiation biomarker and possible intermediary suggesting increased cancer risk. Within a large cohort of U.S. radiologic technologists, 150 provided a blood sample for whole chromosome painting and were interviewed about past X-ray examinations. The number and types of examinations reported were converted to a red bone marrow (RBM) dose score with units that approximated 1 mGy. The relationship between dose score and chromosome translocation frequency was assessed using Poisson regression. The estimated mean cumulative RBM radiation dose score was 49 (range, 0-303). After adjustment for age, translocation frequencies significantly increased with increasing RBM dose score with an estimate of 0.004 translocations per 100 cell equivalents per score unit (95% confidence interval, 0.002-0.007; P < 0.001). Removing extreme values or adjustment for gender, cigarette smoking, occupational radiation dose, allowing practice X-rays while training, work with radioisotopes, and radiotherapy for benign conditions did not affect the estimate. Cumulative radiation exposure from routine X-ray examinations was associated independently with increased chromosome damage, suggesting the possibility of elevated long-term health risks, including cancer. The slope estimate was consistent with expectation based on cytogenetic experience and atomic bomb survivor data.

Nucleotide excision repair polymorphisms may modify ionizing radiation-related breast cancer risk in US radiologic technologists

Exposure to ionizing radiation has been consistently associated with increased risk of female breast cancer. Although the majority of DNA damage caused by ionizing radiation is corrected by the base-excision repair pathway, certain types of multiple-base damage can only be repaired through the nucleotide excision repair pathway. In a nested case-control study of breast cancer in US radiologic technologists exposed to low levels of ionizing radiation (858 cases, 1,083 controls), we examined whether risk of breast cancer conferred by radiation was modified by nucleotide excision gene polymorphisms ERCC2 (XPD) rs13181, ERCC4 (XPF) rs1800067 and rs1800124, ERCC5 (XPG) rs1047769 and rs17655; and ERCC6 rs2228526. Of the 6 ERCC variants examined, only ERCC5 rs17655 showed a borderline main effect association with breast cancer risk (OR(GC) = 1.1, OR(CC) = 1.3; p-trend = 0.08), with some indication that individuals carrying the C allele variant were more susceptible to the effects of occupational radiation (EOR/Gy(GG) = 1.0, 95% CI = <0, 6.0; EOR/Gy(GC/CC) = 5.9, 95% CI = 0.9, 14.4; p(het) = 0.10). ERCC2 rs13181, although not associated with breast cancer risk overall, statistically significantly modified the effect of occupational radiation dose on risk of breast cancer (EOR/Gy(AA) = 9.1, 95% CI = 2.1-21.3; EOR/Gy(AC/CC) = 0.6, 95% CI = <0, 4.6; p(het) = 0.01). These results suggest that common variants in nucleotide excision repair genes may modify the association between occupational radiation exposure and breast cancer risk.

Increased frequency of chromosome translocations associated with diagnostic x-ray examinations

Informative studies of cancer risks associated with medical radiation are difficult to conduct owing to low radiation doses, poor recall of diagnostic X rays, and long intervals before cancers occur. Chromosome aberrations have been associated with increased cancer risk and translocations are a known radiation biomarker. Seventy-nine U.S. radiologic technologists were selected for blood collection, and translocations were enumerated by whole chromosome painting. We developed a dose score to the red bone marrow for medical radiation exposure from X-ray examinations reported by the technologists that they received as patients. Using Poisson regression, we analyzed translocations in relation to the dose scores. Each dose score unit approximated 1 mGy. The estimated mean cumulative red bone marrow radiation dose score was 42 (range 1-265). After adjustment for age, occupational radiation, and radiotherapy for benign conditions, translocation frequencies significantly increased with increasing red bone marrow dose score with an estimate of 0.007 translocations per 100 CEs per score unit (95% CI, 0.002 to 0.013; P = 0.01). Chromosome damage has been linked with elevated cancer risk, and we found that cumulative radiation exposure from medical X-ray examinations was associated with increased numbers of chromosome translocations.

