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Breast cancer is the most commonly diagnosed cancer in women in the United States (1). Accordingly, the lifetime risk of developing breast cancer is a statistic frequently cited in the press. This report presents current estimates of the risk of being diagnosed with breast cancer for California women of four different race/ethnicities: non-Hispanic Asian/Other, non-Hispanic black, Hispanic, and non-Hispanic white. Risk estimates were calculated for invasive tumors, based on statewide breast cancer incidence data from 1988 to 1992 collected by the California Cancer Registry (CCR) and mortality data collected by the Department of Health Service's Office of Vital Records and Statistics.
The risk of developing breast cancer and the risk of being diagnosed with breast cancer are used interchangeably in this report. Lifetime risk is the probability that a newborn female will be diagnosed with breast cancer sometime during her life. It is only applicable from birth and does not assume that a woman will live to reach any particular age (2). Interval risk is the probability that a woman who is cancer-free at a certain age will develop breast cancer within a specified number of years.
Risk estimates were based on age-specific incidence rates (five-year intervals), cancer-related mortality rates, and all-causes mortality rates for each race/ethnic group. Incidence rates were based on the number of first invasive primary breast cancers diagnosed from 1988 to 1992 in California. Second or subsequent primary breast cancers diagnosed during the period were excluded from the calculations. Mortality rates were based on the total number of breast cancer-related deaths or deaths due to other causes occurring during the period. Race- and age-specific population estimates were obtained from the Demographic Research Unit, California Department of Finance.
The probability of developing an invasive breast cancer at different age intervals and over a lifetime were estimated following the methodology of Feuer et al (2,3). Estimates of risk obtained with this method are adjusted for prevalent cases, and are considered more accurate than estimates obtained with previous methodologies. Calculations were based on a hypothetical cohort of 10,000,000 newborn females. During each 5-year age interval, women in this cohort were considered to be at risk for two mutually exclusive events: (i) developing breast cancer for the first time in their lives, or (ii) dying of other causes, without developing breast cancer. Women surviving the interval cancer-free became the population at risk at the beginning of the next age-interval. Probabilities of developing breast cancer or dying of other causes during each interval were derived from the age-specific cancer incidence and non cancer-related mortality rates described above. These probabilities were applied to the hypothetical cohort to generate the expected number of women developing breast cancer or dying of other causes. The risk of developing cancer within a particular age-interval was estimated as the number of expected incident cancers developing during that interval divided by the population at risk and cancer-free at the beginning of that period. Lifetime risk was estimated as the sum of all cancers in the life table divided by the initial birth cohort.< p>
The cumulative risk of developing invasive breast cancer from birth by age and race/ethnicity is presented in Table 4.1. For example, one in every 227 women in California is expected to have been diagnosed with breast cancer by age 40. By age 70, risk has accumulated so that one in every 14 women can be expected to have been diagnosed with breast cancer. Among women under 50 years of age, risk was highest for black women. However, after age 55 white women had cumulative risks that were substantially higher than that of any other race/ethnicity. The cumulative risk for either Asian/Other or Hispanic women at any age group was lower than the risk for white or black women. Similarly to what was observed with estimates for black and white women, the risk for Asian/Other and Hispanic women crossed over between 60 and 65 years of age. Up to age 60, women of Hispanic ethnicity had the lowest cumulative risk, and after age 65, the risk was lowest for Asian/Other women (Figure 4.1).
The estimated lifetime risk of developing invasive breast cancer for white women in California was 13.9%, or one in seven (Figure 4.2). In contrast, the lifetime risk for Asian/Other women (7.6%) was substantially lower, with an expected one in thirteen developing breast cancer over a lifetime. Lifetime risk for black and Hispanic women were 9.6% (one in ten) and 8.6% (one in twelve), respectively. These results are similar to lifetime risk estimates for the same period published by the National Cancer Institute's Surveillance, Epidemiology, and End Results (SEER) Program: 13.2% for white women and 9.5% for black women (1). It should be noted that the SEER Program includes both white non-Hispanic and Hispanic women in the race category of white, but the proportion of Hispanics in the SEER database is smaller than in California. Because Hispanic women have lower breast cancer incidence rates, their inclusion is likely to artificially lower SEER estimates of risk for white women.
The results described above refer to cumulative risk from birth to a specified age. Estimates of risk within the next 10, 20, 30 years or in remaining lifetime, assuming a woman is cancer-free at her current age, are shown in Table 4.2 for both California and SEER. For example, the risk of developing cancer for an Asian/Other woman in California who is now 50 years old would be 1.5%, 3.1%, and 4.8% within the next 10, 20, or 30 years, respectively. For a white woman of the same age, the risk of cancer during the same period would be much higher: 2.7% within 10 years, 6.4% within 20 years, and 10.1% within 30 years. The older the woman, the higher the risk of developing cancer in the next 10 to 30 years, reflecting the fact that incidence rates increase with age (4). However, a 50 year old cancer-free white woman has an estimated 10.1% probability of developing cancer by age 80, while a 70 year old cancer-free white woman has a 4.6% probability of developing cancer by the same age. This is because, having already survived 70 years cancer-free, the risk in her remaining lifetime will be lower than the risk for a 50 year old woman, who is still subject to the probability of developing breast cancer in the 20 additional years. For all race/ethnicities, the risk over the remaining lifetime was highest for women who were currently 30 years old, and decreased progressively with age. Estimates for women of all races combined, based on current age, are displayed in Figure 4.3.
When comparing California and SEER estimates of the risk of developing
breast cancer, it should be kept in mind that exclusions of multiple
primaries were based on very different reporting periods: five
years in California versus twenty years for SEER. The inclusion
of second or any subsequent primary tumors in the calculations
may have a substantial impact on estimates of risk (5).
Because fewer second primary tumors will be identified and excluded
over a 5-year period, risk estimates in California may be artificially
inflated relative to estimates obtained by the SEER program.
Risk estimates are projections of current rates into the future,
and therefore, the underlying assumption is that these rates will
remain constant. Such an assumption is probably not realistic.
On one hand, improvements in screening technology may inflate
incidence rates, since more tumors are detected at an earlier
stage of development. On the other hand, research may lead to
more effective prevention strategies and, therefore, to an actual
decrease in the risk of breast cancer. In addition, it has been
suggested that women are misinterpreting the lifetime risk as
being a short term risk of developing breast cancer (6).
Accordingly, interval risk estimates based on current age may
be more accurate or less prone to misconceptions and, therefore,
more meaningful to the general population.
Based on current data, one in eight California women can be expected to develop invasive breast cancer during her lifetime, an estimate which is very similar to that based on national data. However, the risk accumulates over a woman's entire life, and is not experienced each year. For example, the risk of developing invasive breast cancer for the first time in the next ten years is one in 42 for a 50 year old woman, and one in 29 for a 60 year old woman. It has been shown that many women overestimate their short-term risks (6), which may lead to unjustified fear and anxiety. Accordingly, it is important that women of different age and race/ethnic groups be accurately informed about their projected risks of developing breast cancer.
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