Breast Cancer in California

CHAPTER 2

INCIDENCE AND MORTALITY OF FEMALE BREAST CANCER, 1988-1993

Kurt P. Snipes, Ph.D.

Introduction

This chapter presents female breast cancer incidence and mortality data in California from 1988 through 1993, including data on the incidence of in situ, or non-invasive, breast cancer.

From 1988 to 1993, breast cancer was the leading cancer diagnosed in California women and the second leading cause of cancer-related mortality behind lung and bronchus cancer (1). A total of 103,572 cases of invasive and 13,944 cases of in situ breast cancer were diagnosed, representing approximately one of every three (30.8%) invasive cancers and more than one of every five (21.2%) in situ cancers diagnosed among women. During the same time period, 25,392 women died of breast cancer, representing more than one-sixth (17.7%) of all cancer-related mortality in California women.

Race/Ethnic Differences

Breast cancer was the most commonly diagnosed invasive cancer among women in all four race/ethnic groups examined (1). However, the risk of developing breast cancer varied by race/ethnicity. Figure 2.1 and Figure 2.2 show average annual age-adjusted invasive incidence and mortality rates in California over the five-year period 1989-1993 for all races combined and for the four largest race/ethnic groups. The counts upon which these rates are based are shown in Table 2.1.

Incidence rates of invasive breast cancer varied from a low of 60.7 new cases per 100,000 per year among Asian/Other women to a high of 117.4 among non-Hispanic white women (Fig. 2.1). Black women had the second highest age-adjusted breast cancer incidence rate (96.5 per 100,000), followed by Hispanic women (68.5 per 100,000). Despite the fact that black women had a lower incidence of breast cancer than white women, their age-adjusted mortality rate (32.4 per 100,000) was 18% higher than the mortality rate for white women (27.4 per 100,000). The rate for black women was almost double the rate for Hispanic women (17.9 per 100,000) and two and a half times the rate for Asian/Other women (12.6 per 100,000). Invasive incidence and mortality rates for Asian/Other, black, and Hispanic women were all significantly different from the comparable rates for non-Hispanic white women (p< 0.01). The higher age-adjusted incidence rate observed in white women, and the higher mortality rate reported for black women, resembles comparable data reported by the National Cancer Institute's Surveillance, Epidemiology, and End Results (SEER) Program from 1988 to 1992 (2).

Average annual age-adjusted incidence and mortality rates by race/ethnicity, for California women less than 50 years of age and 50 and older from 1989 to 1993, are shown in Table 2.2. Incidence rates were higher in black women than in white women less than 50 (36.0 per 100,000 vs. 34.7, respectively), followed by Asian/Other women (26.8 per 100,000) and Hispanic women (23.9 per 100,000). However, in women 50 and older, whites had the highest age-adjusted incidence rate per 100,000 (381.2), followed by blacks (294.2), Hispanics (221.0), and Asian/Others (182.2). SEER data from 1988 to 1992 also show a higher age-adjusted incidence rate for black women than for white women under 50 (33.8 per 100,000 vs. 32.9 per 100,000, respectively), and a higher rate for white women 50 and older compared with black women of the same age (360.6 per 100,000 vs. 291.4 per 100,000, respectively). Regardless of age group, black women in California had the highest age-adjusted mortality rate from 1989 to 1993, followed by white, Hispanic, and Asian/Other women. SEER mortality data reported for black and white women of the two age groups from 1988 to 1992 was similar to data from the CCR.

Table 2.3 and Fig. 2.3 show average annual age-specific incidence and mortality rates for invasive female breast cancer by race/ethnicity from 1989 to 1993. Age-specific incidence rates are negligible until age 20-25, at which point they begin to rise sharply with age for all four race/ethnicities. Until age 45-49, age-specific rates are highest for blacks, followed by whites, Asian/Others, and Hispanics. However, after age 50, the rates become higher for whites than for blacks, and higher for Hispanics than for Asian/Others. By age 75, the rate for white women increases to a high of 524.0 per 100,000, 63% higher than the comparable rate for black women (321.0 per 100,000), twice the rate for Hispanic women (263.0 per 100,000), and nearly two and a half times the rate for Asian/Other women (213.0 per 100,000). In general, age-specific breast cancer mortality rates are the highest for blacks for most age categories, followed by whites, Hispanics, and Asian/Others. After age 30, mortality rates for black and white women continue to increase consistently with age, as do those for Hispanics, albeit at a lower rate. After age 45-49, age-specific mortality rates for Asian/Other women do not show a consistent increase, and for women age 85 and older, the rate (44.2 per 100,000) is only about a fourth of the comparable rates for non-Hispanic black and white women (approximately 180 per 100,000).

