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

Objective. To determine if there is an association between occupational exposure to external ionizing radiation and leukemia mortality among civilian Portsmouth Naval Shipyard workers after potential confounders and effect modifiers such as solvent exposure, and time since exposure are considered.;Methods. This study employs a nested case-control design and includes 115 leukemia cases and 460 controls. Among the cases and controls, 201 (35%) were monitored for radiation exposure. Badge doses and exposures to work-required medical x-rays were included in the analysis. Benzene and carbon tetrachloride exposure was assessed using workers' job information and industrial hygiene records. Conditional logistic regression was used to analyze the exposure-response relationship between external ionizing radiation exposure and leukemia mortality.;Results. A significant positive association was found between leukemia mortality and external ionizing whole-body radiation exposure (OR = 1.08 at 10 mSv; 95% CI = 1.01, 1.16) adjusting for gender, radiation worker status and solvent exposure duration. The latter factor (OR = 1.03 at one year of exposure; 95% CI = 1.01, 1.06) was also significantly associated with leukemia mortality. The incorporation of estimated doses from workrelated medical x-ray exposures did not change the leukemia risk estimate.;Significantly different exposure-response trends were observed in the 2.5--5 years since last exposure category compared to the >10 years since last exposure category. The choice of categories for time since last (radiation) exposure (TSLE) classification did not greatly alter these findings.;Conclusion. We observed a positive association between leukemia mortality and increasing external whole-body ionizing radiation exposure, adjusting for radiation worker status and solvent exposure duration. A significant exposure response between leukemia mortality and the duration of time workers were employed in job categories where solvent exposures were probable was also observed. However, solvent exposure was not a confounder of the association between radiation exposure and leukemia risk.;The study findings provide improved precision of exposure-response estimates. The point estimate for leukemia mortality risk from radiation exposure is approximately two times greater for fractionated occupational exposures than that determined from the high-dose rate studies.