Risk of cataract after exposure to low doses of ionizing radiation: a 20-year prospective cohort study among US radiologic technologists

The study aim was to determine the risk of cataract among radiologic technologists with respect to occupational and nonoccupational exposures to ionizing radiation and to personal characteristics. A prospective cohort of 35,705 cataract-free US radiologic technologists aged 24-44 years was followed for nearly 20 years (1983-2004) by using two follow-up questionnaires. During the study period, 2,382 cataracts and 647 cataract extractions were reported. Cigarette smoking for >or=5 pack-years; body mass index of >or=25 kg/m(2); and history of diabetes, hypertension, hypercholesterolemia, or arthritis at baseline were significantly (p <or= 0.05) associated with increased risk of cataract. In multivariate models, self-report of >or=3 x-rays to the face/neck was associated with a hazard ratio of cataract of 1.25 (95% confidence interval: 1.06, 1.47). For workers in the highest category (mean, 60 mGy) versus lowest category (mean, 5 mGy) of occupational dose to the lens of the eye, the adjusted hazard ratio of cataract was 1.18 (95% confidence interval: 0.99, 1.40). Findings challenge the National Council on Radiation Protection and International Commission on Radiological Protection assumptions that the lowest cumulative ionizing radiation dose to the lens of the eye that can produce a progressive cataract is approximately 2 Gy, and they support the hypothesis that the lowest cataractogenic dose in humans is substantially less than previously thought.

Childhood cancer in the offspring born in 1921-1984 to US radiologic technologists

We examined the risk of childhood cancer (<20 years) among 105 950 offspring born in 1921-1984 to US radiologic technologist (USRT) cohort members. Parental occupational in utero and preconception ionising radiation (IR) testis or ovary doses were estimated from work history data, badge dose data, and literature doses (the latter doses before 1960). Female and male RTs reported a total of 111 and 34 haematopoietic malignancies and 115 and 34 solid tumours, respectively, in their offspring. Hazard ratios (HRs) and 95% confidence intervals (CIs) were calculated using Cox proportional hazards regression. Leukaemia (n=63) and solid tumours (n=115) in offspring were not associated with maternal in utero or preconception radiation exposure. Risks for lymphoma (n=44) in those with estimated doses of <0.2, 0.2-1.0, and >1.0 mGy vs no exposure were non-significantly elevated with HRs of 2.3, 1.8, and 2.7. Paternal preconception exposure to estimated cumulative doses above the 95th percentile (82 mGy, n=6 cases) was associated with a non-significant risk of childhood cancer of 1.8 (95% CI 0.7-4.6). In conclusion, we found no convincing evidence of an increased risk of childhood cancer in the offspring of RTs in association with parental occupational radiation exposure.

[Epidemiologic data on radiation-induced breast cancer]

BACKGROUND

Female breast cancer is the most frequent cancer, both in incidence and mortality. It is well known that exposure to ionizing radiation increases the risk, but some questions remain concerning low dose and low-dose rate effects and cofactors. These potential effects have to be taken into account to carry out adequate risk assessment on medically exposed populations. A literature review is proposed on this issue.

METHODS

A Medline search was undertaken. Keywords used were ionizing radiation, breast cancer and epidemiology. More studies were added through references included in the first list of articles. The focus was placed on studies including quantitative dose-effect relationship analyses.

RESULTS

A latency of five to 10 to 13 years is observed in the appearance of risk. The risk diminishes with age at exposure. A diminution with age at risk is also suspected. The excess relative risk per gray varies between 0.3 and 1.5 for an age at first exposure of 25 years. The study of Hiroshima and Nagasaki survivors shows that risk is increased even if doses are restricted to below 0.5Gy. Above high doses (20Gy), the risk no longer increases. This can be interpreted as a cell-killing effect. The excess subsists if doses are fractionated, but a diminution of the effect is suspected.

CONCLUSION

The effects of exposure to levels of doses used for medical diagnostic are very difficult to study in the general population by epidemiological methods. Only studies conducted on very young children could achieve enough power, because of their high radiosensitivity. Available information on the effects of doses above 0.5Gy allows extrapolation on maximal effects. Models deduced from existing cohorts can be used to assess risk, with their limits due to associated uncertainties. Preston et al. proposed an excess absolute-risk model, which makes estimates from the more comprehensive cohorts compatible. This model has been retained by the 2006 committee "Biological effects of ionizing radiation" (report VII).