The median age at diagnosis, or the midpoint of the age distribution of women diagnosed with breast cancer, also varied by race/ethnicity. For non-Hispanic white women, the median age at diagnosis was 65, whereas for the other race/ethnicities it was substantially younger. For black women, the median age at diagnosis was 57, for Hispanic women, 55, and for Asian/Other women, 53. The median age at diagnosis for female breast cancer in white and black women reported by SEER for 1988 to 1992 were quite similar to those reported by the CCR, i.e., 65 for white women and 58 for black women.

Differences in median age at diagnosis, however, are substantially affected by differences in the age distributions of the populations at risk. The median age of non-Hispanic whites during this time in California was 36.8, for blacks was 29.7, for Hispanics 24.6, and for Asian/Other women 31.2. For example, black women in California tend to be, as a group, younger than white women.

Geographic Variation

Table 2.4 shows female breast cancer incidence data and Table 2.5 shows mortality data from 1989 to 1993 by race/ethnicity for counties in California with five-year race-specific population sums of 200,000 or more. Age-adjusted incidence and mortality rates were only calculated for those counties in which at least fifteen cases or deaths due to breast cancer occurred. In order to identify significant differences, race-specific rates at the county level were compared to statewide rates for the same race/ethnic group. The maps in Figure 2.4 and Figure 2.5 summarize geographic variation in invasive female breast cancer rates by identifying counties with rates which were significantly higher or lower than the race-specific statewide rate. When comparing county rates, one should keep in mind that a variety of factors can contribute to variation in cancer rates other than a true difference in the risk of developing cancer. Geographic variation should therefore be interpreted with caution.

Fourteen counties in California met the criteria for calculating age-adjusted breast cancer incidence rates for Asian/Other women and eleven of them met the criteria for calculating mortality rates. Only San Mateo county had invasive breast cancer incidence rates that were significantly higher than the statewide average, and none of the other counties had rates significantly lower. Breast cancer age-adjusted mortality rates ranged from a low of 8.8 per 100,000 for Orange County to a high of 15.7 per 100,000 for Alameda County, but none of the 11 counties had rates that were significantly different from the statewide average of 12.6 per 100,000.

Eleven counties in California met the criteria for calculating breast cancer incidence rates for non-Hispanic black women, and ten of those met the criteria for calculating mortality rates. Age-adjusted incidence rates ranged from a low of 79.7 per 100,000 in Orange County to a high of 112.5 per 100,000 in San Francisco County, but none of the county rates were significantly different from the statewide average. Mortality rates ranged from a low of 24.1 per 100,000 in Santa Clara County to a high of 38.2 per 100,000 in San Francisco County, but none of these rates were significantly different from the statewide average either.

Twenty-four counties in California met the criteria for calculating age-adjusted breast cancer incidence rates among Hispanic women, and 22 of those counties also met the criteria for calculating age-adjusted mortality rates. Incidence rates were significantly higher than the comparable statewide rate (68.5 per 100,000) in Contra Costa (97.7), Alameda (88.5), Sacramento (85.0), and Santa Clara (81.7) counties, and significantly lower in Los Angeles (63.6), Fresno (56.0), and Kern (49.0) counties. Breast cancer mortality rates for Hispanic women ranged from a low of 13.8 per 100,000 in Tulare County to a high of 29.4 per 100,000 in Sacramento County, but were only significantly different from the statewide average of 17.9 per 100,000 in Sacramento County.

