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Mandel et al's1 interesting literature review on trichloroethylene (TCE) exposure and non‐Hodgkin's lymphoma (NHL) has serious limitations.
First, there are three alternative descriptions of their stratification of group 1 studies: (1) population source (multiple industries vs only aerospace); (2) outcome (incidence vs mortality); and (3) location (European vs US). It would have been helpful to have specified a priori the interpretive advantages and disadvantages of each rather than focusing on population source.
Second, in their cohort analyses, they combine incidence and mortality data. Several assumptions the authors have not justified are implicit: (1) exposure to TCE confers quantitatively similar risks for incidence and mortality; (2) temporal changes in risk occur equally in incidence and mortality; (3) because mortality lags incidence, exposure to TCE must have been relatively constant in each study longitudinally (even though technological improvements probably decreased exposure); and (4), although they state, “The increases for NHL in the general population over the past several decades have been for both morbidity and mortality” (p 20), surveillance, epidemiology and end results data show that incidence increased more than mortality (fig 11),), especially in white men (fig 22),), resulting in a nearly two‐thirds increase in percentage surviving at least 5 years from 1960 to 1995 (from 31% to 51% in white men).2
Third, although the authors conclude that, “…there is insufficient evidence to suggest a causal link between TCE and NHL [abstract]”, all eight group I studies showed increased risks, with two being statistically significant, and all four summary risk estimates were increased, with three being statistically significant. Group I comparisons of highest and lowest, cumulative, and duration of, exposures show similar patterns. Group II data are less compelling, although the case–control data show some support.