Richard B. Hayes, D.D.S., Ph.D.*
|The prostate gland, located at the base of the penis,
surrounds the urethra and produces seminal fluid. Cancer of the
prostate is one of the most common cancers among American men, with
an incidence rate exceeding that for lung cancer. It is primarily a
disease of the elderly: The median age at diagnosis is 72. While
Europeans and American whites have high prostate cancer mortality
rates, perhaps the highest reported mortality in the world for
prostatic cancer is among American blacks. The lowest rates are
found in Asians (Kurihara et al., 1989).
From 1973 to 1991, prostate cancer mortality in the United States increased at a rate of 1.0 percent per year among white males and 1.8 percent among blacks (Ries et al., 1994). However, among American blacks over the age of 65, the overall rate rose 45.2 percent for blacks--more than twice the rate for whites (22.4 percent).
Since the late 1940s, the rate of identification of prostate cancer cases has increased 67 percent or about 1.8 percent per year (Devesa et al., 1987). This dramatic increase is in part due to the greater frequency of operations for benign disease of the prostate, with the subsequent incidental finding of asymptomatic prostatic tumors, as well as the escalation in the use of new diagnostic technology including transrectal ultrasound guided needle biopsy, computer tomography, and serum testing for prostate-specific antigen (PSA). However, the steady increase in the mortality rates implies that the escalation in incidence is not solely attributable to incidental discovery and early detection, but to a real change in the risk of developing the disease (Miller et al., 1993).
While prostate cancer is uncommon among Japanese in Japan, Japanese in Hawaii have prostate cancer rates intermediate between those in Japan and the high incidence among Hawaiian whites. These results, and other studies that show migrant populations tending toward the prostate cancer risk pattern of their host country, strongly suggest that environmental factors contribute to the large differences in risk found between countries (Bosland, 1988). Many such factors are under investigation.
Prostate cancer rates are generally greater in countries where the population consumes more animal fat. Comparison of dietary habits of prostate cancer cases and controls has shown that cases overall consume more animal fats (Kolonel et al., 1988; Graham et al., 1983; Ross et al., 1987). No correlation has been noted with the consumption of fat from vegetable sources (Rose et al., 1986). Greater dietary intake of vitamin A has also been associated with an excess risk for prostate cancer in some studies (Kolonel et al., 1988; Graham et al., 1983), although high blood levels of retinol appear related to a decreased risk for this disease (Reichman, 1990).
The evidence for an important role of diet in prostate cancer development has increased over the last decade. Further studies supported by NCI are designed to specify these associations and to determine the contribution of dietary factors to ethnic differences in the United States in prostate cancer risk.
Other factors may also be involved in the development of this disease. Reports of greater sexual activity and frequency of venereal disease in prostate cancer cases than controls raises the possibility that some cases may be the result of a sexually transmitted agent (Ross et al., 1987) although no likely microorganism has been identified. A history of some benign prostatic disease, including prostatitis (Honda et al., 1985) and some types of hyperplasia (Kovi et al., 1988), may increase the risk of prostate cancer. Studies of occupational groups have shown farmers to be consistently at higher risks for prostate cancer (Blair et al., 1988), although it is unclear if this finding is the result of occupational factors or to concomitant lifestyle factors. Other studies weakly suggest associations with work in rubber manufacturing, iron and steel foundries, and some other manufacturing occupations (Bosland, 1988).
Although the mechanism of prostate cancer development is not understood, hormones, including the male androgenic hormone, testosterone, could play an important role. These hormones are essential in normal prostate development and function; their manipulation is important in prostate cancer treatment and in the development of prostate cancer in experimental animals. Diet (Howie and Schultz, 1985) and other factors (Dai et al., 1988) may influence hormone levels, and people at high risk for prostate cancer may have different hormone patterns than those at low risk (Ross et al., 1992). Recent studies (Barrett-Conner et al., 1990; Nomura et al., 1988) implicate various steroidal and related hormones, suggesting that the relationship of hormone status with prostatic cancer risk is likely complex.