A nested case-control study of leukemia mortality and ionizing radiation at the Portsmouth Naval Shipyard

Evaluation of the carcinogenicity of carbon tetrachloride

Carbon tetrachloride (CCl4) has been extensively used and reported to produce toxicity, most notably involving the liver. Carbon tetrachloride metabolism involves CYP450-mediated bioactivation to trichloromethyl and trichloromethyl peroxy radicals, which are capable of macromolecular interaction with cell components including lipids and proteins. Radical interaction with lipids produces lipid peroxidation which can mediate cellular damage leading to cell death. Chronic exposure with CCl4 a rodent hepatic carcinogen with a mode of action (MOA) exhibits the following key events: 1) metabolic activation; 2) hepatocellular toxicity and cell death; 3) consequent regenerative increased cell proliferation; and 4) hepatocellular proliferative lesions (foci, adenomas, carcinomas). The induction of rodent hepatic tumors is dependent upon the dose (concentration and exposure duration) of CCl4, with tumors only occurring at cytotoxic exposure levels. Adrenal benign pheochromocytomas were also increased in mice at high CCl4 exposures; however, these tumors are not of relevant importance to human cancer risk. Few epidemiology studies that have been performed on CCl4, do not provide credible evidence of enhanced risk of occurrence of liver or adrenal cancers, but these studies have serious flaws limiting their usefulness for risk assessment. This manuscript summarizes the toxicity and carcinogenicity attributed to CCl4, specifically addressing MOA, dose-response, and human relevance.

The United States Navy and Employees with Cancer: The Time for Change Is Now

ABSTRACT

The US Navy, including the US Marine Corps and Naval Nuclear Propulsion Program (NNPP), has a robust radiological protection and monitoring program meeting (and typically exceeding, in the name of conservatism) federal law requirements. The program covers the variety of ways in which the Navy produces and uses ionizing radiation and radioactive sources: in medicine, nuclear ship propulsion and repair, industrial and aircraft radiography, and myriad other unique uses in carrying out its vital mission. In executing these programs, thousands of people across the world are employed as active-duty Sailors and Marines, government civilians, and government contractors. These workers include physicians, reactor operators, radiation safety officers, and nuclear repair workers, to name but a few. The health protection standards for these workers are promulgated in the publicly available Navy Medicine P-5055 Radiation Health Protection Manual (NAVMED P-5055), published February 2011 with Change 2 published December 2022, and are applicable to Navy and Marine Corps and NNPP radiation protection programs. The NAVMED P-5055 outlines the individual medical requirements for those qualified and able to receive exposure to ionizing radiation as part of their duties and requires that "Radiation workers receive focused medical examinations to establish whether or not cancer is present which would medically disqualify a person from receiving occupational radiation exposure." Additionally, without scientific or medical basis, the NAVMED P-5055 requires disqualifying those employees who have a history of cancer, cancer therapy, radiation therapy including radiopharmaceuticals received for therapeutic purposes, or bone marrow suppression from drawing dosimetry, entering radiation areas, or handling radioactive material. This policy, which exists regardless of lifetime occupational radiation dose or projected future radiation dose, applies to all cancers except adequately treated basal cell carcinoma. The policy is not supported by relevant scientific and medical literature; does not align with reasonable professional ethical standards; does not conform to US Navy radiological training, which stipulates the assumed increased risk of developing cancer from Navy and Marine Corps and NNPP occupational radiation exposure is small; and removes critical leadership and mentoring capability from the workforce unnecessarily. This article discusses in detail (1) this policy and its ramifications to the Navy and Marine Corps and NNPP workforce and (2) recommendations, benefits, and impacts for the Navy and Marine Corps and NNPP to remove this policy and still maintain a robust radiation protection program.

Strengths and Weaknesses of Dosimetry Used in Studies of Low-Dose Radiation Exposure and Cancer

BACKGROUND

A monograph systematically evaluating recent evidence on the dose-response relationship between low-dose ionizing radiation exposure and cancer risk required a critical appraisal of dosimetry methods in 26 potentially informative studies.

METHODS

The relevant literature included studies published in 2006-2017. Studies comprised case-control and cohort designs examining populations predominantly exposed to sparsely ionizing radiation, mostly from external sources, resulting in average doses of no more than 100 mGy. At least two dosimetrists reviewed each study and appraised the strengths and weaknesses of the dosimetry systems used, including assessment of sources and effects of dose estimation error. An overarching concern was whether dose error might cause the spurious appearance of a dose-response where none was present.

