Brain tumor and exposure to pesticides in humans: a review of the epidemiologic data

JOURNAL

OF THE

NEUROLOGICAL SCIENCES ELSEVIER

Journal of the Neurological Sciences132 (1995) 110-121

Review article

Brain tumor and exposure to pesticides in humans: a review of the epidemiologic data Nicolaas I. Bohnen, Leonard T. Kurland * Departments

of Neurology and of Health Sciences Research, Section

of Clinical Epidemiology, Mayo Clinic and Mayo Foundation, Rochester, MN 55905,

USA

Received 19 June 1994; revised 24 April 1995; accepted27 April 1995

Abstract We examined the relationship between exposure to pesticides and the subsequent development of brain tumors in adults through a critical review of the literature. The results of retrospective case-control studies are conflicting, in part because of biases in the selection of patients and controls, poor definition and ascertainment of the nature and extent of the exposure to pesticides, and a non-uniform approach to the collection of antecedent information. A number of the studies evaluated farmers as a group exposed to pesticides; however, inference about cancer incidence in farmers may reflect not only their possible exposure to pesticides, but also exposure to petrochemical products, exhaust fumes, mineral and organic dusts, and biological exposure to animals and microbes. The great majority of the cohort studies of chemical workers employed in the manufacture of pesticides did not indicate an excess of brain cancer mortality. There have been few cohort studies of pesticide applicators and these revealed elevated but non-significant relative risks for excess mortality due to brain cancer. Existing data are insufficient to conclude that exposure to pesticides is a clear risk factor for brain tumors. Given the conflicting results reported for farmers and pesticide applicators and their contrast to chemical workers, it seems more plausible that exposure to multiple agents and/or other factors, such as genetic predisposition, are most relevant with respect to brain tumor pathogenesis. Keywords:

Brain tumor; Herbicides; Pesticides;Human

1. Introduction In this presentation, we review and critique the epidemiologic literature that addresses the relationship between exposure to pesticides (Table 1) and the risk of subsequent development of brain tumors in adults. Little is known about the etiologic risk factors for brain tumors. Few analytic studies have identified specific environmental risk factors. Prior radiation exposure has often been mentioned as a possible risk factor for brain tumor (Hodges et al., 1992), as have been a variety of occupational exposures (Thomas and Waxweiler, 1986). Some

Abbreviations: CI = confidence interval; PMR = proportionate mortality ratio; OR = odds ratio; RR = relative risk; SMR = standardized mortality ratio * Correspondingauthor. Tel.: (507) 284 5540; Fax: (507) 284 1731. 0022-510X/95/$09.50 0 1995 Elsevier Science B.V. AI1 rights reserved SSDI 0022-510X(95)00151-4

blue-collar occupational groups, including rubber workers, oil refinery workers, chemical plant workers, polyvinyl chloride workers, and others, have been reported to have an elevated risk of brain tumors (Thomas et al., 1986; Thomas and Waxweiler, 1986; Teta et al., 1991). The possible etiologic importance of exposure to pesticides has been suggested by a case-control study on childhood brain tumors by Gold et al. (1979) by case reports of childhood brain neoplasms arising after exposure to chlordane and heptachlor (Infante et al., 1978) and by a study on patients who died from malignant tumors, among whom a high level of organochlorine compounds was found in the adipose tissue of those who had glioblastoma (Unger and Olsen, 1980). In addition, there are studies suggesting a possible role of the agricultural environment, where pesticides are commonly used, in the etiology of brain neoplasms but published results have been equivocal (Thomas and Waxweiler, 1986).

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of the NeurologicalSciences

2. Epidemiologic studies reporting an association between exposure to pesticides and brain tumor In order to comprehend the controversy regarding pesticides and brain tumors, it is helpful to rank or prioritize epidemiologic research designs according to their ability to validly assess causal relationships between antecedent exposures and development of disease. We will discuss different study designs in order of increasing hierarchy, from case series, ecologic studies, case-control studies to cohort studies. We will further discuss the results of cohort studies for three occupational groups which have directly or indirectly increased exposure to pesticides: chemical workers, farmers, and pesticide applicators. 2.1. Case reports and case series Anecdotal or case reports are observations in which an association may be suggested and provide general information for hypothesis generation. They are subject to numerous types of bias, including selection, sampling, response, and observer biases (Cornfield and Haenszel, 1960; Sackett, 1979); they have no denominators and provide no rates, which are essential for valid comparisons. There are numerous reports of cases and series of cases where possible exposures to pesticides are associated with brain tumors; most of them deal with children and will not be discussed here (Chaddock et al., 1987; Infante et al., 1978). With respect to adult patients, Morantz et al. (1985) found that 2 of 7 patients from a cluster of primary brain tumors were exposed to pesticides, but the same authors later reported that 6 of the 7 patients had had dental x-rays (Neuberger et al,, 1991). In the absence of comparison series, such reports are generally not helpful in the identification of specific risk factors. 2.2. Registry and ecologic studies In such studies, cancer rates for specified populations of counties or census tracts are compared with rates from other groups. Registry studies differ from case-control designs in that exposure and confounder information are collected for groups of people rather than for individuals. As we do not know the distribution of the confounding

