Cancer Mortality after Protracted Low-level Radiation Exposure for Early and Contemporary Workers in Two Large Occupational Cohorts in the U.S. Million Person Study

An evaluation is presented of differences in radiation-related solid cancer mortality risk for early versus contemporary sub-groups of radiation workers in both of the two constituent Million Person Study (MPS) cohorts. The two previously analyzed MPS cohorts are 123,401 industrial radiographers monitored from 1939–2011 and followed through 2019 and 135,193 nuclear power plant workers monitored from 1957-1984 and followed through 2011. The rationale behind this extended new analysis is to investigate if these two MPS cohorts support recently published increased risks for contemporary workers in a different cohort, The International Nuclear Workers Study (INWORKS) with pooled U.S., French and UK nuclear worker data, particularly for the U.S. component. The US-INWORKS contributed about one-third of the workers to the full-INWORKS study based on 309,932 workers. For all solid cancer mortality, the US-INWORKS study reported a low and non-significant excess relative risk (ERR) per Sv cumulative equivalent dose for the whole cohort of 0.19 (95% CI: –0.10; 0.52), whereas for contemporary workers the ERR per Sv was 2.23 (95% CI: 1.13, 3.49), approximately 10 times higher than the entire US-INWORKS cohort. The risk for the full INWORKS cohort was 0.52 (90% CI: 0.27; 0.77) per Gy colon dose whereas, for contemporary workers, the risk was 1.44 (90% CI: 0.65, 2.32), nearly 3 times higher. These risks for contemporary workers are both larger than risks informing radiation protection and much higher (7.0 and 4.5 times) than the Japanese A-bomb survivor’s risk for males exposed acutely between the ages of 20 and 60 years of 0.32 (95% CI: 0.01; 0.50). Limitations include missing information on organ doses from radionuclide intake, neutrons and the absence of adjustment for non-radiation risk factors (notably asbestos exposure). The analysis of the MPS cohorts addresses these dosimetric- and asbestos-related limitations. For all solid cancer mortality, industrial radiographers showed equal Poisson ERRs per 100 mGy colon dose for early and contemporary workers: 0.06 (95% CI: 0.00; 0.12) and 0.07 (95% CI: 0.01; 0.13), respectively. The results for nuclear power plant workers were 0.10 (95% CI: –0.09; 0.29) and 0.02 (95% CI: –0.02; 0.06), respectively. It appears premature to conclude that there is generally a difference in excess risk between early and contemporary workers from radiation exposures.