Rocky Flats Plant: Workers (Livingston et al. 2006) 

Scenario 

The Rocky Flats Plant, a former U. S. nuclear weapons production facility in Colorado, U. S. A., caused radioactive contamination mainly of plutonium, americium and uranium, within and outside the facility. A significant number of Rocky Flats workers were exposed externally and internally due to work-related radioactivity caused by air-born plutonium leaked from the barrels of the radioactive wastes as well as plutonium fires, during the years from 1952 to 1990. In 1992, the U. S. Department of Energy initiated medical monitoring program for workers who were exposed during the course of their employment.  

Chromosome aberrations blood lymphocytes were examined in 47 volunteer participants (in age from 48 to 89 years) selected from former plutonium workers. The participants were assigned to two groups, high-dose and low-dose groups, based on their total (external plus internal) occupational dose at the plant. The high-dose group consisted of 30 workers with external and internal dose greater than 0.5 Sv, and the low-dose group comprised 17 workers exposed to below 0.1 Sv of external dose only. The 21 controls were selected from among the workforce at the plant over a 16-month period (1997-1998) who had no history of occupational exposure to radiation during their lifetime. Additional 5 controls were selected from the laboratory personnel. The combined control group was age matched to the exposed groups. The effects were studied for chromosome translocation frequencies by FISH paining using whole chromosome painting probes for chromosomes 1, 4 and 12, and for micronucleus assay by cytokinesis-block procedure. 

Reference 

Livingston, G. K., Falk, R. B. and Schmid, E. (2006): Effect of occupational radiation exposures on chromosome aberration rates in former plutonium workers. Radiat. Res., 166:89-97. 
Occupational dose and chromosome aberrations 
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Study group N Genome equivalent total translocations per 1000 cells (Consecutive list in rank order)
Controls* 26 0 1.05 1.05 1.05 2.1 2.1 2.1 2.1 2.1 2.1 2.1 3.15 3.15 3.15 3.15 3.15 4.19 5.24 6.29 6.29 7.34 7.34 8.39 8.39 9.43 9.43 - - - -
Low-dose group 17 3.15 3.15 5.24 5.24 6.29 6.29 6.29 6.29 8.39 9.43 10.48 11.53 12.58 12.58 13.63 13.63 18.87 - - - - - - - - - - - - -
High-dose group 30 4.19 5.24 6.29 8.39 8.39 9.43 9.43 10.48 10.48 11.53 11.53 11.53 12.58 12.58 12.58 13.41 13.63 13.63 14.68 14.68 15.72 16.77 19.92 19.92 30.4 30.68 32.5 36.69 46.12 46.12
*) Controls: unexposed controls and laboratory controls.
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Commentary at data compling (Editorial) 

     Alternative visualization of the impact of radiation exposure to the study populations and complexityy of dose-response response kinetics of translocations. Low doses of low LET radiation in low dose-rate could suppress the spontaneously occurring (replication-dependent, hematopoitic origin) translocations (General feature in low dose and low dose-rate exposure to low LET radiation)..