Breast cancer is the most common invasive malignant condition affecting women in the United States; it is estimated to account for 32% of all cancers diagnosed in this population in 1994 [1, 2]. The most recent results from the Surveillance, Epidemiology, and End Results program of the National Cancer Institute [1] documented a 5-year disease-specific survival rate of 80% for white women diagnosed with breast cancer between 1983 and 1989. The corresponding rate for black women was only 64%. Although improvements in the detection and treatment of breast cancer in the last 30 years have substantially increased the 5-year survival rates for both races, no evidence indicates that these advances have lessened the difference in rates. In fact, this difference may actually be increasing.

To improve the survival of black women with breast cancer in the United States, the factors that contribute to the development of breast cancer and the poorer prognosis in this group must be understood. The purpose of this article is to review current knowledge about the risk factors for and the clinical manifestations and pathologic characteristics of breast cancer in black women and to use this information to identify areas in which intervention may improve survival rates.

Methods

The MEDLINE database (1966 to 1995) was searched to identify all English-language articles related to breast cancer in black women; the following medical subject headings were used: African American, black, negro, breast neoplasms, breast diseases, cancer screening, and mammography. In addition, the reference sections of all identified articles were reviewed for additional pertinent sources of information. In the review of the collected literature, results from large cohort and case–control studies were emphasized whenever such studies were available. Information from case series and case reports was used only when other data was not available.

Epidemiology and Risk Factors
The age-adjusted incidence of breast cancer in U.S. women has steadily increased over the past 50 years, and a particularly sharp increase was seen in the mid-1980s, partially because of increased use of mammography. During this period, white women have had a consistently higher risk for developing breast cancer, with an age-adjusted incidence in 1990 of 112.7 new cases per 100 000 women. The age-adjusted incidence for black women in 1990 was 95.8 new cases per 100 000 women. Closer examination of incidence statistics shows that the 20% higher incidence rate in white women is not uniform among all age groups but has followed a crossover pattern in which the risk for breast cancer in young women is slightly but consistently greater in the black population and the risk of developing breast cancer in the middle-aged and elderly populations is substantially higher among white women (Figure 1). As the incidence of breast cancer in young black women has increased faster than the incidence in young white women, the age at which the crossover in incidence occurs has gradually increased over the past 20 years and is now between 45 and 49 years of age. By reviewing the acknowledged risk factors for the development of breast cancer, investigators have proposed several hypotheses to explain the racial variation in breast cancer incidence.

Large epidemiologic studies of the U.S. population have identified various potential risk factors for breast cancer. The factors most consistently found to alter the risk for breast cancer development include various fertility and reproductive measurements, family history of cancer, presence or absence of certain benign breast diseases, and several socioeconomic indicators.

Figure 1. Age-specific breast cancer incidence rates, 1986-1990. Data obtained from Miller and colleagues .Three reports have been published that examined risk factors for the development of breast cancer in black women (Table 1). These case–control studies have evaluated 1) factors that are known to contribute to the development of breast cancer in the general population and 2) various other conditions hypothesized to contribute to the differences in incidence rates seen between various population groups. Overall, risk factors for breast cancer in black women appear to be similar to those in the general population: early age at menarche, late age at menopause, nulliparity, late age at first full-term pregnancy, history of breast cancer in first-degree relatives, and a history of benign breast disease. In addition, the possibility that prolonged (> 10 years) use of oral contraceptives may adversely affect breast cancer risk in black women was raised by one large population-based study. Higher level of education, higher socioeconomic status, and higher body mass index—characteristics often identified as associated with elevated risk for breast cancer - have not been as consistently documented to alter risk in black women.

These well-established risk factors have been examined for their possible contribution to the differences in rates of breast cancer incidence seen between black and white women. Early age at menarche, late age at menopause, late age at first full-term pregnancy, and nulliparity all appear to contribute to a higher risk for breast cancer. The median age at menarche is slightly lower in black than in white girls: 12.5 years compared with 12.8 years. Black women also have an earlier median age of natural menopause, 49.3 years compared with 50.0 years in white women, and are more likely than white women to have had surgical menopause. Finally, the age at first full-term pregnancy is consistently about 2 years less in black women. Although these differences in reproductive values seem small, it has been estimated that they can alter the risk for breast cancer enough to be clinically evident in large populations. It is also interesting to note that the combination of reproductive factors seen in black women results in an expected risk pattern for breast cancer development similar to that documented in the actual incidence data. Early menarche increases the risk for cancer throughout a woman’s life; early pregnancy results in a transient (10- to 15-year) increase in the risk for breast cancer, followed by a significant reduction in breast cancer risk; and early menopause contributes to a decreased risk for breast cancer in older women. These factors contribute to higher than expected rates in young women and substantially lower rates after menopause.

The use of oral contraceptives, particularly at a young age and for prolonged periods of time, is a less firmly established reproductive factor affecting the risk for breast cancer. Some studies have also suggested that use before the first pregnancy is a risk factor for breast cancer development. Many characteristics of oral contraceptive use in black women are similar to those seen in white women. Approximately 80% of women of childbearing age in either race have used oral contraceptives at some time in their reproductive life. The average duration of use is similar in black and white women (5 years), and the proportion of long-term (> 10 years) users is also similar (15% to 20%). The only notable differences in usage patterns are a consistently earlier age at the start of oral contraceptive use by black women and a lower incidence of use before the first pregnancy by black women. These two characteristics would have potentially opposite effects on the development of breast cancer. Currently, the actual influence of oral contraceptive use on the risk for breast cancer in black women compared with white women awaits further epidemiologic studies that more clearly define the importance of specific usage patterns on cancer risk.

Hormone replacement therapy has also been associated with an increased risk for the development of breast cancer, especially in long-term users. Demographic data on the patterns of use of postmenopausal hormone replacement therapy are not available, making it impossible to comment on racial differences in hormone use and their possible contribution to breast cancer risk.

History of breast cancer in a first-degree relative increases a woman’s risk for breast cancer 1.5- to 3.0- fold. This excess risk has been determined by studying groups of predominantly white women. Two case-control studies that analyzed black women with breast cancer (identified through the Surveillance, Epidemiology, and End Results program of the National Cancer Institute) found an approximately twofold increase in the risk for breast cancer in black women who had a first-degree relative with breast cancer. This finding was consistent with what would be predicted in a similar group of white women. No further studies have been reported, and no racial or demographic information is yet available on the prevalence of germline mutations in the recently discovered genes associated with familial breast cancer (BRCA1 and p53) in black women, although one black family with a high prevalence of breast cancer has been associated through linkage analysis with a probable mutation of the BRCA1 gene.

Finally, the presence of certain benign breast lesions has also been associated with an increased risk for the development of breast cancer in the general population. Specifically, proliferative disease without atypia is generally associated with a relative risk of 1.5 to 1.9 compared with the risk of control populations, and proliferative disease with atypical hyperplasia is associated with a relative risk of 2.5 to 4.5. Unfortunately, little is known about the pattern of benign breast disease in black women. Fibroadenomas are diagnosed more frequently in black women and at an earlier age (15 to 30 years of age); the frequency of proliferative and nonproliferative benign breast disease in black women is unknown.