Affiliations of authors: Department of Preventive Medicine, Keck School of Medicine of the University of Southern California (USC), and USC/Norris Comprehensive Cancer Center, Los Angeles, CA.
Correspondence to: Lihua Liu, Ph.D., Cancer Surveillance Program, Department of Preventive Medicine, Keck School of Medicine, University of Southern California, 1540 Alcazar St., CHP-204, Los Angeles, CA 90033 (e-mail: lihualiu{at}hsc.usc.edu).
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ABSTRACT |
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INTRODUCTION |
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A number of studies have examined patterns of SES among prostate cancer patients. However, the methodologic approaches of these studies have varied, and they produced inconsistent results, including reports of positive associations (510), negative associations (11,12), and no associations (1323) between SES and prostate cancer incidence. This inconsistency in results is likely to reflect the difficulty of obtaining an objective and reliable measurement of SES for cancer incidence data.
One factor that could, in theory, affect the relationship between SES and prostate cancer incidence in recent years is the prostate-specific antigen (PSA) blood test. In 1986, the U.S. Food and Drug Administration (FDA) approved the PSA test as a method to monitor the progression of prostate cancer. The potential utility of PSA testing as a cancer-screening tool was quickly recognized and led to its widespread use in early detection programs. The nationwide incidence of prostate cancer subsequently increased dramatically (24).
We were particularly interested to know if the relationship between SES and prostate cancer incidence would change with the advent of the PSA test. We have used data from the Los Angeles Cancer Surveillance Program (CSP), the population-based cancer registry for Los Angeles County, CA, to examine the relationship between SES and prostate cancer incidence among men who were diagnosed with prostate cancer during the period from 1972 through 1997.
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SUBJECTS AND METHODS |
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The CSP collected information on all male residents of Los Angeles County who were diagnosed with primary invasive prostate cancer during the period from 1972 through 1997 (n = 83 068). We examined this information for the date of cancer diagnosis, the race/ethnicity and the SES of the affected individual, and the stage of the disease at diagnosis to evaluate patterns of prostate cancer incidence among different subpopulations in Los Angeles County. Prostate cancer patients were classified into five mutually exclusive racial/ethnic groups (white, black, Hispanic, Asian, and "other") on the basis of information obtained from their medical records and by the comparison of their surnames with those contained on the U.S. Bureau of the Census 1980 Spanish surname lists. We used the same criteria employed by the CSP to establish the race/ethnicity of white men who were diagnosed with prostate cancer: Only those men whose surnames appeared on the list of Spanish surnames were classified as Hispanic. Black men were not further distinguished by Hispanic origin or Spanish surnames. The Asian subgroup consisted of Chinese, Japanese, Korean, and Filipino men. The remaining prostate cancer patients, who included American Indians, Pacific Islanders, and other very small groups of Southeast Asians, were grouped together in the "other" category of race/ethnicity. The racial/ethnic composition of the prostate cancer patients in Los Angeles County from 1972 through 1997 was 72.0% white, 14.1% black, 9.9% Hispanic, 3.3% Asian, and 0.7% "other." We excluded the men comprising the latter subgroup (n = 546) from further analyses because it was heterogeneous and contained less than 1% of the prostate cancer patients in Los Angeles County.
Measurement of SES.