RESULTS

The review included 8 environmental, 4 medical, and 14 occupational studies that varied in properties relative to evaluation criteria. Treatment of dose estimation error also varied among studies, although few conducted a comprehensive evaluation. Six studies appeared to have known or suspected biases in dose estimates. The potential for these biases to cause a spurious dose-response association was constrained to three case-control studies that relied extensively on information gathered in interviews conducted after case ascertainment.

CONCLUSIONS

The potential for spurious dose-response associations from dose information appeared limited to case-control studies vulnerable to recall errors that may be differential by case status. Otherwise, risk estimates appeared reasonably free of a substantial bias from dose estimation error. Future studies would benefit from a comprehensive evaluation of dose estimation errors, including methods accounting for their potential effects on dose-response associations.

Mortality among Coast Guard Shipyard workers: A retrospective cohort study of specific exposures

In a previous analysis of a cohort of shipyard workers, we found excess mortality from all causes, lung cancer, and mesothelioma for longer work durations and in specific occupations. Here, we expand the previous analyses by evaluating mortality associated with 5 chemical exposures: asbestos, solvents, lead, oils/greases, and wood dust. Data were gathered retrospectively for 4,702 workers employed at the Coast Guard Shipyard, Baltimore, MD (1950-1964). The cohort was traced through 2001 for vital status. Associations between mortality and these 5 exposures were calculated via standardized mortality ratios (SMRs). We found all 5 substances to be independently associated with mortality from mesothelioma, cancer of the respiratory system, and lung cancer. Findings from efforts to evaluate solvents, lead, oils/greases, and wood dust in isolation of asbestos suggested that the excesses from these other exposures may be due to residual confounding from asbestos exposure.

Cohort mortality study of garment industry workers exposed to formaldehyde: update and internal comparisons

BACKGROUND

To further evaluate the association between formaldehyde and leukemia, we extended follow-up through 2008 for a cohort mortality study of 11,043 US formaldehyde-exposed garment workers.

METHODS

We computed standardized mortality ratios and standardized rate ratios stratified by year of first exposure, exposure duration, and time since first exposure. Associations between exposure duration and rates of leukemia and myeloid leukemia were further examined using Poisson regression models.

RESULTS

Compared to the US population, myeloid leukemia mortality was elevated but overall leukemia mortality was not. In internal analyses, overall leukemia mortality increased with increasing exposure duration and this trend was statistically significant.

CONCLUSIONS

We continue to see limited evidence of an association between formaldehyde and leukemia. However, the extended follow-up did not strengthen previously observed associations. In addition to continued epidemiologic research, we recommend further research to evaluate the biological plausibility of a causal relation between formaldehyde and leukemia.

A meta-analysis of leukaemia risk from protracted exposure to low-dose gamma radiation

CONTEXT

More than 400,000 workers annually receive a measurable radiation dose and may be at increased risk of radiation-induced leukaemia. It is unclear whether leukaemia risk is elevated with protracted, low-dose exposure.

OBJECTIVE

We conducted a meta-analysis examining the relationship between protracted low-dose ionising radiation exposure and leukaemia.

DATA SOURCES

Reviews by the National Academies and United Nations provided a summary of informative studies published before 2005. PubMed and Embase databases were searched for additional occupational and environmental studies published between 2005 and 2009.

STUDY SELECTION

We selected 23 studies that: (1) examined the association between protracted exposures to ionising radiation and leukaemia excluding chronic lymphocytic subtype; (2) were a cohort or nested case-control design without major bias; (3) reported quantitative estimates of exposure; and (4) conducted exposure-response analyses using relative or excess RR per unit exposure.

METHODS

Studies were further screened to reduce information overlap. Random effects models were developed to summarise between-study variance and obtain an aggregate estimate of the excess RR at 100 mGy. Publication bias was assessed by trim and fill and Rosenthal's file drawer methods.

RESULTS

We found an ERR at 100 mGy of 0.19 (95% CI 0.07 to 0.32) by modelling results from 10 studies and adjusting for publication bias. Between-study variance was not evident (p=0.99).

CONCLUSIONS

Protracted exposure to low-dose gamma radiation is significantly associated with leukaemia. Our estimate agreed well with the leukaemia risk observed among exposed adults in the Life Span Study (LSS) of atomic bomb survivors, providing increased confidence in the current understanding of leukaemia risk from ionising radiation. However, unlike the estimates obtained from the LSS, our model provides a precise, quantitative summary of the direct estimates of excess risk from studies of protracted radiation exposures.

The relationship between multiple myeloma and occupational exposure to six chlorinated solvents

OBJECTIVES

Few studies have examined whether exposure to chlorinated solvents is associated with multiple myeloma. We evaluated associations between multiple myeloma and occupational exposure to six chlorinated solvents: 1,1,1-trichloroethane, trichloroethylene (TCE), methylene chloride (DCM), perchloroethylene, carbon tetrachloride and chloroform.

