To identify new causes of breast cancer, epidemiological studies should fully characterize and, ideally, maximize the distribution of candidate risk factors in the population under study. With this goal in mind, we examined recent breast cancer incidence characteristics and trends over a 17-year period in one of the highest-risk populations in the breast cancer literature: non-Hispanic white women in the SFBA. We found that rates of invasive breast cancer increased from 1988 to approximately 1999 and decreased thereafter in women 40 years old or older. From 2000–2001 to 2003–2004, decreases in invasive breast cancer rates also occurred in women with all histologic subtypes, tumor sizes, hormone receptor-defined tumors, and localized and regional disease. A recent analysis in SEER also found decreases in tumors less than 2 cm, hormone receptor-defined tumors, and local and regional disease from 1999/2000 to 2003 [13
]. Although similar trends were observed among non-Hispanic white women in the rest of California in our study, incidence and mortality rates of breast cancer were consistently higher in the SFBA than in the rest of California or in other SEER regions across all time periods. Even after the recent declines in 2003–2004, incidence rates of breast cancer in situ
and invasive breast cancer were 12.5% and 4.2% higher, respectively, as compared with 16.7% and 5.5% higher in 2000–2001, among SFBA women than among women in the rest of California. Our age-specific incidence trends are similar to recent reports of declines in US [12
] and German [11
Contemporaneous population-based data on breast cancer risk factors from CHIS and case-control studies provide further evidence that in SFBA non-Hispanic white women, there is a higher prevalence of certain breast cancer risk factors [30
], including advanced education, lower BMI among women younger than 50 years old, nulliparity, late age at first birth, use of estrogen plus progestin HT in 2001, and alcohol consumption, as compared with non-Hispanic white women living in other parts of California. Other breast cancer risk factors, including use of combined estrogen and progestin HT in 2003, physical inactivity, and obesity in women 50 years old or older, were less common in SFBA women or were similar in SFBA women and women in the rest of California and are therefore unlikely to have contributed to the higher incidence rates of breast cancer in the SFBA than in the rest of California.
The most notable risk factor changes we observed from 2001 to 2003 were the 76% and 72% relative decreases in the percentages of women reporting use of combined estrogen and progestin HT in the SFBA and the rest of California, respectively. Other risk factors did not appear to change substantially during this time period, but education level and mammographic screening history did increase modestly. Decreases in HT use are comparable to those noted in our recent report of a 68% decrease in the use of HT among middle-aged Northern California women after 2002 [10
] and are consistent with findings in the US [12
] and Germany [11
]. Increasing HT use from 1994 to approximately 1999, the plateau in use from 1999 to 2001 following the 1998 release of findings from the Heart and Estrogen/Progestin Replacement Study [39
], and the dramatic decrease in Northern California [10
] after the 2002 WHI findings [14
] closely mirror the trends we observed in both invasive breast cancer and breast cancer in situ
incidence in the SFBA and the rest of California. This pattern, in addition to the decreases in ER+
tumors, though limited by the high percentage of missing values, further supports the notion of a strong influence of the population prevalence of HT use on breast cancer incidence patterns.
It is unclear to what extent mammographic screening patterns, which have been associated with breast cancer incidence increases in the US (particularly in the late 1980s and early 1990s [30
]), explain the elevated incidence rates in the SFBA. Our observations of higher incidence rates of breast cancer in situ
, detected exclusively by mammography, and excess rates of localized and regional disease or tumors less than 2 cm, as well as excesses in women targeted by mammography screening guidelines (40 years old or older), suggest that the SFBA excess could be due in part to higher levels of screening, a finding supported by CHIS data that find a somewhat higher prevalence of mammographic screening in 2001, but not 2003, in the SFBA. The continued assessment of future trends in incidence rates will help us to understand whether a plateau in mammography screening [13
] is playing a role in the observed trends.
Some, but not all [7
], prior studies using ecologic and cohort study designs have found that sociodemographic characteristics [5
] and risk factor distributions [4
] explained the higher incidence rates of breast cancer in the SFBA compared with other regions. However, without information on residential mobility, these studies could not address the reasons why high-risk populations concentrate in certain geographic areas. Demographic change in the US appears to be favoring the migration of educated workers into certain geographic areas, including the SFBA, out-migration of less-educated persons to other parts of the country, and migration of service workers to suburbs on the periphery of the educated urban cores [43
]. These patterns of migration are supported by 2000 to 2004 census data that list San Francisco and San Jose among the slowest-growing metropolitan areas and that list metropolitan areas outside the Greater SFBA as some of the fastest-growing [44
]. The extent to which these migration patterns concentrate women with multiple established breast cancer risk factors in particular areas over time may help explain past and future breast cancer incidence trends. The breast cancer incidence patterns observed in SFBA women may be representative of patterns occurring in subpopulations with high breast cancer incidence, but for whom routine surveillance is challenging, such as women residing in West Los Angeles [45
], women of high socioeconomic status [46
], or female teachers in California [47
]. Cancer surveillance efforts are further limited by the lack of population counts defined by individual characteristics, such as educational attainment within small geographic areas, that are necessary for estimation of incidence rates stratified by these characteristics. Therefore, non-Hispanic white women in the SFBA, where surveillance is ongoing, may serve as bellwethers for cancer trends occurring in similar subpopulations living in more heterogeneous areas.
A limitation of this analysis is that the data are ecological. That is, the data are available only at a geographic (that is, county) level rather than at the individual level. In addition, we did not adjust breast cancer incidence rates for known risk factors, nor did we have population prevalence estimates of risk factor changes over the 17-year study period or in the rest of the SEER regions. We were able to present risk factor changes between variables measured similarly in the 2001 and 2003 CHISs, but these data are limited by low response rates that could result in selection bias. However, response rates in CHIS were comparable to those in other population-based surveys [29
], and response rates among the population-based controls included in the present analysis, though not directly comparable to CHIS, were higher. Even with these limitations, the ability to examine the prevalence of established breast cancer risk factors from two population-based sources allowed us to compare the prevalence of breast cancer risk factors in SFBA non-Hispanic white women with similar women in the rest of California. Furthermore, our data provided sufficient power for examining incidence trends by age, histologic subtype, stage at diagnosis, and hormone receptor status.