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variables among the exposed and unexposed individuals, the magnitude of risk inferred for the exposed group may be in error. Thus, interpretation of results from registry studies, and inference about an individual’s exposure and its risk, are problematic. Another limitation arises from registry studies using death certificates to identify tumor cases (Percy et al., 1981). As most death certificates do not contain information about histologic verification of a brain tumor, the accuracy of the diagnoses is questionable and may lead to misclassification bias, especially with respect to brain metastases. Smith-Rooker and co-workers (Smith-Rooker et al., 1992) reviewed the charts of 100 patients with glioblastoma multiforme and found that more than one-third of the patient sample came from just three counties in which rice, cotton, or wood products were produced. However, the authors were unable to provide an answer to why certain other similar agricultural counties did not show an excess of patients with this type of brain tumor. In a large Swedish cancer registry it was found that the risk ratio of observed to expected brain tumors among agricultural workers was only 1.08 (Wiklund, 1983). Similarly, Burmeister (1981) did not find a significant excess of brain cancer mortality among 121101 farmers in Iowa. Godon et al. (1991) failed to show a positive relationship between mortality data for cancers of the brain and geographic areas classified into three categories of exposure to pesticides sold in a Canadian province. In addition, they found weak correlations between cancer mortality in males 15-64 years of age and females 35-64 years of age with the sales of benzonitriles, quaternary ammonium, and phenol derivatives. However, these correlations were found in the context of at least 380 correlation tests, which raises the question whether these findings may have been due to chance alone. Lastly, McLaughlin et al. (1987), in a population-based registry study, did not find a significant elevation in risk for gliomas among farmers (RR = 1.1). 2.3. Case-control studies In case-control studies, controls are compared with cases on specific characteristics and thus, ideally, differ only in the presence of a given disease and those factors which contribute to the disease (Sackett, 1979). Such

Table 1 Listing of common pesticides by licensing category Termites and other wood-infesting General household pests Lawn and ornamental pests: Insecticides Herbicides Fungicides Rodents Fumigants

organisms

aldrin, chlordane, chlorpyrifos, DDT, heptachlor, propoxur, pentachlorophenol aldrin, bendiocarb, carbaryl, chlordane, chlorpyrifos, dieldrin, DDT, diazinon, dichlorvos, tachlor, lindane, malathion, propoxur, pyrethrins, toxaphene most pesticides listed under general household pests 2,4-D, 2,4,5-T, paraquat, silvex captan, folpet, pentachlorophenol, phenylmercuric acetate strychnine, zinc phosphide, warfarin, coumafuryl, chlorophacinone, arsenicals calcium cyanide, methylbromide, paradichlorobenzene

hep-

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studies can be executed quickly and at low cost, even when the disorder of interest is rare. The retrospective nature of the research design requires careful selection of an appropriate control group and appraisal of the potential for differential recall of events by patients and controls. The problems inherent in such case-control studies include identification of patients, selection of controls, and the lack of completeness of the inquiry about risk factors that may

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be remote in time, as well as confounding and information biases (Cornfield and Haenszel, 1960; Sackett, 1979; Checkoway et al., 1989). Publicity about pesticides and disease and the tendency of individuals with cancer to try to identify events in their lives that may have caused their disease could result in recall bias, with false-positive results. A search of the English-language medical literature

Table 2 Retrospective case-control studies that have directly or indirectly evaluated the pesticide-brain

tumor relationship

Investigator

Number and source of cases

Number and source of controls

Type of exposure or occupation

Method

Results

Musicco et al. (19821

42 hospitalized glioma patients

agricultural occupation

questionnaire/ interview

Olin et al. (1987)

78 astrocytoma patients

42 patients with nonneoplastic neurologic diseases or benign tumors 197 clinical controls and 92 population controls

occupational and residential environment

interview

Brownson et al. (1990)

312 brain cancer registry patients

1248 non-brain cancer patients

occupation

cancer registry

Musicco et al. (19881

420 hospitalized glioma patients

465 non-glioma brain tumor patients and 277 patients with non-neoplastic neurologic disease

occupation and residence

interview

Reif et al. (1989)

452 registered brain cancer patients

19452 non-brain cancer patients

occupation

interview

Schlehofer et al. (19901

226 patients with primary brain tumors in Germany

418 population trols

occupation

questionnaire

Thomas et al. (19861 Giustu et al. (19871

718 brain deaths 344 brain deaths

tumor

738 controls

occupation

cancer

1157 controls

occupation

death certificates death certificates

Ahlbom et al. (19861

78 astrocytoma patients from Sweden

exposure to herbicides or insecticides

questionnaire

Speers et al. (1988)

202 Texas males who died from glioma 1674 male cancer deaths from Italian agricultural region

197 clinical controls with meningioma, pituitary adenoma or cerebral aneurysm and 92 population controls 238 male controls

occupation

death certificates

random samples of 480 individuals selected from same regional mortality file as being deceased from all causes

occupation

regional mortality file (death certificates)

