University of California and other educational institutions published the report to prove low level of exposure causes leukemia.
The details are like below.
Radiation and the Risk of Chronic Lymphocytic and Other Leukemias among Chornobyl Cleanup Workers
Lydia B. Zablotska,1 Dimitry Bazyka,2 Jay H. Lubin,3 Nataliya Gudzenko,2 Mark P. Little,3 Maureen Hatch,3 Stuart Finch,4 Irina Dyagil,2 Robert F. Reiss,5 Vadim V. Chumak,2 Andre Bouville,3 Vladimir Drozdovitch,3 Victor P. Kryuchkov,6 Ivan Golovanov,6 Elena Bakhanova,2 Nataliya Babkina,2 Tatiana Lubarets,2 Volodymyr Bebeshko,2 Anatoly Romanenko,2 Kiyohiko Mabuchi3
1Department of Epidemiology and Biostatistics, School of Medicine, University of California, San Francisco, San Francisco, California, USA, 2National Research Center for Radiation Medicine, Kyiv, Ukraine, 3Division of Cancer Epidemiology and Genetics, National Cancer Institute, National Institutes of Health, Department of Health and Human Services, Bethesda, Maryland, USA, 4Robert Wood Johnson Medical School, Camden, New Jersey, USA, 5Departments of Pathology and Medicine, College of Physicians and Surgeons, Columbia University, New York, New York, USA, 6Burnasyan Federal Medical Biophysical Centre, Moscow, Russia
November 8, 2012
The present study provided evidence of increased risk of leukemia associated with chronic protracted exposure to low doses of ionizing radiation. The finding from our primary analysis of similar radiogenic risks both for CLL and non-CLL was based on a well-defined population-based cohort, rigorous case ascertainment and expert hematological review, coupled with well characterized radiation dose estimates. In our cohort of cleanup workers from 1986 through 2006, about 16 % (19 cases) of all leukemia were attributed to radiation exposure, with similar estimates for non-CLL (15%) and CLL (18%). CLL is the most common type of leukemia in this cleanup worker population and, as they age, CLL cases will rapidly increase, raising concerns for medical consequences. The radiogenic risk for CLL also has important public health implications in other populations as it is the most prevalent type of leukemia in Western populations, with approximately 16,000 cases estimated to be diagnosed in the U.S. in 2012 (Howlader et al. 2012). Further investigations are needed to develop a better understanding of the association between radiation and CLL.
The age at diagnosis of 137 cases ranged from 25 to 78 years (median=56) and the corresponding age for 863 controls ranged from 25 to 79 years (median=55). Mean estimated bone marrow radiation doses and standard deviations (SD) for cases and controls were 132.3 mGy (342.6) and 81.8 mGy (193.7), respectively (Table 1). Seventy-eight percent of study participants had bone marrow doses below 100 mGy and 87% below 200 mGy. Cases and controls did not differ significantly by urban vs. rural residential status at the time of interview, age at first radiation exposure in the 30-km Chernobyl zone, or education; however, more cases than controls were proxy-interviewed (p<0.001) (Table 1). Cases and controls did not differ significantly by
calendar year of first cleanup mission, type of work or total number of missions, or by self-reported smoking, alcohol consumption, medical or diagnostic radiation exposures, or occupational exposures to chemicals or radiation before and after the Chernobyl accident (results not shown). Thirty-eight percent of cleanup workers were in the 30-km zone around the Chernobyl nuclear power plant for over two months (median time in the zone for all workers=35 days, range 1-1,711 days, similar for cases and controls p Wilcoxon=0.729).
For all leukemias, we found a significant positive association with continuous radiation dose with an estimated ERR/Gy=1.26 (95% CI: 0.03, 3.58, p=0.041) (Table 2). However, preliminary analysis identified a significant (p=0.021) difference in the dose-response for 20 cases (6 non-CLL and 14 CLL) with direct in-person interviews <2 years from start of chemotherapy compared with other cases (ERR/Gy=-0.47, 95% CI: <-0.47, 1.02, p=0.244 for 20 cases vs.ERR/Gy=2.38, 95%CI: 0.49, 5.87, p=0.004 for the remaining 117 cases, Table 2 and Supplemental Material, Table S2). Due to this marked disparity, we limited our primary analyses to cases who were interviewed 2-15 years after start of chemotherapy, did not have chemotherapy, or for whom proxy interviews were used and their matched controls (85% of all cases and 83% of all controls).
Relative risks increased with increasing radiation dose for all leukemia (Figure 1). Tests for quadratic, exponential or power deviations from the linear dose-response shown in Figure 1 were not significant (p=0.927, p=0.917, p=0.267, respectively). The dose-responses increased significantly for both non-CLL (ERR/Gy = 2.21, 95% CI: 0.05, 7.61, p=0.039) and CLL (ERR/Gy = 2.58, 95% CI: 0.02, 8.43, p=0.047) subtypes, with tests for interaction consistent with homogeneity (p=0.888) (Table 2).
There was no significant difference in ERR/Gy estimates by proxy or direct interviews (p=0.420), calendar period of diagnosis (p=0.141) or type of work performed in the 30-km Chernobyl zone (p=0.711) (Table 2). Although also not significant, ERR/Gy estimates tended to decrease with increasing time (years) from first radiation exposure in the Chernobyl zone and to increase with increasing age at first exposure (p=0.162, p=0.249, respectively) (Table 2). The proportion of proxy vs. direct interviews decreased over time (60.0%, 73.9%, 55.6%, and 54.2% for cases diagnosed in 1986-1989, 1990-1994, 1995-2000, and 2001-2006, respectively). We estimated that approximately 16 percent of all leukemia cases in our Chernobyl cleanup worker population over a period of 20 years of follow-up (PAR=16.4%, 95% CI: 3.9, 32.6) were attributable to radiation exposure from the Chernobyl accident. The majority of the PAR arose from dose groups of <200 mGy in which there were large numbers of cleanup workers (Figure 2). Proportions of non-CLL and CLL cases attributable to radiation were similar, with a PAR of 15.4% (95% CI: 0.4, 38.5) and 17.5% (95% CI: 0.2, 41.0), respectively. For completeness, we evaluated modifications of the ERR/Gy presented in Table 2 using all case and control data (Supplemental Material, Table S2). In general, results using the full dataset were consistent with the primary analysis. However, the ERR/Gy for CLL (0.76, 95% CI: <-0.38, 3.84, p=0.352) was lower than the estimated ERR/Gy for CLL from our primary analysis excluding 14 CLL cases (2.58, 95% CI: 0.02, 8.43, p=0.047). In the analysis using the full dataset, as in the primary analysis, the ERRs were not significantly different between CLL and non-CLL outcomes (p=0.536).