Forty-one counties in California met the criteria for calculating invasive breast cancer incidence and mortality rates for non-Hispanic white women. Counties with age-adjusted incidence rates significantly higher than the comparable statewide rate of 117.4 per 100,000 included Marin (132.5), San Francisco (128.2), San Mateo (125.9), Contra Costa (125.7), Orange (124.7), and Los Angeles (121.6). Nine counties reported incidence rates significantly lower than the statewide average, including Stanislaus (105.7), San Bernardino (104.9), Santa Cruz (103.3), Butte (100.8), Kern (95.6), Tulare (95.2), Madera (93.9), Merced (89.0), and Tehama (80.8). The only county with age-adjusted mortality rates significantly higher than the state as a whole was Los Angeles County (30.1 per 100,000), and this county also had significantly higher incidence rates. Counties with significantly lower mortality rates than the comparable statewide rate of 27.4 per 100,000 included Riverside (24.6), Ventura (23.6), Santa Barbara (22.3), and Stanislaus (21.5). As noted above, Stanislaus County also reported significantly lower incidence rates.

In Situ Breast Cancer

Between 1988 and 1993, 11.9% (13,944) of all breast cancers diagnosed in California women were diagnosed at the in situ stage (Table 2.6). In 1993, 13.3% of all breast cancer cases among Asian/Other women, 11.3% of cases among non-Hispanic black women, 11.1% of breast cancer diagnoses among Hispanic women, and 12.4% of breast cancer cases among non-Hispanic white women were diagnosed at the in situ stage.

Average annual age-adjusted in situ breast cancer incidence rates by race/ethnicity in California, 1989 to 1993, are shown in Table 2.6 and Figure 2.1. As was the case for invasive breast cancer, the highest age-adjusted rates are found among non-Hispanic white women (17.5 per 100,000) and non-Hispanic black women (12.0 per 100,000). When compared to non-Hispanic white women, all three other race/ethnicities had significantly lower in situ incidence rates.

Age-specific in situ incidence rates by race/ethnicity for California from 1989 to 1993 are shown in Table 2.7 and Figure 2.3. Age-specific rates are highest among non-Hispanic white women, followed by non-Hispanic black women. Age-specific rates for Hispanic and Asian/Other women are generally lower than those for black women, and are fairly equal in magnitude.

In situ female breast cancer incidence counts and average annual age-adjusted rates by county and race/ethnicity for California women, 1989-1993, are shown in Table 2.8 and Figure 2.6. Nine counties met the criteria for calculating age-adjusted rates for Asian/Other women, 7 met the criteria for non-Hispanic black women, 13 met the criteria for Hispanic women, and 41 counties met the criteria for non-Hispanic white women. In situ breast cancer rates among Asian/Other women in San Mateo County (16.8 per 100,000), Alameda County (15.6 per 100,000), Contra Costa County (15.0 per 100,000), and San Francisco County (12.8 per 100,000) were all significantly higher than the statewide rate of 8.9 per 100,000. Among non-Hispanic black women, age-adjusted in situ county incidence rates ranged from a low of 10.9 per 100,000 in Los Angeles County to a high of 20.2 per 100,000 in Solano County, but only San Francisco County (19.4 per 100,000) reported rates significantly different from the statewide average for black women of 12.0 per 100,000.

Among Hispanic women, three counties had in situ breast cancer rates significantly higher than the comparable statewide rate of 8.1 per 100,000: Sacramento County (16.2 per 100,000), San Francisco County (13.1 per 100,000), and Alameda County (12.8 per 100,000). Los Angeles County reported significantly lower rates among Hispanic women at 6.6 per 100,000. Among non-Hispanic white women, six counties recorded in situ breast cancer rates significantly higher than the comparable statewide rate (17.5 per 100,000), and five counties reported rates that were significantly lower that the statewide average. The six counties with higher rates per 100,000 white women were: San Francisco (27.2), Marin (24.7), San Mateo (24.2), Contra Costa (20.7), Santa Clara (20.6), and Orange (19.3). The five counties reporting significantly lower rates among white women were: Tulare (9.6), Kern (10.1), Stanislaus (10.5), San Bernardino (11.6), and Riverside (13.6).