Farmers had a S-fold risk of glioma (RR = 5.0; p < 0.0051, but this result was not statistically significant in a stratified analysis (RR = 1.9; nonsignificant). Persons living by a petrochemical plant (RR = 13.5; 95% Cl = 2.4-76.31, working at an airfield (RR = 13.4; 95%CI = 2.572.21, living near a sewage treatment plant (RR = 2.7; 95% CI = 1.0-7.5) had an increased risk of astrocytoma. A crop farmer had an OR of 1.5 (95% CI = 1.0-2.4) for brain cancer, but no significantly increased risk was found for the group of general farmers (OR = 1.1; 95% CI = 0.6-1.7). Farmers had a RR of 1.6 (95% CI = 1.062.42) for glioma which was statistically significant when compared with the total control group, but not when compared with the tumor control group. Farmers who used insecticides and fungicides had a RR of 1.0 compared with both control groups. Farmers had an increased OR of 1.3 (95% CI= 1.0-1.71 for brain cancer, but subgroup analysis did not reveal a significant effect for the major group of general farmers (OR = 1.1; 95% CI = 0.8-1.55). Agricultural workers were not at a significantly higher risk of brain tumor when compared to population controls (RR = 1.1; 95% CI = 0.7-1.9). Farmers did not have an increased risk of brain cancer (OR = 0.8; 95% CI = 0.4-1.8). The OR for the association between farming and brain cancer was not increased (OR = 0.8; 95% CI = 0.5-1.11. There was a statistically non-significant increased risk (OR = 2.4; 95% CI = 0.96.5) when compared with clinical controls, and a slightly increased, but non-significant risk (OR = 1.3; 95% CI = 0.5-3.5) when compared with normal population controls. Agricultural workers were found to be at a decreased risk of glioma (OR = 0.61; 95% CI = 0.3-1.221. Farmers with more than 10 years of experience did not have an increased risk of brain cancer (OR = 1.04; 95% CI = 0.43-2.44).

Forastiere et al. (19931

con-

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revealed a series of case-control studies on pesticides and brain tumor utilizing a variety of case ascertainment methods with assessment of direct or indirect exposures to pesticides. Few studies could directly assess exposure to pesticides in terms of dose, duration, and specific type of chemicals, whereas the majority of the studies used less precise exposure parameters, such as categorization on the basis of specific occupations where workers may be at increased exposure to pesticides. Table 2 provides a summary overview of case-control studies that will be discussed in more detail below. Musicco et al. (1982) showed a higher proportion of Italian farmers among hospitalized patients with glioma than among controls; however, the results did not achieve statistical significance in a stratified analysis for age and sex. Referral bias is potentially another problem in this study, as patients with brain tumor were predominantly tertiary referral patients in contrast to the control subjects, most of whom resided locally. In a later study, Musicco et al. (1988) reported a RR for glioma of 1.6 for farmers, which was statistically significant when compared with a combined control group of non-glioma patients and patients with other neurologic disorders, but not when compared with a tumor control group. In addition, they found that farmers who reported the use of insecticides and fungicides had a risk of about 2.0. No significantly increased risks were found among farmers who used herbicides and fertilizers. Based upon a post-hoc analysis of the commercial compounds used by these Italian farmers, which appeared to have been rich in alkyl ureas and copper sulphate, the authors interpreted their results as a probable effect of the exposure to alkyl ureas, which are precursors of N-nitroso alkyl ureas, the most powerful chemicals in inducing neurogenic tumors in animals, rather than a general effect of the pesticides that were applied (IARC, 1979). No data were provided about individual dose or duration of exposure among farmers. It should be noted that the tumor control group consisted of patients with non-gliomatous brain tumors, such as meningiomas, pituitary adenomas and brain metastases. If there were a hypothetical relationship between pesticides and these control tumors, then the statistical analysis would be biased with a risk estimated toward 1. Olin et al. (1987) compared 78 astrocytoma patients with both hospital and population control groups and found a few items associated with astrocytoma, such as “living near a petrochemical plant,” “living near a municipal sewage treatment plant,” and “working at an airfield.” Yet, no increased risk was found for the items “working with organic chemicals” or “living near any chemical industry.” A case-control study from southern Georgia failed to note an excess risk of brain tumor among farmers, and the risk of brain cancer did not correlate with county herbicide use (Giustu et al., 1987). Thomas and colleagues (Thomas et al., 1986) found that farmers were not at an increased

