Occupation - Epidemiology and Nongenetic Causes of Breast Cancer

A review of 115 studies conducted between 1971 and 1994 found little support for an association between specific occupations and breast cancer risk. Limited evidencesuggested that cosmetologists, beauticians, and pharmaceutical manufacturing workers had a modestly elevated risk of breast cancer, but conclusions were not possible due to lack of adequate exposure data.

Although ionizing radiation is a recognized risk factor for breast cancer, none of the studies of radiation workers, including those of x-ray technicians and workers at uranium fuel plants and atomic energy plants, found an elevation of breast cancer risk among women in these occupations. The few studies carried out on specific occupational agents have not provided any evidence of association. In particular, although organic solvents may increase risk of various cancers in animals, women who worked in dry cleaning or shoe manufacturing, or who were exposed to trichloroethylene did not have an elevated risk of breast cancer.

Despite the large literature on occupation as a risk factor, most studies have simply examined associations between occupational title and breast cancer risk; specific information on exposure to potential carcinogens was collected in only a few studies.

Although some studies collected detailed information on lifetime occupational history, often broad occupational groupings representing only the most recent occupation were used in analyses. Furthermore, most studies have not controlled adequately for known breast cancer risk factors - reproductive factors, in particular - that are likely to confound any observed association with occupation. Employment outside the home, and in a specific occupation, is likely to be highly correlated with educational attainment and socioeconomic status, and thus with reproductive characteristics. In the few studies that have controlled for sociodemographic and reproductive risk, breast cancer risk was not found to vary across occupational groups.

In contrast, a consistent finding across studies that were unable to control for important confounding factors has been an increased breast cancer risk among more highly educated women, rather than a consistently observed association with any specific occupation. Thus, further analyses of occupational titles without consideration of known breast cancer risk factors or actual workplace exposures are unlikely to be informative.

Medical History
A variety of diseases and medications are known or suspected to cause or be associated with modifications of hormones or growth factors and thus may influence breast cancer risk. Type 2 diabetes mellitus has been suggested to increase risk of breast cancer. Hyperinsulinemia, as occurs in type 2 diabetes mellitus, may promote breast cancer, because insulin may be a growth factor for human breast cancer cells.

Furthermore, insulin levels are inversely related to levels of sex hormone-binding globulin, and thus are positively related to levels of available estrogens and androgens. Many studies have lacked information about the type and severity of diabetes, which makes the interpretation of the various findings difficult. In a case control investigation of subclinical diabetes, hyperinsulinemia with insulin resistance was a significant risk factor for breast cancer, independent of weight or body fat distribution.