Temporal Trends

Table 2.1 and Figure 2.7 show average annual age-adjusted invasive female breast cancer incidence rates from 1988 to 1993, and the average percent increase or decrease per year in the age-adjusted rate over that time period (Estimated Annual Percent Change, EAPC). The invasive breast cancer incidence rate for all races combined declined from 111.8 per 100,000 in 1988 to 105.6 per 100,000 in 1989, remained between 106 and 108 per 100,000 over the next three years, and then dropped to 102.0 in 1993. The EAPC in invasive breast cancer incidence for all races combined over that time period was slightly negative (-1.3%), but was not statistically significant. Race-specific incidence trends were also generally downward, but none were statistically significant (Fig. 2.7).

Table 2.6 and Figure 2.7 show average annual age-adjusted in situ female breast cancer incidence rates from 1988 to 1993, and associated EAPC. For all races combined, in situ rates increased from 14.0 per 100,000 in 1988 to 16.3 in 1992, and then declined to 15.1 per 100,000 in 1993. The EAPC in in situ female breast cancer incidence for all races combined was positive (3.3%), but not statistically significant. Race-specific in situ incidence trends were also generally upward, with EAPCs ranging from 2.4% for non-Hispanic whites to 8.6% for Asian/Other women. However, none of the race-specific in situ trends were statistically significant (Fig. 2.7).

Temporal trends in age-adjusted female breast cancer mortality rates from 1973 to 1994 are discussed in the following chapter. Therefore, a discussion of mortality rates is not included here.

Summary

A total of 103,572 cases of invasive and 13,944 cases of in situ female breast cancer were diagnosed in California from 1988 to 1993, representing approximately one-third of all invasive cancer diagnoses and approximately one of every five in situ diagnoses. During the same time period, 25,392 women died of breast cancer, or more than one sixth of all cancer related mortality in California women.

Average annual invasive and in situ female breast cancer age-adjusted incidence rates for 1989 to 1993 were highest among non-Hispanic white women and lowest among Asian/Other women. Average annual breast cancer mortality rates for the same time period were highest among non-Hispanic black women, and, as with incidence, lowest among Asian/Other women. Race-specific age-adjusted invasive incidence rates were also calculated and compared for two age groups, women less than 50 and 50 and older. In the younger age group, black women had the highest age-adjusted rates, followed by non-Hispanic whites, Asian/Others, and Hispanics. For women 50 and older, the same order was observed as for all ages combined, i.e., whites had the highest incidence rates, blacks the next highest, Hispanics the next highest, and Asian/Others the lowest. Black women had the highest age-adjusted breast cancer mortality rates regardless of age group, whites the next highest, then Hispanic and Asian/Other women.

In general, for women under the age of 50, age-specific incidence rates were highest for blacks and lowest for Hispanics, while for women over the age of 50, age-specific rates were highest for whites and lowest for Asian/Others, mirroring what was seen when comparing age-adjusted incidence rates for these two age groups. The median age at diagnosis for invasive breast cancer in white women was 65, for black women 57, for Hispanic women 55, and for Asian/Other women 53.

Among those counties which met the criteria for calculating race-specific age-adjusted incidence and mortality breast cancer rates, significant differences from respective statewide race-specific averages were primarily observed for non-Hispanic white and Hispanic women. Among Hispanic women, incidence rates were higher than the statewide average in Contra Costa, Alameda, Sacramento and Santa Clara counties, and mortality rates were significantly higher in Sacramento county. Among non-Hispanic white women, the urbanized counties surrounding the San Francisco Bay and Los Angeles County had incidence rates significantly higher than the state, and Los Angeles and Orange counties had age-adjusted mortality rates that were significantly higher than the state. In general, the variation observed in county age-adjusted in situ incidence rates among the race/ethnicities was similar to that noted for invasive breast cancer.

Race-specific temporal trends in invasive female breast cancer incidence were all generally downward in California from 1988 to 1993, while race-specific trends for in situ breast cancer in California during that time were all generally upward. Mortality trends are discussed in the following chapter.