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risk of brain tumor (OR = 0.8) in a case-referent death certificate analysis of 718 cases and 738 matched controls. A case-control study using glioma mortality data for 202 Texas males revealed a non-significant decreased OR (0.6) for agricultural workers (Speers et al., 1988). Brownson and co-workers (Brownson et al., 1990) found an increased risk of brain cancer for crop farmers (OR = 1.5; 95% CI = 1.0-2.4) in a case-control study of 312 cases and 1248 controls selected from a cancer registry, but there was not a statistically significant increased risk for farmers in general. Reif and colleagues (Reif et al., 1989) reported another registry-based case-control study of 425 brain cancer patients and 19452 non-brain cancer patients with a higher proportion of farmers in the brain tumor group. However, a post-hoc analysis revealed that the major group of farmers with a general type of farming practice did not have a statistically significant increased risk (OR = 1.1; 95% CI = 0.8-1.55). Results from registry-based case-control studies should be viewed with caution unless the population at risk is completely identified. Another major problem encountered in the above-mentioned studies was the lack of correction for error inflation. Multiple tests were performed while still considering a level of p < 0.05 as significant. It should be noted, however, that correction for error inflation is less important when studies are of a purely observational nature. Furthermore, studies in which farmers or residence in an agricultural area serve as an indirect parameter of exposure to pesticides are subject to many confounding factors. First, contrary to general belief, many farmers typically use only a few pesticides during their lifetimes and make only a few applications per year (Blair and Zahm, 1990). Second, and perhaps more importantly, farmers are exposed to a multitude of chemicals other than pesticides, such as petrochemical products, exhaust fumes, ultraviolet radiation, mineral and organic dusts, and biological exposure to microbes (Schuman et al., 1967). 2.4. Cohort studies In general, cohort studies provide the best opportunity to test hypotheses concerning disease etiology because they avoid response and observer bias. In this type of study, cohorts or groups of individuals with documented exposure to a suspected risk factor, such as pesticides, are followed for years to determine if the subsequent incidence of or mortality from brain tumors exceeds that of a non-exposed cohort or some appropriate matched sample of the general population. Cohort studies may be prospective if the cohort is defined at a given time and has periodic follow-up or a terminal point, or they may be historical if the cohort was defined in the past and followed by medical documentation or terminal events to the present. Historical cohort studies generally have uncertain data on intensity and frequency of exposure with regard to specific pesti-

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tides; however, in spite of such difficulties, they have been conducted and are, in fact, the only cohort studies on exposure to pesticides and brain tumor which have been published to date in the English literature. Bertazzi et al. (1989) reported the mortality experience of the Seveso area in Italy which was contaminated with dioxins following an industrial accident in 1976. These authors found an increase in brain cancer mortality for females from this area in the period 1982-1986 (RR = 5.8; 95% CI = 1.228.9), but there was also an unexplained increased rate, particularly among females, prior to the Seveso industrial accident. An overview of cohort studies on brain tumor and pesticides is provided in Table 3. Studies are categorized into the three major occupations that have been implicated as having an increased direct or indirect exposure to pesticides, i.e., chemical workers in the manufacture of pesticides, pesticide applicators, and farmers. Chemical workers employed in the manufacture of pesticides Coggon and co-workers (Coggon et al., 1986) did not find a significant excess of mortality of brain cancer among workers exposed to phenoxy herbicides with high versus low grade of exposure (SMRS 116 and 80, respectively). A study of mortality among a cohort of 5172 workers exposed to TCDD (dioxin) revealed five deaths from brain and nervous system cancer, which was less than

expected (SMR = 0.68) (Fingerhut et al., 1991). Similar findings were reported in a cohort of employees involved in the production of phenoxy herbicides (Lynge, 1985). Bond et al. (1988) followed the health status of 878 chemical workers who were exposed to 2,4-dichlorophenoxyacetic acid (2,4-D) during the 1945-1982 period, with follow-up extending through 1982. No cases of brain tumor were observed in the cohort. Wang and MacMahon (1979) found only one case of brain tumor in a retrospective mortality study among 1403 workers employed in the manufacture of chlordane and heptachlor. Similarly, Wong et al. (1984) did not find a statistically significant excess of brain tumor mortality in workers potentially exposed to brominated chemicals such as DBCP, TRIS, PBB, and DDT. Results from a mortality study of 800 workers employed in the manufacture of chlordane revealed that the overall death rate tended to be less than that for the United States population, and that production workers with higher pesticide blood levels had lower SMRs for cancer than non-production employees (Shindell and Ulrich, 1986). No specific mention about brain tumor mortality was made, but according to the data provided, this number could not be more than one. Bloemen and colleagues (Bloemen et al., 1993) were unable to find a significant relationship between exposure to 2,4-D and its derivatives and mortality from brain tumors in a cohort of chemical workers who were poten-

Table 3 Historical cohort studies that have directly or indirectly evaluated the pesticide-brain

tumor relationship

Investigator

Source of diagnosis

Cohort

Number of cases

Total number

Risk estimate (95% CI)

