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.