Breast Cancer Facts & Figures, 1996

How has the occurrence of breast cancer changed over time?

Incidence Trends

Overall: The incidence of breast cancer over the past 50 years shows three distinct phases:

• between 1940 and 1982, there was a steady, annual rate of increase of about 1% per year;
• between 1982 and 1987 the increase was about 4% per year;
• between 1987 and 1992, the incidence of developing breast cancer appeared to stabilize at approximately 109.6 cases per 100,000 women.

White vs. African-American Women: Breast cancer incidence rates have increased for both white and African-American women-from 84.3 per 100,000 in 1973 to 113.1 in 1992 in white women and from 68.7 to 101.0 in African-American women over the same time period. (Figure 2).

Older vs. Younger Women: Breast cancer incidence rates in white women younger than age 50 increased only slightly from 29.3 per 100,000 in 1973 to 31.7 in 1992. African-American women under age 50 also experienced an increase in breast cancer incidence from 25.8 in 1973 to 34.0 in 1992.

Tumor size and stage: Increase use of mammography has resulted in breast cancers being found earlier in their development when they are smaller and at less advanced stages.

Incidence data for the years 1980 through 1987 indicate that the sharp incidence increase that occurred during this period was restricted to localized tumors of less than 2 cm in diameter. Rates for localized tumors increased by 52% between 1982 and 1989, while rates for breast cancers diagnosed at regional or distant stages decreased 9% and 10% respectively. In 1983, 51% of all localized tumors were less than 2 cm compared with 62% in 1988.

Reasons for trends: Researchers speculated that the increase in breast cancer incidence rates since 1940 may be due to the rising prevalence of breast cancer risk factors, such as the modern reproductive patterns in women of having fewer children and delaying childbirth until a later age. Environmental factors, such as exposure to DDT and similar chemicals, may also have contributed to the increase rate. The increase and later stabilization of rates in the 1980s is believed to be related to increased use of breast cancer screening methods, particularly mammography.

Mortality Trends

Overall: Overall, breast cancer mortality has been relatively stable since 1950. However, between 1989 and 1992, breast cancer mortality decined 4.7%, the largest short-term rate decrease since 1950.

White vs. African-American Women: From 1973 to 1988, breast cancer mortality rates increased 1.1% among white women and 19.4% among African-American women (Figure 2). The recent decline in breast cancer mortality has been restricted to white women. Between 1989 and 1992, mortality rates among whites decline approximately 5.5%, while rates among African-American women increased 2.6%.

Older vs. Younger Women: Although overall breast cancer mortality rates have remained relatively stable since 1950, trends for older and younger women differ markedly. Mortality for women under the age of 50 decreased slightly between 1973 and 1992. During this period, rates for white women declined from 6.8 to 5.5 per 100,000, while rates for African-American women remained stable at approximately 9.0 per 100,000. Among women over age 50, mortality increased for African-American women and remained relatively stable for whites between 1973 and 1992. Recently rates for white women have declined, decreasing from 93.9 per 100,000 in 1989 to 89.3 per 100,000 in 1992. During this same period, rates for African Americans increased from 96.0 to 99.7 per 100,000.

Reasons for trends: The recent decline in breast cancer mortality has been attributed to improvements in breast cancer treatment, increases in breast cancer awareness and screening, and changes in the prevalence of breast cancer risk factors. As more breast cancers are diagnosed at earlier stages, researchers expect mortality rates to continue to decline.

Comments On The Breast Cancer "Epidemic":

The increasing rates of breast cancer have been described by some as representing an epidemic. Technically the increases in breast cancer incidence rates during the 1980s do not constitute an epidemic, but instead are mostly related to increases in the use of mammography.

One reason for the impression that an epidemic of breast cancer may exist is the increase in the actual numbers of women being diagnosed with the disease, a trend which largely reflects the aging of the US population. For example, from 1970 to 1990, the number of women in the population who were 20-39 years old increased substantially, leading to a subsequent increase in the actual number of breast cancers diagnosed at those ages. If age-specific breast cancer incidence rates are applied to women of these ages in the population for the years 1970, 1980, and 1990, the results are: 5,120 breast cancer cases diagnosed in 1970; 7,800 diagnosed in 1980; and 10,050 diagnosed in 1990. These increasing numbers could give the impression of an epidemic if the underlying increase in population was not considered.

Who is at risk of developing breast cancer?

Epidemiologists use the word "risk" in different ways. Lifetime risk refers to the probability that an individual, over the course of a lifetime, will be diagnosed or die from cancer. Relative risk is a measure of the strength of the relationship between risk factors and cancer. With respect to breast cancer, it compares the risk of developing breast cancer in women with a certain exposure or trait to risk in women who do not have this exposure or trait. For example, women who have a mother, sister, or daughter with breast cancer have a relative risk of about 2 compared to women with no family history of breast cancer. This means that they are about two times or 100% more likely to develop breast cancer than women who do not have a family history of the disease. As a second example, the relative risk of developing breast cancer may be about 1.3-1.8 among women who have used estrogen replacement therapy long-term compared with women who have never used this therapy. This means that estrogen users may be 30%-80% more likely to develop breast cancer than women who have never used these hormones.

Lifetime Risk

A woman in the United States has a 12.6%, or a 1 in 8, risk of developing breast cancer and a 3.6%, or 1 in 28, risk of dying from breast cancer over her lifetime. The following table examines a woman's risk in relation to her age, rather than over a lifetime:

Table 2: Age specific probabilities of developing breast cancer

American Cancer Society, Surveillance Research, 1995, Data from SEER, 1995

Risk Factors for Breast Cancer

Cancer risk factors can either be conditions that directly cause the disease (e.g., chemical or radiation exposures) or personal characteristics that are only indirectly associated with cause (e.g., age, family history, reproductive factors, etc.). Established risk factors for breast cancer include:

• Increasing age. The single most important risk factor for the development of breast cancer is age.
• Family history of breast cancer. Although any family history of breast cancer is associated with an increase in risk, the history of breast cancer in a first-degree relative (mother, sister, or daughter) is associated with the largest increase. Most family history of breast cancer probably does not represent a direct genetic link, but rather lifestyle similarities and the inheritance of selected risk factors, e.g., menstrual patterns, endogenous hormonal milieu, obesity, etc. Genetically determined breast cancers occur at younger ages, are more likely to be bilateral, and appear in multiple family members over three or more generations.
• A personal history of breast cancer. Invasive and in situ breast cancers increase the lifetime risk of developing a new breast cancer in any remaining breast tissue in either breast.
• History of benign breast disease. Biopsy-confirmed benign breast disease is associated with an increased risk of breast cancer. Several studies have suggested that the increased risks associated with benign breast disease may be confined primarily to women with diagnoses of atypical hyperplasia.
• Hormonal factors. Early age at menarche, late age at menopause, late age at first live birth, and few pregnancies all may increase a woman's risk of breast cancer by affecting her lifetime exposure to hormones. Some scientists postulate hormones may promote cell division in breast tissue and increase the risk of mutation. Oral contraceptive use may be related to breast cancer diagnosed at young ages, and long-term estrogen replacement therapy also may increase risk.

In addition, white race, high socioeconomic status, and exposure to ionizing radiation are risk factors for breast cancer.