Invasive breast cancer: Incidence rates of invasive breast cancer over the past six decades show three distinct phases:

The steady increase in incidence from 1940 to 1982 is attributed to the gradual increase in underlying risk factors for breast cancer, such as delayed childbearing and having fewer children. The more rapid increase between 1982 and 1988 is due largely to increased detection through greater use of mammography screening, with diagnosis of smaller, more easily treatable cancers than would have occurred otherwise. During this short period, incidence rates of smaller tumors (<2.0 cm) more than doubled, while rates of larger tumors (>3.0 cm) decreased 27%.3 The stabilization of rates in the 1990s probably results from the increased detection and earlier diagnosis from mammography screening between 1982 and 1988.

In situ breast cancer: Incidence rates of in situ breast cancer have increased considerably over the past 25 years. Much of this increase is due to increases in ductal carcinoma in situ (DCIS), which from 1992 to 1996 accounted for 87% of the in situ breast cancers diagnosed among women in SEER areas. Over the entire period 1973 through 1996, incidence rates of DCIS increased 7 times faster than incidence rates of invasive breast cancer:

Most cases of DCIS are detectable only by mammography, and the large increases in DCIS incidence rates since 1982 are a direct result of mammography screening practices.4 Although invasive breast cancer incidence rates have remained level since 1988, DCIS incidence rates have continued to increase. This, however, reflects a shift in the stage of disease at diagnosis toward earlier, more curable cancers rather than a true increase in occurrence.

Lobular carcinoma in situ (LCIS) is less common, accounting for 13% of female in situ breast cancers diagnosed in recent years. Between 1982 and 1988, increases in LCIS incidence rates were smaller than increases for DCIS, and since 1988, incidence rates of LCIS have remained level.

Age: Between 1973 and 1996, incidence rates of invasive breast cancer increased for women aged 40 and over (Figure 3A). Rates grew more rapidly among women aged 50 and older than for women in their 40s. Incidence rates of invasive breast cancer did not increase for women under age 40 during this time. Incidence rates of DCIS increased for women of all ages during this same time period, although rates grew fastest in women over age 5. In the late 1980s and early 1990s, the numbers of breast cancer cases in young women increased, largely due to the growth and aging of the US population, as many “baby boomer” women reached ages 25-40. However, since 1985, breast cancer incidence rates among women under age 40 have actually declined significantly at an average of –1.3% per year.

Race/Ethnicity: Between 1990 and 1996, incidence rates remained relatively unchanged in women of all racial and ethnic groups, except for American Indian/Alaska Native women.

Mortality Trends

There has been an important reduction in breast cancer death rates in recent years:

This decline in breast cancer mortality has been attributed both to improvements in breast cancer treatments and the benefits of mammography screening. As more breast cancers are diagnosed while in situ or at earlier stages of invasive disease, death rates should continue to decline. During the 1990s, death rate declines have been most notable in white women. Death rates for women under age 50 declined an average –2.2% per year between 1992 and 1996, slightly faster than the average decline for older women, –1.6% per year.

Source: American Cancer Society, Inc.