Background: Occupational exposures to ionizing radiation mainly occur at low dose rates and may accumulate effective doses of up to several hundred milligray. It is presently assumed that the cancer risk per dose after such exposures is smaller than for acute, high-dose exposures.
Objective: To evaluate the evidence of cancer risks from low-dose-rate, moderate-dose (LDRMD) exposures to ionizing radiation.
Methods: A literature search for primary epidemiological studies on cancer incidence and mortality risks from LDRMD exposures included publications from 2002 to 2007, and an update of the UK National Registry for Radiation Workers study. The analysis was restricted to studies reporting excess relative risk (ERR) estimates per dose for all or large groups of cancers. For each study we calculated the risk for the same types of cancer among the atomic bomb survivors with the same gender proportion and matched quantities for dose, mean age-attained and mean age-at-exposure. A combined estimator of the ratio of the ERR per dose from the LDRMD to the corresponding value for the atomic bomb survivors was calculated.
Results: Generally, the ERR per dose was larger than or similar to the corresponding estimate for the atomic bomb survivors. Overall, the ratio of the ERR per dose in the LDRMD studies to the corresponding quantity in the atomic bomb survivors was 1.21 (90% CI: 0.51; 1.90).
Conclusions: The present analysis does not confirm that the cancer risk per dose for LDRMD exposures is lower than for acute, high-dose exposures. Although the individual epidemiological studies all have methodological and statistical limitations, the results of the present analysis challenge the cancer risk values currently assumed for occupational exposures. This particularly applies to the use of a dose and dose-rate effectiveness factor (DDREF) for low-dose-rate exposures.
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