About 90-95% of stomach cancers are adenocarcinomas, although other, less common tumors do occur. Most adenocarcinomas are thought to develop slowly—over many years. Premalignant lesions and early-stage cancers generally begin in the mucosa or inner lining of the stomach. The cells become progressively abnormal as they accumulate genetic damage. With malignant transformation, the cancers develop the capacity to invade the submucosa and muscle wall of the stomach, to extend from there into the subserosa and the outermost serosa that wrap the stomach, or to metastasize to other organs, such as the liver, lungs, and bones. Some types of lymphoma, a cancer of the immune system, also occur in the wall of the stomach; they account for about 4% of stomach cancers. Included among the other tumors are the slow-growing (indolent) lymphoma of mucosa-associated lymphoid tissue (MALT) and carcinoid, a hormone-producing tumor of the stomach and other organs.

In the United States, the median survival of persons with stomach cancer is less than 1 year after diagnosis, although the 5-year survival rate has increased slightly from 15.1% for patients diagnosed in 1974-1976 to 23.2% for those diagnosed in 1995-2001.

EPIDEMIOLOGIC EVIDENCE CONSIDERED

The committee deemed 36 papers on occupational cohorts and five population-based case-control studies to be informative for assessing the association of asbestos and stomach cancer. In that several citations on the cohorts reported separate findings for subgroups (by sex or by separate factory workforces), a total of 42 cohorts were included in this review.

Cohort Studies

The cohorts that presented usable information on the risk of stomach cancer were indicated in Table 6.1. Their histories and design properties are described in Table B.1, and the details of their results concerning cancer at this site are abstracted in Table D.4. The results of the cohort and case-control studies are summarized in Table 10.1, and Figures 10.1 and 10.2 are plots of RRs for overall exposures and for exposure-response gradients from the cohort studies reviewed.

Most of the occupational cohort studies were conducted in predominantly white populations in North America, Europe, and Australia. Two studies in China (Pang et al. 1997, Zhu and Wang 1993) were also included in the review. The reported associations between stomach cancer and occupational asbestos exposure in cohort studies were based on somewhat larger numbers of cases than those related to esophageal cancer, but statistical power in individual studies was generally low



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