Furthermore, the incidence of most cancers increases with age. Two-thirds of all cases of cancer are in individuals over age 65 (Longo, 1998). Lung cancer is largely a disease of men and women past the age of 55, with the peak incidence occurring between ages 55 and 65 (Minna, 1998).
In order to accumulate enough cases to avoid false-negative conclusions, it is important for the study to have adequate statistical power, which is a function of both the follow-up time and the size of the population. Longer follow-up time will allow an examination of a range of latency periods between the exposure and the diagnosis of disease.
Table 4.7 provides information on the follow-up periods of the uranium worker studies examined in this chapter. Otherwise well-designed studies such as those of Checkoway et al. (1988) and Hadjimichael et al. (1983) suffer from a relatively short period of follow-up. The studies of Ritz (1999), Dupree et al. (1995), Frome et al. (1990), and Polednak and Frome (1981) have 25 to 30 years of follow-up.
Collectively, the occupational cohorts of workers exposed to uranium, both to relatively insoluble oxides and to more soluble forms, provide a substantial body of evidence to judge the effects of exposure to uranium on cancer risk. Many of these workers may have had high levels of exposure during the early years of the nuclear industry and have now been followed for more than 30 years.
There is strong evidence that the levels of exposure in these occupational settings have not increased overall cancer mortality (Table 4.8). The SMRs are all close to or less than 100, indicating that cancer mortality in uranium workers was similar to the comparison group, which was either the entire U.S. male population or the population of the region near the work site. However, cancer is very heterogeneous, and organs and organ systems vary in their cumulative exposure to inhaled or ingested uranium and probably also in their susceptibility to carcinogenesis. Thus, the committee looked carefully at those cancers that were most likely to be related to internal exposure to uranium. However, we cannot exclude the possibility that exposure to uranium increases the risk of some relatively uncommon cancers.
Lung cancer has received the greatest attention in past studies of uranium exposure, and it is the disease about which the committee can make its most extensive analysis of the relationship between uranium exposure and disease. The reason for attention to lung cancer is due to the long residence of inhaled uranium dust in lung tissue and regional lymph nodes. In addition, lung cancer is a common disease and the number of cases is often sufficient to permit analysis