Wang and MacMahon (1979) Wong et al. (1984) Lynge (1985) Coggon et al. (1986) Shindell and Uhich (1986) Bond et al. (1988) Alberghini et al. (1991) Fingerhut et al. (1991) Saracci et al. (1991) Bloemen et al. (1993) Cantor and Booze (1991) Swaen et al. (1992) Figa-Talamanca et al. (1993) Pesatori et al. (1994) Alavanja et al. (1988) Alavanja et al. (1989) Littorin et al. (1993) Wigle et al. (1990) Morrison et al. (1992) Ronco et al. (1992)

death certificates death certificates registry death certificates death certificates registry death certificates death certificates registry death certificates death certificates death certificates death certificates death certificates death certificates death certificates registry registry registry death certificates

chlordane and heptachlor workers brominated chemical workers phenoxy workers phenoxy workers chlordane workers 2,4-D workers farmers licensed to apply dioxin workers phenoxy and chlorophenol workers 2,4-D workers aerial applicators herbicide applicators pesticide applicators pesticide applicators agricultural extension workers forest and soil conservationists horticulturists farm operators farmers Italian farmers

1 5 4 11
< 1 (not provided) 1.32 (95% CI = 0.4-3.1) 0.37 (not provided) 1.27 (95% CI = 0.6-2.3) < 1 (not provided) < 1 (not provided1 1.39 (95% CI = 0.7-2.5) 0.68 (95% CI = 0.2-1.6) 0.38 (95% CI = 0.1-0.8) < 1 (not provided) 0.63 (95% CI = 0.1-1.81 3.18 (95% CI = 0.6-9.3) 2.7 (95% CI = 1.1-5.6) 2.2 (95% CI = 0.9-4.4) 2.08 (95% CI = 1.2-3.7) 1.7 (95% CI = 0.6-3.7) 1.5 (95% CI = 0.8-2.7) 1.03 (95% CI = 0.8-1.3) 0.98 (95% CI = 0.8-1.11 0.9 (not provided)

Ronco et al. (1992)

registry

Danish farmers

1403 3579 3 390 5 754 800 878 4580 5 172 18390 878 9 677 1341 2319 3 805 1495 1411 2370 73 538 156 242 (not provided) (not provided)

5

0.95 (not provided)

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tially exposed to the herbicide. Saracci et al. (1991) analyzed the pooled data of a large international cohort of workers exposed to phenoxy herbicides and chlorophenols and found a statistically decreased SMR of brain cancer (SMR = 0.38; 95% cr = 0.14-0.83; N = 18390). Licensed pesticide applicators Blair et al. (1983) reported the mortality experience of 3827 licensed pesticide applicators. Disease information was obtained from death certificates. Excess deaths were observed for leukemia and lung cancer. Five cases of brain cancer were found, which yielded an SMR of 2.0. It should be realized, however, that the SMR was based upon comparison with the general population (all ages) rather than with age-and sex-matched control subjects or age-and sexspecific rates matched with the exposed population. Moreover, the presumed excess of brain cancer was not identified as a single type histologically. Cantor and Booze (1991) found a decreased SMR for brain cancer among aerial pesticide applicators (SMR = 0.63), with no significant difference in SMR between aerial pesticide applicators and flight instructors. Pesatori et al. (1994) found a non-significant excess of brain cancer mortality among pest control workers in Florida (SMR = 2.2; 95% CI = 0.7-4.4). Swaen et al. (1992) reported a non-significant excess of brain cancer mortality in a cohort of 1341 licensed herbicide applicators in the Netherlands (SMR = 3.18; 95% CI = 0.64-9.30), but the number of brain tumor deaths was small. MacMahon et al. (1988), in their intensive follow-up of a cohort of 16 124 pesticide applicators, found only the tumor category for lung cancer to be significantly elevated, whereas no other types of cancer had a significantly increased SMR. Figa-Talamanca et al. (1993) recently reported an increase in brain tumor mortality in an Italian cohort of licensed pesticide users. However, a significant number of licensed pesticide users (n = 1002, 29%) were excluded from the study because of missing information. Furthermore, there was no histologic verification of the tumor diagnosis and no data were available on the quantity or types of pesticides used. Specifically, given the fact that the only significant excess of brain cancer was confined to the subgroup of those 65 years and older, the presence of intracerebral metastases from systemic cancer cannot be ruled out. Agricultural workers A historical cohort study of male Canadian prairie farmers, using record-linkage from population, agricultural and cancer registries, revealed a marginally significant association between the risk of dying of glioblastoma and increasing fuel/oil expenditures by male farmers (RR = 2.1; 95% CI = 0.9-5.0) (Morrison et al., 1992). However, there was not a significant association between brain cancer mortality and other indirect exposure estimates such as

