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Although mortality is only one of many important measures of the impact of breast cancer at the individual and community level, it is an important endpoint for disease surveillance in that it measures the combined effects of advances in disease prevention, screening, and treatment. This chapter examines trends in female breast cancer mortality in California over the twenty-two year period 1973 to 1994, the most recent year for which mortality data are currently available.
During this time period, the likelihood of developing breast cancer increased in California and nationwide. Data from the Northern California Cancer Center (NCCC), which has population-based cancer data from the five-county San Francisco Bay Area since 1973 collected as part of the National Cancer Institute's Surveillance, Epidemiology and End Results (SEER) Program, show that the female breast cancer incidence rate in the Bay Area was fairly stable during the early- to late-1970's. It then increased by about 35% from the late 1970's until 1987, when it appears to have peaked (1). As in the rest of the state, breast cancer incidence rates in the Bay Area since 1988 appear to have declined somewhat. Data from the SEER program, which covers about 10% of the United States population, show a similar trend in the breast cancer incidence rate, although the decline since 1987 is not as clear (2). It is likely that some of the increase in breast cancer incidence was due to more aggressive breast cancer screening. The extent to which other factors may be involved is unclear and is the subject of research as well as concern among scientists and the public.
Although statewide data on breast cancer incidence were not collected prior to 1988, information on deaths due to breast cancer are available on death certificates, which are abstracted and coded by the California Department of Health Services Office of Vital Records and Statistics.
Table 3.1and Figure 3.1 show the total number of female breast cancer deaths each year in California from 1973 to 1994 and the annual age-adjusted rate. The number of deaths increased steadily from about 3,100 in 1973 to 4,400 in 1994. However, the age-adjusted rate, which takes into account differences in the size and age-distribution of populations, changed very little from 1973 (28.4 deaths per 100,000 women) to 1985 (28.1). From 1985 to 1992, the female breast cancer mortality rate declined fairly steadily to 23.8 per 100,000, and has remained about the same for the last three years (24.3 in 1993 and 24.0 in 1994). The age-adjusted mortality rate declined by about 14% over the ten-year period 1985-1994, with a statistically significant average decrease of 2.0% per year.
Age-specific rates by ten-year interval from age 30-39 to age 80 and older are shown by year in Table 3.1 and Figure 3.2. It is apparent that the decrease in breast cancer mortality described in the above paragraph has not been experienced uniformly by women of all ages.
Breast cancer mortality among women less than 60 years old decreased by 25% to 30% over the twenty-two year period, with statistically significant decreases in the age-specific mortality rate of 2% to 4% per year from 1985 to 1994 (Table 3.2). From 1973 to 1985, the mortality rate among women 60 years old and older increased somewhat, especially among women 70 to 79 years old. Since 1985, the mortality rate among women 60 to 69 years old has declined by about 20%, with a statistically significant decrease of about 2.5% per year. The mortality rate among women 70-79 years old decreased by 10% and showed a statistically significant downward trend of about 1.5% per year since 1985, while women 80 years old and older are still showing a slight, but not statistically significant, increase in mortality.
It is difficult to examine long-term trends in mortality by race/ethnicity in California, since death certificates in this state did not consistently identify persons of Hispanic ethnicity until 1985. Race-specific trends in breast cancer mortality are shown in Table 3.3 and Figure 3.3 for the ten-year period 1985-1994. In general, lower mortality rates among Asian/Other and Hispanic women reflect lower breast cancer incidence rates in these two groups compared to black and non-Hispanic white women (3).
Over the ten-year period, non-Hispanic white women showed a fairly consistent and statistically significant decrease in breast cancer mortality of about 2.1% per year. Rates among the other three race/ethnic groups are somewhat unstable, and none of the ten-year trends are statistically significant. However, breast cancer mortality among black and Hispanic women decreased by about one percent per year on average.
Historically, breast cancer mortality rates in California have been about five percent higher than in the United States as a whole (Figure 3.4). Since the mid-1980's, however, the rates have been approaching each other, as the nationwide rate increased slightly and the California rate began to decrease. Since 1988, the breast cancer mortality rate in California has been lower than in the nation as a whole, although data for the U.S. are not available for 1993 and 1994. This is consistent with data from the NCCC which show that breast cancer incidence rates in the Bay Area were about ten percent higher than in the SEER program until very recently (1).
Despite substantial increases in the breast cancer incidence rate during the early- and mid-1980's, the breast cancer mortality rate in California and in the United States as a whole has remained fairly stable over the last two decades. In California, the age-adjusted breast cancer mortality rate decreased by a statistically significant 2% per year on average from 1985 to 1994, and was about 15% lower in 1994 than in 1973. Women of all ages except those 80 years old and older have shown statistically significant decreases in breast cancer mortality in the last ten years.
A concerted public health effort has been undertaken in the past decade to improve knowledge about the importance of, access to and utilization of breast cancer screening. While more detailed analyses would be necessary to assess the relative contribution of changes in incidence, stage at diagnosis, treatment and cohort effects to the decrease in breast cancer mortality, it is likely that the successes of this public health effort have played a key role.
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