Breast cancer is the most common cancer among women both in the United States and in the world. It is the second leading cause of cancer death among women in the United States. Among U.S. women in 2009, approximately 192,370 new cases of invasive breast cancer and 62,280 carcinoma in situ will occur and 40,170 will die from breast cancer.1 In terms of its global burden, there are about 1.05 million new cases and 373,000 deaths each year worldwide.2
There is a 4- to 5-fold variation in breast cancer incidence rates worldwide, with the highest in North America (99.4/100,000) and the lowest in Asia (22.1/100,000) and Africa (23.4/100,000).3 However, mortality rates are relatively less variable, with Africa (16.2/100,000) being similar to North America (19.2/100,000). These international variations are partly due to differences in environmental and lifestyle factors, screening practices, and treatment strategies.
In the past 50 years, breast cancer incidence has increased worldwide, including in the United States, where the highest rate is found.4 Data from the Surveillance, Epidemiology, and End Results (SEER) program show that incidence increased in the 1980s and 1990s in the United States but decreased after 2002 mainly in white women4 (Fig. 4-1). During the same period, 5-year survival also increased, reaching 90% at year 1997 for women 50 years or older and 87% for women under 50 at diagnosis (Fig. 4-2).4 The continued improvement in prognosis may be attributable to both screening, which increases the diagnosis of small, localized breast tumors, and treatment effectiveness.
Trends in female breast cancer incidence and death rates by race and ethnicity, United States. Rates are age-adjusted to the 2000 U.S. standard population. (Data from Ries L, Melbert D, Krapcho M, et al. SEER Cancer Statistics Review, 1975-2005. Bethesda: National Cancer Institute; 2008.)
Breast cancer survival trends in the United States. Values shown are 5-year relative survival (survival adjusted for life expectancy—an approximation to breast cancer-specific survival). (Data from Ries L, Melbert D, Krapcho M, et al. SEER Cancer Statistics Review, 1975-2005. Bethesda: National Cancer Institute; 2008.)
Breast cancer incidence rates increase with age, as shown by the log scale plot of age-specific cancer incidence rates in 5-year age groups (Fig. 4-3). For estrogen receptor (ER)-positive cancer, the incidence rates increase rapidly until approximately age 50 (proportional to the seventh power of the age), and then rise slowly (proportional to the first power of the age). However, the incidence rate of ER-negative cancer increases rapidly before age 50 (proportional to the fifth power of the age) and then remains constant. As a result, ER-positive tumors are more likely to occur in postmenopausal women.
Breast cancer incidence rate by age and estrogen receptor status, United States, 2000-2005.4(Data from Ries L, Melbert D, Krapcho M, et al. SEER Cancer Statistics Review, 1975-2005. Bethesda: National Cancer Institute; 2008.)
Race, Ethnicity, and Socioeconomic Status
There are racial and ethnic differences in breast cancer incidence and mortality in the United States (Fig. 4-1). In the past 30 years, incidence rates were higher in white women than in African American women. However, African American women had higher mortality rates than white women, and this racial disparity has been widening in recent years. The incidence and mortality rates in Asian, Hispanics, and Native American women are lower than those of non-Hispanic white and African American women.
African Americans are more likely to have regional and distant disease, contributing to the survival disparity (Fig. 4-4A). However, African Americans also have lower 5-year survival rates than whites within stage strata (Fig. 4-4B). In addition to later stage at diagnosis, socioeconomic factors such as limited access to quality health care and comorbidities contribute substantially to outcome disparities.5,6 However, some degree of difference in outcomes persists between blacks and whites after accounting for these factors, suggesting that cancer biological differences might also be relevant.7,8 Breast cancers in African Americans are more likely to be early-onset, higher-grade, and ER-negative compared with those in whites.5,6
A. Distribution of breast cancer stage at diagnosis in African-American and white women, United States, 2000-2005. B. Five-year relative survival for African-American and white women by breast cancer stage, United States, 1975-1979 and 1996-2004.4(Data from Ries L, Melbert D, Krapcho M, et al. SEER Cancer Statistics Review, 1975-2005. Bethesda: National Cancer Institute; 2008.)
Early age at menarche is a well-established risk factor for breast cancer in both premenopausal and postmenopausal women, with a reduction in risk of 5% to 10% for each year delay in age at menarche.9 Inaccurate recall of menarcheal age, especially in older women, may underestimate the strength of this association. Several mechanisms may explain the protective effects of late menarche. Early menarche may be associated with more rapid onset of regular, ovulatory menstrual cycles and hence longer duration of lifetime exposure to endogenous hormones.10 Estrogen levels are higher several years after menarche and remain so throughout the reproductive years in women with early menarche.11,12
Parous women have lower risk of breast cancer compared with nulliparous women, but the relationship ...