Unfortunately, most of the known risk factors for breast cancer, such as reproductive and family history, are not easy to change, and are likely to explain only 30 to 50 percent of all breast cancers. Despite intensive research, ways to prevent breast cancer remain elusive.
Nonetheless, progress is being made. A successful public health effort over the last decade has increased the proportion of women who receive regular mammograms. The proportion of women in California who report having a mammogram in the previous two years has increased markedly among all income groups since 1987.
Accompanying the increase in mammography utilization has been a decrease in the rate of late-stage disease.
This graph shows trends in incidence rates for early and later-stage breast cancers. Stage 0 are in situ tumors, and Stage I are small (less than two centimeters in largest diameter) tumors with no evidence of spread of the cancer to lymph nodes or other organs.
Although the invasive breast cancer incidence rate has been fairly stable over this time period, the incidence of early stage (Stage 0-I) tumors significantly increased over the last eight years, and the rate of later-stage tumors (any invasive tumor larger than two centimeters or with nodal involvement) significantly decreased.
Breast cancers are also being diagnosed when smaller, which is another good sign for survival. The proportion of breast cancers one centimeter or less in largest diameter increased from about 24 percent in 1988 to 29 percent in 1996, while the proportion of tumors larger than two centimeters in diameter decreased.
Most importantly, the breast cancer mortality rate is now beginning to decline. This graph shows breast cancer mortality rates in California since 1973.
Breast cancer mortality rates in California began decreasing in about 1987, and have been decreasing steadily since that time. Mortality rates in California are now 23.1 deaths per 100,000 women, which is about 20 percent lower than in 1973 (28.2). Similar declines in breast cancer mortality are also seen nationwide.
Breast cancer survival has improved over the last two decades for all stages combined, and for localized and regional cancers examined separately.
This graph shows that among women 50 to 69 years old, the three-year relative survival rate for localized breast cancer increased from about 95 percent in 1975 to about 99 percent in 1989. This is probably due to the diagnosis of smaller tumors with a better prognosis, and to the increased use of breast conserving surgery.
Three-year relative survival for regional disease in this age group increased as well, from less than 80 percent for cases diagnosed in 1975 to nearly 90 percent in 1989. This is probably related to the progressively wider use of adjuvant therapy in breast cancer treatment, including both chemotherapy and tamoxiphen.
Similar improvements in survival occurred for women less than 50 years old and for women ages 70 and older.
The conclusion of the authors of this study was that “the recent drop in breast cancer mortality is too rapid to be explained only by the use of mammography; likewise, there has been no equivalent dramatic increase in survival rates that would implicate therapy alone. Thus, indications are that both are involved in the recent rapid decline in breast cancer mortality in the US.”
Finally, the Breast Cancer Prevention Trial recently showed that the prescription medication tamoxifen may reduce the risk of developing breast cancer by 50 percent among women who are already at increased risk for breast cancer due to family history or other well-established risk factors.
Women who participated in the study had an estimated risk of developing breast cancer that was about two to five times higher than women in the general population. Women should discuss with their health care provider their individual risks for breast cancer, and the potential risks and benefits of taking tamoxifen. In addition, a clinical trial began in 1999 that will compare tamoxifen with raloxifene, an osteoporosis drug which may also reduce the risk of breast cancer. For more information on tamoxifen and the Study of Tamoxifen and Raloxifene (STAR), visit the NCI website http://www.nci.nih.gov.