Occupational radiation doses to operators performing cardiac catheterization procedures

Cardiac catheterization procedures using fluoroscopy reduce patient morbidity and mortality compared to operative procedures. These diagnostic and therapeutic procedures require radiation exposure to patients and physicians. The objectives of the present investigation were to provide a systematic comprehensive summary of the reported radiation doses received by operators due to diagnostic or interventional fluoroscopically-guided procedures, to identify the primary factors influencing operator radiation dose, and to evaluate whether there have been temporal changes in the radiation doses received by operators performing these procedures. Using PubMed, we identified all English-language journal articles and other published data reporting radiation exposures to operators from diagnostic or interventional fluoroscopically-guided cardiovascular procedures from the early 1970's through the present. We abstracted the reported radiation doses, dose measurement methods, fluoroscopy system used, operational features, radiation protection features, and other relevant data. We calculated effective doses to operators in each study to facilitate comparisons. The effective doses ranged from 0.02-38.0 microSv for DC (diagnostic catheterizations), 0.17-31.2 microSv for PCI (percutaneous coronary interventions), 0.24-9.6 microSv for ablations, and 0.29-17.4 microSv for pacemaker or intracardiac defibrillator implantations. The ratios of doses between various anatomic sites and the thyroid, measured over protective shields, were 0.9 +/- 1.0 for the eye, 1.0 +/- 1.5 for the trunk, and 1.3 +/- 2.0 for the hand. Generally, radiation dose is higher on the left side of an operator's body, because the operator's left side is closer to the primary beam when standing at the patient's right side. Modest operator dose reductions over time were observed for DC and ablation, primarily due to reduction in patient doses due to decreased fluoroscopy/cineradiography time and dose rate by technology improvement. Doses were not reduced over time for PCI. The increased complexity of medical procedures appears to have offset dose reductions due to improvements in technology. The large variation in operator doses observed for the same type of procedure suggests that optimizing procedure protocols and implementing general use of the most effective types of protective devices and shields may reduce occupational radiation doses to operators. We had considerable difficulty in comparing reported dosimetry results because of significant differences in dosimetric methods used in each study and multiple factors influencing the actual doses received. Better standardization of dosimetric methods will facilitate future analyses aimed at determining how well medical radiation workers are being protected.

Polymorphisms in apoptosis- and proliferation-related genes, ionizing radiation exposure, and risk of breast cancer among U.S. Radiologic Technologists

BACKGROUND

Although genes involved in apoptosis pathways and DNA repair pathways are both essential for maintaining genomic integrity, genetic variants in DNA repair have been thought to increase susceptibility to radiation carcinogenesis, but similar hypotheses have not generally been raised about apoptosis genes. For this reason, potential modification of the relationship between ionizing radiation exposure and breast cancer risk by polymorphic apoptosis gene variants have not been investigated among radiation-exposed women.

METHODS

In a case-control study of 859 cases and 1,083 controls within the U.S. Radiologic Technologists cohort, we assessed breast cancer risk with respect to 16 candidate variants in eight genes involved in apoptosis, inflammation, and proliferation. Using carefully reconstructed cumulative breast dose estimates from occupational and personal diagnostic ionizing radiation, we also investigated the joint effects of these polymorphisms on the risk of breast cancer.

RESULTS

In multivariate analyses, we observed a significantly decreased risk of breast cancer associated with the homozygous minor allele of CASP8 D302H [rs1045485, odds ratio (OR), 0.3; 95% confidence interval (95% CI), 0.1-0.8]. We found a significantly increased breast cancer risk with increasing minor alleles for IL1A A114S (rs17561); heterozygote OR 1.2 (95% CI, 1.0-1.4) and homozygote OR 1.5 (95% CI, 1.1-2.0), P(trend) = 0.008. Assuming a dominant genetic model, IL1A A114S significantly modified the dose-response relationship between cumulative personal diagnostic radiation and breast cancer risk, adjusted for occupational dose (P(interaction) = 0.004).

