Risk of lymphohematopoietic malignancies in uranium miners.

Rericha et al. (2006) analyzed data from Czech uranium miners with respect to incidence of malignancies of the lymphohematopoietic system. Their results, however, do not correspond with those of two recent studies on German uranium miners (Kreuzer et al. 2004; Mohner et al. 2006). Rericha et al. (2006) used a case-cohort design, in which the subcohort was stratified by attained age and duration of employment. Stratification by age is a standard approach in case-cohort studies to optimize data ascertainment in the subcohort. However, stratification by duration of employment is problematic, because in occupational epidemiology it should be assumed that the duration of employment is highly correlated with cumulative exposure. Therefore, this kind of stratification contradicts the general demand for a random selection of controls with respect to exposure under study. Comparing the ratios of sampling fractions (< 12 months vs. [greater than or equal to] 12 months duration of employment) between age groups results in a heterogeneous picture (Table 1).

It is not uncommon in occupational cohort studies to include only subjects with a duration of employment of at least a certain number of months into the cohort. An analysis of only those miners with an employment duration of at least 12 months would be in line not only with the standard methodology but also with earlier published results of the authors (Rericha et al. 1998). Hence, the authors should have at least explained their reasoning for including the remaining miners in a second set of strata. In addition, they should have presented separate results for both duration strata to validate the result of the combined analysis.

Given the above-mentioned assumption concerning the relationship between duration of employment and cumulative exposure, I calculated crude incidence rate ratios using data from Table 1 of Rericha et al. (2006). The age-specific odds ratios cover a wide range (0.29-7.16), and a corresponding test yields only borderline homogeneity. Consequently, completeness of matching with the cancer registry needs to be discussed.

According to the study design, the time period between last exposure and begin of follow-up can span up to 27 years; therefore, the healthy-worker survivor effect could be an important issue in this study (Rericha et al. 2006). In light of the discussion on the magnitude of the latency period for leukemia, more detailed results would be useful to get an impression on, for example, the effect of the year of last exposure.

The author declares he has no competing financial interests.

Matthias Mohner

Federal Institute for Occupational Safety and Health Berlin, Germany

E-mail: [e-mail]

REFERENCES

Kreuzer M, Schnelzer M, Tschense A, Grosche B. 2004. Risk of lung cancer and other cancers in the German uranium miners cohort study. 11th IRPA-Congress. Available: http://irpa11.irpa.net/pdfs/1b16.pdf [accessed 8 March 2007].

Mohner M, Lindtner M, Otten H, Gille HG. 2006. Leukemia and exposure to ionizing radiation among German uranium miners. Am J Ind Med 49:238-248.

Rericha V, Kulich M, Rericha R. Shore DL, Sandler DP. 2006. Incidence of leukemia, lymphoma, and multiple myeloma in Czech uranium miners: a case-cohort study. Environ Health Perspect 114:818-822.

Rericha V, Sandler DP, Shore DL, Solansky I, Hnizdo E, Sram R. 1998. Non-lung cancer incidence in Czech uranium miners [Abstract]. Epidemiology 9:S99.

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Table 1. Rate ratios (RRs) of sampling fractions and incidence rate
ratios (IRRs) for lymphohematopoietic malignancies (95% confidence
intervals), calculated from Rericha et al. (2006).

Age (years)       RR sampling fractions  IRR

19-35             0.52 (0.40-0.69)       1.06 (0.47-2.61)
36-45             1.11 (0.91-1.37)       0.56 (0.27-1.19)
46-55             1.16 (1.00-1.34)       1.08 (0.63-1.95)
56-65             1.54 (1.21-2.00)       7.16
(1.18-292.75)
66-90             1.24 (0.77-2.07)       0.29 (0.05-1.99)
M-H combined      1.09 (0.99-1.20)       1.00 (0.71-1.41)
Homogeneity test  p = 0.000              p = 0.055