Brain and other central nervous system (CNS) tumors (ICD-9 191.0–191.9, 192.0–192.3, 192.8–192.9) are among the most deadly adult cancers (ACS, 2002a). Prognosis and treatment depend heavily on the location of the tumor in the CNS and the type of cell in which it develops. Survival rates vary with age. The 5-year relative survival rate of people 15–44 years old is close to 55%. Those 45–64 years old, however, have a 5-year survival rate of 16%, and those over 65 years old have a 5-year survival rate of 5%.
Histologic types of brain and other CNS cancers in the United States vary in their incidence and mortality by age, sex, and race. Rates are higher in males than in females, and whites have higher rates than blacks, followed by Hawaiians, Chinese, Japanese, and Filipino Americans, and Alaskan natives. Age-specific incidences show a peak under the age of 10 years, an exponential rise from the early 20s to 70 years, and then a decline with increasing age thereafter (Inskip et al., 1995).
The only established environmental risk factor for the development of brain and CNS cancers is exposure to radiation, which usually occurs during treatment for other cancers. Other people at risk include those with impaired immune systems or those with a family history of such disorders as neurofibromatosis type 2, tuberous sclerosis, and Von Hippel-Lindau disease. However, the risk factors for brain and CNS cancers remain largely unknown (ACS, 2002f; NCI, 2002u).
As pointed out by others (Blair and Zahm, 1995; Bohnen and Kurland, 1995), a major limitation in investigation of insecticides and brain cancer risk has been that most of the studies lack information regarding agents of exposure. Often, no specific exposure determination is undertaken, and analyses are based on job title or industry, such as farmer or agriculture. Another limitation is that many studies relied on death certificates to identify both exposure and disease (as reviewed by Khuder et al., 1998), and pathologic confirmation of the specific type of cancer is lacking. Brain and other CNS cancers can take many forms (such as meningioma, anglioblastoma, astrocytoma, and ganglioglioma), and most epidemiologic studies do not identify the histologic type, because of the lack of pathologic review. As a result, studies often analyze the broader category of “brain or CNS cancers.” Epidemiologic studies of brain and other CNS cancers must also address and consider the relatively low survival rate that limits the number of cases to interview and the aggressive nature of the disease, which adversely affects memory and the recall ability of patients. The key studies reviewed by the committee are discussed below, and relevant results are included in Table 5.8.
An ecologic study by Mills (1998) found no correlation between age-adjusted incidences of brain cancer and specific insecticide use, including diazinon, for all 58 California counties. The greatest limitation of this study is the lack of individual measurement of exposure and outcome, as well as the lack of information on other exposures and confounding variables, which could provide alternative explanations for the findings.
A few studies have examined the relationship between insecticide use and brain and other CNS cancers among farmers and agricultural workers. However, most studies are