An “epidemic” is a sudden increase in the number of new cases of a disease, usually measured by incidence rates. The incidence rate is calculated by dividing the number of new cases over a specific period of time, usually a calendar year, by the population in the geographic area during the same time period. The rates examined are usually either for a specific age group, or are standardized to the age distribution of a chosen year (age-adjusted).
The California Cancer Registry began collecting statewide cancer information in 1988, so assessing long-term statewide trends in incidence is not possible. However, cancer data has been collected in the Bay Area since 1973 as part of the National Cancer Institute’s cancer registry, called the Surveillance, Epidemiology, and End Results (SEER) program.
As shown above, the age-adjusted breast cancer incidence rate in the Bay Area increased by about 35 percent during the early- to mid- 1980s, and peaked in 1987. Since 1988, incidence rates have been fairly stable, both in the Bay Area and statewide. Very similar trends are seen in data from the SEER program, which covers about 14 percent of the US population.
No one is certain why breast cancer rates increased during the 1980s. We know that more and more women were being screened for breast cancer during that time, and that some of the cancers diagnosed then would have been discovered at a later date, in the absence of screening. Thus, improvements in screening may have made it appear, temporarily, that rates were increasing. It is also known that the proportion of women at increased risk for breast cancer due to delayed childbearing and smaller family size was increasing. The possibility that increases in other or unknown risk factors played a role is the subject of ongoing research.
Nonetheless, the public and many physicians have the impression that they are seeing more and more cases of breast cancer, especially among women under the age of 50. And although it may seem contradictory, both things are true -- the number of cases among relatively young women is increasing, while the incidence rate is fairly constant, or in some age groups, even decreasing somewhat.
This graph compares the number of new cases diagnosed (the bars) and the corresponding incidence rate (the line) among women ages 45 to 49. The number of new cases diagnosed each year increased by more than 50 percent over this relatively short period, from about 1,560 cases in 1988 to 2,400 cases in 1996. This means that for every two women diagnosed with breast cancer in this age group in 1988, three were diagnosed in 1996. However, the incidence rate, the number of new cases adjusted for population size, was about the same in 1988 (214 per 100,000) as in 1996 (217 per 100,000). The number of cases increased without a corresponding increase in the incidence rate because the number of women ages 45 to 49 increased rapidly each year as baby boomers matured and reached the age where breast cancer becomes more common.
Among 40 to 44 year olds, the number of cases increased by about 25 percent while the incidence rate decreased by 5 percent. Among 35 to 39 year olds, the number of cases increased by five percent while the incidence rate decreased by 15 percent.
Another reason that breast cancer makes a deep impression on both the public and health care professionals is that breast cancer strikes at a younger age than many other cancers. This graph compares the cumulative number of breast and prostate cancers diagnosed over the five-year period 1991-1995 by age category. “Cumulative” means that the number of cases in a given age group is added to all cases diagnosed at an earlier age, so the number increases as you move from younger to older ages, and the final bar on the right (85+) shows the total number of cancers diagnosed.
Over this five-year period, about the same number of prostate and female breast cancers were diagnosed, about 100,000 cases in each. However, about 25 percent of breast cancers were diagnosed among women less than 50 years old, compared to only one percent of prostate cancers. For every one prostate cancer diagnosed among men under the age of 50, nearly 30 breast cancers were diagnosed among women, despite the fact that the total number of cases among men and women was almost identical. The fact that breast cancer disproportionately affects the relatively young may also contribute to the impression that breast cancer is reaching epidemic proportions.
In sum, data from the CCR indicate that while breast cancer is a serious concern, there is no evidence of an ongoing epidemic. Incidence rates did increase markedly during the 1980s, but have been stable since the CCR was implemented in 1988.