METHODS

In-person interviews obtained occupational histories and information on jobs with likely solvent exposure. We assigned exposure metrics of probability, frequency, intensity and confidence using job-exposure matrices modified by job-specific questionnaire information. We used logistic regression to estimate ORs and 95% CIs for associations between multiple myeloma and ever exposure to each, and any, chlorinated solvent and analysed whether associations varied by duration and cumulative exposure. We also considered all occupations that were given the lowest confidence scores as unexposed and repeated all analyses.

RESULTS

Risk of multiple myeloma was elevated for subjects ever exposed to 1,1,1-trichloroethane (OR (95% CI): 1.8 (1.1 to 2.9)). Ever exposure to TCE or DCM also entailed elevated, but not statistically significant, risks of multiple myeloma; these became statistically significant when occupations with low confidence scores were considered unexposed (TCE: 1.7 (1.0 to 2.7); DCM: 2.0 (1.2 to 3.2)). Increasing cumulative exposure to perchloroethylene was also associated with increasing multiple myeloma risk. We observed non-significantly increased multiple myeloma risks with exposure to chloroform; however, few subjects were exposed.

CONCLUSIONS

Evidence from this relatively large case-control study suggests that exposures to certain chlorinated solvents may be associated with increased incidence of multiple myeloma; however, the study is limited by relatively low participation (52%) among controls.

Summary of retrospective asbestos and welding fume exposure estimates for a nuclear naval shipyard and their correlation with radiation exposure estimates

In support of a nested case-control study at a U.S. naval shipyard, the results of the reconstruction of historical exposures were summarized, and an analysis was undertaken to determine the impact of historical exposures to potential chemical confounders. The nested case-control study (N = 4388) primarily assessed the relationship between lung cancer and external ionizing radiation. Chemical confounders considered important were asbestos and welding fume (as iron oxide fume), and the chromium and nickel content of welding fume. Exposures to the potential confounders were estimated by an expert panel based on a set of quantitatively defined categories of exposure. Distributions of the estimated exposures and trends in exposures over time were examined for the study population. Scatter plots and Spearman rank correlation coefficients were used to assess the degree of association between the estimates of exposure to asbestos, welding fume, and ionizing radiation. Correlation coefficients were calculated separately for 0-, 15-, 20-, and 25-year time-lagged cumulative exposures, total radiation dose (which included medical X-ray dose) and occupational radiation dose. Exposed workers' estimated cumulative exposures to asbestos ranged from 0.01 fiber-days/cm(3) to just under 20,000 fiber-days/cm(3), with a median of 29.0 fiber-days/cm(3). Estimated cumulative exposures to welding fume ranged from 0.16 mg-days/m(3) to just over 30,000 mg-days/m(3), with a median of 603 mg-days/m(3). Spearman correlation coefficients between cumulative radiation dose and cumulative asbestos exposures ranged from 0.09 (occupational dose) to 0.47 (total radiation dose), and those between radiation and welding fume from 0.14 to 0.47. The estimates of relative risk for ionizing radiation and lung cancer were unchanged when lowest and highest estimates of asbestos and welding fume were considered. These results suggest a fairly large proportion of study population workers were exposed to asbestos and welding fume, that the absolute level of confounding exposure did not affect the risk estimates, and that weak relationships existed between monitored lifetime cumulative occupational radiation dose and asbestos or welding fume.

What can epidemiology tell us about risks at low doses?

Puskin, J. S. What Can Epidemiology Tell Us about Risks at Low Doses? Radiat. Res. 169, 122-124 (2008). Limitations on statistical power preclude direct detection and quantification of radiogenic cancer risks at very low (environmental) levels of low-LET radiation through epidemiological studies. Given this limitation and our incomplete understanding of cellular processes leading to radiation carcinogenesis, an "effective threshold" in the dose range of interest for radiation protection cannot yet be ruled out. Ongoing epidemiological studies of chronically exposed individuals receiving very low daily doses of radiation can be used, however, together with radiobiological data, to critically test whether such a threshold is plausible.