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the number of acres sprayed with herbicides, the number of acres treated with insecticides, or the number of acres fertilized. Results of tests for dose-response relationships were not significant. In addition, no consistent pattern was observed between specific categories of brain tumors and exposures. Cancer risk for farmers in Denmark and Italy was studied by Ronco et al. (1992) who found that the risk of brain cancer mortality was significantly reduced among Italian farmers but not for Danish farmers (Table 3) notwithstanding the fact that herbicides were applied by Italian farmers themselves, whereas this was done by professional applicators for Danish farmers. Alberghini et al. (1991) reported a non-significant mortality excess due to brain cancer among 4580 Italian farmers who used pesticides (SMR = 1.39; 95% CI = 0.69-2.46) the excess of brain cancer becoming statistically significant in the age-group 65-78 years (SMR = 4.09; 95% CI = 1.658.43). However, results were obtained in the light of multiple testing with a small number of decedents, with no information about individual exposures. Results of a Swedish cohort study of 2370 horticulturists revealed a SMR of 1.5 (95% CI = 0.8-2.7) for brain tumor (Littorin et al., 1993). Subgroup analysis revealed an excess of brain tumor particularly among young and middle-aged workers (SMR = 3.2; 95% CI = 1.6-5.7). It should be noted that if brain tumors were related to exogeneous factors, one would expect a more pronounced effect among elderly workers, given the biological lag time for a tumor to develop and increased cumulative exposure to a noxious factor. Alavanja et al. (1988) analyzed the data on the mortality of a cohort of agricultural extension agents and found a PMR for brain tumor of 2.08 (95% CI = 1.18-3.65). However, a secondary case-control study revealed an OR of 1.0 for brain cancer. A similar study from the same group on the mortality among forest and soil conservationists found a non-significant proportional excess for brain cancer (PMR = 1.7; 95% CI = 0.6-3.7) (Alavanja et al., 1989). Finally, a cohort study of the mortality experience of 73538 male farm operators in Canada failed to find an excess of brain tumor in this population (SMR = 1.03; 95% CI = 0.83-1.25) (Wigle et al., 1990).

3. Biologic considerations Experimental studies on animals have shown that a number of chemicals (not limited to pesticides) may induce the development of tumors of the nervous system. The nitrosoureas are the neurocarcinogenic chemical substances most often used experimentally and have also been found in variable quantities in cigarette smoke (IARC, 1979). Some fungicides and herbicides used in agriculture may contain alkyl ureas (Musicco et al., 1988). Most of the experimental work has been done with rats and has been reviewed by Maekawa and Mitsumori (1990). Gliomas have been induced by direct implantation of polycyclic

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aromatic hydrocarbons and by intravenous administration of nitroso compounds such as N-methyl-N-nitrosourea and N-ethyl-N-nitrosourea. Oral administration of ethylurea and nitrite caused sufficient formation of N-ethyl-N-nitrosourea in the stomachs of pregnant rats to cause brain tumors in all their offspring. Other chemicals shown to induce brain tumors in rats include ethylene oxide, vinyl chloride, acrylonitrile, propane sulfates and aryl dialkyltriazenes. It should be noted that rats’ susceptibility to chemically-induced brain tumors is significantly influenced by strain, gestational age, and fetal versus adult status. Despite substantial biologic plausibility, the nature and strength of the role of specific chemicals in neuro-oncogenesis in humans remain controversial (Ames and Gold, 1992).

4. Confounding factors in the assessment of pesticides and brain tumor 4.1. Multiplicity

of exposures to different chemical agents

Pesticide applicators are frequently exposed to a number of herbicides, insecticides, and fungicides. It will be clear that exposure to numerous pesticides poses problems of interpreting risk associated with particular chemicals, and multiple comparisons increase the chance of falsepositive findings. Pesticide applicators are also exposed to petrochemical solvents or carriers of the pesticides. In addition, pesticide applicators working within buildings and closed ventilator systems can be exposed to organic solvents, decomposition products, and microbes such as Legionella. 4.2. Natural pesticides found in food Ames and Gold (1992) have recently emphasized the toxicologic significance of exposures to naturally occurring chemicals. They calculated that 99.99% by weight of the pesticides in the average American diet are chemicals that plants produce to defend themselves (nature’s pesticides). Only 52 of these natural pesticides have been tested in high-dose animal cancer tests, and 27 were found to be rodent carcinogens; these 27 were found to be present in many common foods. These authors estimated that Americans eat about 1.5 g of natural pesticides per person per day, which is about 10000 times more than they consume of synthetic pesticide residues. It is clear that the assessment of toxic effects of synthetic pesticides is diluted and likely to be obscured by the differential toxic effects from naturally occurring pesticides (Gold et al., 1994). 4.3. Extrapolation human condition

from high dose animal testing to the

The administration of chemicals at the maximum tolerated dose in standard animal cancer tests is postulated to