CONCLUSION

The U.S. Radiologic Technologists breast cancer study provided a unique opportunity to examine the joint effects of common genetic variation and ionizing radiation exposure to the breast using detailed occupational and personal diagnostic dose data. We found evidence of effect modification of the radiation and breast cancer dose-response relationship that should be confirmed in studies with more cases and controls and quantified radiation breast doses in the low-to-moderate range.

Polymorphisms in DNA repair genes, ionizing radiation exposure and risk of breast cancer in U.S. Radiologic technologists

High-dose ionizing radiation exposure to the breast and rare autosomal dominant genes have been linked with increased breast cancer risk, but the role of low-to-moderate doses from protracted radiation exposure in breast cancer risk and its potential modification by polymorphisms in DNA repair genes has not been previously investigated among large numbers of radiation-exposed women with detailed exposure data. Using carefully reconstructed estimates of cumulative breast doses from occupational and personal diagnostic ionizing radiation, we investigated the potential modification of radiation-related breast cancer risk by 55 candidate single nucleotide polymorphisms in 17 genes involved in base excision or DNA double-strand break repair among 859 cases and 1083 controls from the United States Radiologic Technologists (USRT) cohort. In multivariable analyses, WRN V114I (rs2230009) significantly modified the association between cumulative occupational breast dose and risk of breast cancer (adjusted for personal diagnostic exposure) (p = 0.04) and BRCA1 D652N (rs4986850), PRKDC IVS15 + 6C > T (rs1231202), PRKDC IVS34 + 39T > C (rs8178097) and PRKDC IVS31 - 634C > A (rs10109984) significantly altered the personal diagnostic radiation exposure-response relationship (adjusted for occupational dose) (p < or = 0.05). None of the remaining 50 SNPs significantly modified breast cancer radiation dose-response relationships. The USRT genetic study provided a unique opportunity to examine the joint effects of common genetic variation and ionizing radiation exposure on breast cancer risk using detailed occupational and personal diagnostic exposure data. The suggestive evidence found for modification of radiation-related breast cancer risk for 5 of the 55 SNPs evaluated requires confirmation in larger studies of women with quantified radiation breast doses in the low-to-moderate range.

Retrospective biodosimetry among United States radiologic technologists

Measurement of chromosome translocations in peripheral blood lymphocytes has been used to quantify prior exposure to ionizing radiation, including for workers exposed to low, chronic doses. We assessed translocation frequencies in a subset of U.S. radiologic technologists to substantiate ionizing radiation dose estimates developed for 110,418 technologists who worked between 1916 and 1984. From 3,441 cohort members known to have begun working before 1950, we selected a sample of 152, stratified by estimated cumulative dose, over-sampling from higher-dose categories and excluding persons with a prior cancer diagnosis, a personal or family history of chromosomal instability disorders, or a current history of smoking. Estimates of film-badge dose ranged from less than 10 cSv to more than 30 cSv. Blood samples, obtained in 2004, were analyzed by fluorescence in situ hybridization (FISH) whole chromosome painting by simultaneously labeling chromosomes 1, 2 and 4 in red and 3, 5 and 6 in green. Translocations were scored in 1800 well-spread metaphase cells and expressed per 100 cell equivalents (CE) per person. Linear Poisson regression models with allowance for overdispersion were used to assess the relationship between estimated occupational red bone marrow absorbed dose in cGy and translocation frequency, adjusted for age, gender and estimated red bone marrow absorbed dose score from personal diagnostic procedures. We observed 0.09 excess translocations per 100 CE per cGy red bone marrow dose (95% CI: -0.01, 0.2; P = 0.07), which is similar to the expected estimate based on previous cytogenetic studies (0.05 excess translocations per 100 CE per cGy). Despite uncertainty in the estimates of occupational red bone marrow absorbed doses, we found good general agreement between the doses and translocation frequencies, lending support to the credibility of the dose assessment for this large cohort of U.S. radiologic technologists.