Cancer risks and low-level radiation in U.S. shipyard workers

The risks for four cancers, leukemia, lymphopoietic cancers (LHC), lung cancer and mesothelioma, were studied in workers from shipyards involved in nuclear powered ship overhauls. The population represented a sample of all workers based on radiation dose at study termination. The final sample included 28,000 workers with > or = 5.0 mSv, 10,462 workers with < 5.0 mSv and 33,353 non-nuclear workers. Nuclear workers had lower mortality rates for leukemia and LHC than US white males but higher rates of lung cancer and a significant five-fold excess of mesothelioma. Dose-dependent analyses of risks in the high exposure group indicated that for each cancer the risk increased at exposures above 10.0 mSv. An internal comparison of workers with 50.0 mSv exposures to workers with exposures of 5.0-9.9 mSv indicated relative risks for leukemia of 2.41 (95% CI: 0.5, 23.8), for LHC, 2.94 (95% CI: 1.0,12.0), for lung cancer, 1.26 (95% CI: 0.9, 1.9) and for mesothelioma, 1.61 (95% CI: 0.4, 9.7) for the higher exposure group. Except for LHC, these risks are not significant. However, the increasing risk with increasing exposure for these cancers, some of which are known to be related to radiation, suggests that low-level protracted exposures to gamma rays may be associated with these cancers. Other agents such as asbestos, which are common to shipyard work, may play a role especially in the risk of mesothelioma. Future follow up of the population would identify bounds on radiation risks for this population for comparison with similar risks estimated from other populations.

A Nested Case-Control Study of Lung Cancer Risk and Ionizing Radiation Exposure at the Portsmouth Naval Shipyard

Results have been inconsistent between studies of lung cancer risk and ionizing radiation exposures among workers at the Portsmouth Naval Shipyard (PNS). The purpose of this nested case-control study was to evaluate the relationship between lung cancer risk and external ionizing radiation exposure while adjusting for potential confounders that included gender, radiation monitoring status, smoking habit surrogates (socioeconomic status and birth cohort), welding fumes and asbestos. By incidence density sampling, we age-matched 3,291 controls selected from a cohort of 37,853 civilian workers employed at PNS between 1952 and 1992 with 1,097 lung cancer deaths from among the same cohort. Analyses using conditional logistic regression were conducted in various model forms: log-linear (main), linear excess relative risk (ERR), and categorical. Lung cancer risk was positively associated with occupational dose (OR = 1.02 at 10 mSv; 95% CI 0.99- 1.04) but flattened after the inclusion of work-related medical X-ray doses (OR = 1.00; 95% CI 0.98-1.03) in multivariate analyses. Similar risk estimates were observed in the linear ERR model at 10 mSv of cumulative exposure with a 15-year lag.

Chronic lymphocytic leukemia radiogenicity: a systematic review

OBJECTIVE

Chronic lymphocytic leukemia (CLL) is generally considered to be non-radiogenic and is excluded from several programs that compensate workers for illnesses resulting from occupational exposures. Questions about whether this exclusion is justified prompted a Congressional mandate to the National Institute for Occupational Safety and Health (NIOSH) to, further, examine the radiogenicity of CLL. This study revisits the question of CLL radiogenicity by examining epidemiologic evidence from occupationally and medically-exposed populations.

METHODS

A systematic review of radiation-exposed cohorts was conducted to investigate the association between radiation and CLL. Exploratory power calculations for a pooled occupational study were performed to examine the feasibility of assessing CLL radiogenicity epidemiologically.

RESULTS

There is a bias against reporting CLL results, because of the disease's presumed non-radiogenicity. In medical cohort studies that provide risk estimates for CLL, risk is elevated, though non-significantly, in almost all studies with more than 15 years average follow-up. The results of occupational studies are less consistent.

CONCLUSIONS

Studies with adequate follow-up time and power are needed to better understand CLL radiogenicity. Power analyses show that a pooled study might detect risk on the order of radiation induced non-CLL leukemia, but is unlikely to detect smaller risks.

Risk of Chronic Myeloid and Acute Leukemia Mortality after Exposure to Ionizing Radiation among Workers at Four U.S. Nuclear Weapons Facilities and a Nuclear Naval Shipyard

A nested case-control study was conducted among workers at five U.S. nuclear facilities to evaluate leukemia mortality risk (excluding chronic lymphocytic) from ionizing radiation using worksite doses and adjusting for potential confounding. Conditional logistic regression was used to estimate the relative risk (RR) of exposed workers and the excess relative risk (ERR) per unit of radiation among 206 cases and 823 age-matched controls. Adjusting for sex and benzene, the RR of leukemia for workers receiving more than 10 mSv was higher compared to those receiving lower or no dose; however, the risk increase was attenuated in the highest dose group. The ERR per 10 mSv was 1.44% (95% CI: < -1.03%, 7.59%) but was higher for workers born after 1921 compared to workers born earlier or when excluding leukemias of uncertain type. Excluding the 7% who were high-dose workers (> 100 mSv), the sex- and benzene-adjusted ERR per 10 mSv was 6.82% (95% CI: -2.87%, 24.1%). The results suggest that risks among these nuclear workers are comparable to those observed in high-dose populations, although no evidence was observed of a positive quadratic dose-response term in this study. This large study is among the first to jointly evaluate benzene and ionizing radiation risk.