increase cell division (mitogenesis), which in turn increases rates of mutagenesis and, thus, carcinogenesis (Ames and Gold, 1992). These authors found that about 50% of both naturally occurring pesticides and synthetic pesticides are indeed rodent carcinogens when tested at high doses. However, the same authors also indicated that at the low doses of most human exposures, in which mitogenesis may not occur, the hazards to humans of rodent carcinogens may be much less than is commonly believed. 4.4. Pesticides and tumors outside the nervous system which potentially may metastasize to the brain Studies on the human carcinogenicity of pesticide exposure suggest an increased risk for tumors of mesenchymal origin, such as soft tissue sarcoma, leukemia, non-Hodgkin’s lymphoma, and multiple myeloma, although results among studies are not consistent (Epstein and Ozonoff, 1987; Blair and Zahm, 1990; Burmeister, 1990; Brown et al., 1990; Fingerhut et al., 1991; Cantor et al., 1992; Swaen et al., 1992; Blair and Zahm, 1993). Ibrahim et al. (1991), in a critical review, concluded that there was a suggestive relationship between exposure to the phenoxy herbicide 2,4-D and non-Hodgkin’s lymphoma but that a cause-effect relationship is far from being established. Contaminants of phenoxy herbicides (dioxins) have been reported to be associated with soft tissue sarcomas, but results again are controversial (Eriksson et al., 1990; Hardell and Sandstroem, 1979; Smith et al., 1984; Woods et al., 1987). As mentioned earlier, there are studies reporting an increased incidence of lung cancer in farmers or pesticide applicators, whereas other authors negate these findings. For example, MacMahon et al. (1988) found an increased SMR for lung cancer in a cohort of pesticide applicators, whereas McDuffie et al. (1990) reported an absence of correlation of lung cancer risk with occupational exposure to any specific pesticide or pesticides grouped by chemical composition in a population-based tumor registry. 4.5. Multifactorial

pathogenesis

In spite of considerable and varied research efforts, the etiology of brain tumors remains obscure (Codd et al., 1990). A relationship between brain neoplasm, and head trauma, scalp irradiation, viruses, and other environmental exposures has been suggested (Schoenberg, 1982), but when faced with the question of causation of a primary brain tumor in an individual, one may speculate on the relative roles of genetic susceptibility, immunologic degradation with aging, and chance exposure to one or more of a variety of exogenous factors, some of which are still to be identified. The multifactorial issue becomes even more complex when considering the possibility that certain exogenous agents may interact with other endogenous or

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exogenous conditions as tumor promoters rather than as inducers.

5. Considerations for future epidemiological tions on a pesticide-brain tumor association

investiga-

Epidemiological studies on occupational risk and exposure to toxic agents are difficult to assess (Blondell, 1990). Major problems encountered in the design of studies of cancer in pesticide users are selection of appropriate controls, quantitation of exposure, and multiple exposures to different pesticides. Blair and Zahm (1990,, 1993) reviewed methodologic issues and identified a number of weaknesses in case-control studies which assessed exposure by means of interview techniques; foremost among these was inaccurate recall by the subject or surrogate respondent for type and frequency of exposure. Future studies directed toward clarifying the relationship between exposure to pesticides and brain tumor should adhere to time-tested and proven epidemiologic principles. 5.1. Diagnostic criteria for brain tumor should be explicit It is advantageous to study as homogeneous a group as can be assembled on clinical grounds. A histologic diagnosis is preferred rather than an unspecified diagnosis of ‘ ‘ brain tumor’ ’ as so often recorded on the death certificate. 5.2. Select controls by more than one method It is desirable to select controls in more than a single way to reduce the potential bias inherent in a particular method of selection. For example, multiple control groups can be used, such as age-, sex-and race-matched healthy controls or one consisting of patients with cancer but excluding brain tumor or cancers that are potentially associated with pesticides. If differences occur between the control groups, a careful search should be made for possible confounding factors (Blettner and Sauerbrauer, 1993).

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haps in a subsequent case-control study) information that is clearly a result of a special research effort or self analysis by a patient who, with or without reported physician inquiry or prompting, has convinced himself that an uncommon event is the cause of his tumor. Third, it is desirable to ask patients or informants if they have any specific hypotheses about the cause of the disease in order to determine the effect of such impressions on their interview responses. 5.4. Monitor and specify the type of exposure Exposure assessment is a difficult but critical component in the epidemiology of cancer and pesticides. The nature of exposure should be clearly defined and the time of exposure specified. Monitoring to determine air, skin, biopsy, blood and urinary levels of pesticides should be incorporated into prospective epidemiologic studies whenever possible. 5.5. Compare with other sources of information When feasible, compare exposure data obtained from interviews with objective data from supplier’s records. 5.6. Avoid diagnostic sensitivity bias Diagnostic sensitivity bias should be avoided. Greenwald et al. (1981) concluded that an excess of brain tumors among Kodak employees may have resulted from a ‘diagnostic sensitivity bias.’ Kodak employees, with their comprehensive medical surveillance programs, had brain tumor diagnoses that were more often confirmed histologically than did other brain tumor patients in New York. 5.7. Compare with baseline rates The observed rates of cancer should not be compared solely with those of the general population, but should be compared in a specific fashion for the characteristics of the population or occupation group under study, such as sex, race, age and calendar time (Loomis, 1992).

5.3. Minimize recall bias 5.8. Duration of follow-up should be long enough The problem of recall bias in case-control studies can be addressed in at least three ways. First, a sequential questionnaire/interview design may be used to standardize the information gathering process (Armon and Kurland, 1990). Ideally, all interviewers should not only be trained in uniform data collection techniques, but also blinded to the information obtained by other interviewers, to the results of past data analyses, and to the study hypotheses. Unrelated questions may be included in the questionnaire to estimate the effect of recall bias and suggestibility. Second, it may be desirable to analyze separately (per-

As many tumors, such as oligodendrogliomas, lower grade astrocytomas and meningiomas, have a long lag or latency time before they become clinically manifest, studies should take that into account in terms of prior exposure and should incorporate a follow-up duration of sufficient length. 5.9. Validate the association The method of establishing association must be valid, but the distinction between association and causation should

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be maintained. The demonstration of an association between exposure and disease may indicate that there is an increased risk of disease in exposed individuals and communities, but an association alone is not sufficient to prove that the exposure is the cause of the disease. If associations are found, these should be subjected to criteria which permit conclusions about causative relationships. These criteria have been succinctly described by Sartwell and Last (1980) and include strength of association, consistency, dose-response relationship, chronological relationship, specificity, and biological plausibility. There should be consistent findings across different studies, especially across studies of equal validity, and the association should be a strong one (for example, as measured by the RR); a disease-specific response is more convincing than a general effect. A number of environmental agents that are accepted as carcinogens fit these rules of causality, most notably tobacco use; however, many environmental exposures that are frequently considered hazardous do not fulfill these requirements to show causation.

6. Discussion From this review of the epidemiologic literature on exposure to pesticides and subsequent development of brain tumors, it is our view that the weight of current scientific evidence does not support an etiologic role for specific pesticides in the pathogenesis of primary brain tumor in adults. Most of the studies suffered from methodological problems and poor documentation of vital information, such as length and quantity of exposure to specific pesticides, the histologic nature of the brain tumors, and general medical information on the individual patient. Information about exposure was often obtained by interview of farm workers or their next of kin. The reliability and validity of information on pesticide use has been questioned since it was subject to recall, often years after the event (Blair and Zahm, 1990). We were unable to perform a valid meta-analysis because many studies are non-comparable. Even in positive studies of farmers, the increase in risk was small (usually < 2), the increase usually did not reach statistical significance (could have occurred by chance alone), and, most importantly, study results were generally inconsistent. Increased risk of brain cancer confined to those who performed agricultural work has often been cited as suggesting a possible etiologic role of exposure to organic pesticides; however, farmers are also exposed to a multitude of other chemical agents, environmental stresses such as sunlight, mineral and soil dust, and biological microbes. These other agents may interact with each other, and become relevant in oncogenesis. In this respect, Creagan and Fraumeni (1972) emphasized the similarity between the naturally occurring soil-related carcinogens elaimycin and nitrosourea, and Ronco et al. (1992) re-

ported that specific occupations in agriculture having a high risk for cancers of the lymphopoietic system mostly entailed contact with animals. The great majority of the cohort studies of chemical workers employed in the manufacture of pesticides did not demonstrate an excess of brain cancer mortality. Most of the cohort studies of pesticide applicators revealed non-significant tendencies for excess mortality due to brain cancer. However, we realize the shortcomings of making conclusions solely on statistical significance; therefore, we have tabulated all cohort studies in Table 3 and have considered statistically non-significant tendencies as well. We feel that existing data are insufficient to conclude that exposure to pesticides is a clear risk factor for brain tumors. Given the conflicting results reported for farmers and pesticide applicators in contrast to chemical workers, it may be possible that exposure to multiple agents and/or other factors may be more relevant than a single type of exposure. It should be noted that animal studies and most human studies do not take into account important factors such as multiplicity of exposures, smoking and alcohol consumption, interactions with endogenous or other exogenous factors, and, for humans, genetic susceptibility and general medical condition, which may ultimately have determined his/her susceptibility to cancer. The National Academy of Sciences’ Institute of Medicine conducted an independent investigation to evaluate the strength of evidence for human health effects among veterans who may have been exposed to herbicides used as military defoliants in Vietnam and could find no association between exposure to herbicides and brain tumors (Goetz et al., 1994). Finally, recent reports in the literature have indicated that the incidence of brain tumors is rising, and that this increasing incidence might be related to increased exposure to environmental factors, such as pesticides or electromagnetic radiation (Davis et al., 1991; Polednak, 1991; Larsen, 1993). However, the study of brain tumors in the population of Olmsted County, Minnesota (1950-1992), revealed that although the incidence rate for brain tumors increases with increasing age, the incidence rate for symptomatic brain tumors did not change significantly over the entire period 1950-1992 (Radhakrishnan et al., 1995). The increase in age-specific incidence rates with increasing age is believed to reflect better medical care for the elderly with more access to neuroimaging studies (Radhakrishnan et al., 1995). Although there is insufficient evidence for a positive association between exposure to pesticides and brain tumors by present data, further studies are needed which should apply methodologic standards that minimize biases. After all, lack of proof. is not proof of lack. Prospective cohort studies, such as that of Ranch Hand (Wolfe et al., 1990), involving large samples of well-defined and clearly documented individuals with known exposure to pesticides followed for long periods to determine outcome with

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respect to cancer such controversy. made to develop while controlling food sources.

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and other diseases would help resolve Most importantly, an attempt should be detailed occupational exposure indices, for naturally occurring exposures from

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