We previously developed a method for assigning an SES classification to each cancer patient in the CSP database by using census tract-specific population census data on median household income and educational attainment for adults aged 25 years and older who resided in each census tract at the time of the 1970 census, the 1980 census, and the 1990 census (25). Census tracts in the county were ranked separately by each of these two characteristics for each census year. As a result, each census tract had two ranking scores, one relating to income and one relating to educational level, from each census. These two ranking scores were equally weighted and summed for each census tract for each census year to yield a composite ranking score that defined SES. All census tracts in Los Angeles County for each census year were divided into quintiles according to this composite ranking score. The top quintile of census tracts has the highest composite ranking score for SES and thus represents the group of men with the highest SES (SES1). Likewise, the bottom quintile of census tracts includes men with the lowest SES (SES5). The division of census tracts into quintiles does not result in an equal distribution of population of Los Angeles County by quintiles. The classification of census tracts by SES was irrespective of race/ethnicity. Consequently, the population distribution by SES also varies by race/ethnicity and does not necessarily represent quintile distribution. Each prostate cancer patient in our study was assigned the SES classification of the census tract in which he lived at the time of his diagnosis. Men who were diagnosed with prostate cancer during the period from 1972 through 1974 were assigned SES values derived from the 1970 census; men who were diagnosed with prostate cancer during the period from 1975 through 1984 were assigned SES values derived from the 1980 census; men who were diagnosed with prostate cancer in 1985 and later years were assigned SES values derived from the 1990 census. Because this method for assigning an SES classification requires that the patient's address at diagnosis be geographically coded into a census tract, we further excluded from our analysis 1818 prostate cancer patients (2.2%) whose addresses could not be geographically coded.
Identification of stage of disease at prostate cancer diagnosis.
Complete information on the stage of disease at prostate cancer diagnosis was obtained from the CSP for men who were diagnosed in 1977 or later. Because the information on the stage of disease at prostate cancer diagnosis for men who were diagnosed during the period from 1972 through 1976 was incomplete, we did not use it in these analyses. The stage of prostate cancer at diagnosis was classified according to the Summary Staging Guide of the National Cancer Institute's Surveillance, Epidemiology, and End Results (SEER) Program1 (26). Prostate cancer was classified as localized if it was confined to the prostate gland and had a tumor stage designation of T1 or T2; as regional if it extended beyond the prostate and had a tumor stage of T3, T4, or N1; as distant if it had metastasized and had a tumor stage designation of M1; or as unknown if insufficient information was available to assign a tumor stage.
Population estimates.
We obtained annual estimates of the Los Angeles County population classified by sex, age, race/ethnicity, and SES for the intercensal years of 1972 through 1979 and 1981 through 1989 by linearly interpolating results obtained from the 1970 census, the 1980 census, and the 1990 census. We based our population estimates for 1991 through 1997 on the 1990 census results and the age-, sex-, and race-specific annual population growth rates estimated by the Department of Finance of the State of California (27).
Statistical analysis.
We calculated the age-adjusted incidence rates for prostate cancer by race/ethnicity, SES, and stage of disease over time, using the sexagerace/ethnicitySES-specific annual population estimates as the denominators. Rates were standardized to the 1970 U.S. population. We performed a linear regression analysis to test for trends in the age-adjusted incidence rates that were associated with increasing levels of SES. All P values were two-sided.
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RESULTS |
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DISCUSSION |
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Our data support a familiar social phenomenon; i.e., individuals with greater resources are more likely to benefit from improvements in the detection of disease, in treatments of disease, or in knowledge about disease risks than individuals with limited resources, who are disadvantaged with respect to access, quality, and utilization of medical services (28,29). Despite having higher prostate cancer incidence rates, men of higher SES have a better prostate cancer survival than men of lower SES (10,30), almost certainly due in large part to the early detection provided by PSA testing.
It is not clear if the widespread use of the PSA test and the dramatic increase in prostate cancer incidence rate have contributed directly to any changes in prostate cancer mortality. Prostate cancer mortality increased nationwide during the late 1980s. That trend decreased substantially from 1991 through 1995 (24), even though the magnitude of the decline in mortality during this period was small when compared with the dramatic increase in prostate cancer incidence. The effect of PSA screening on prostate cancer mortality is not known (24,3133). However, in Los Angeles County, the age-adjusted prostate cancer mortality rate was relatively stable from 1988 through 1997 among different racial/ethnic populations; i.e., no statistically significant changes were observed (our unpublished data, available upon request). These findings imply that there has been no effect of PSA screening on prostate cancer mortality in Los Angeles County. Whether differential PSA screening by SES will eventually affect mortality in these groups differently is a matter of conjecture at this point.
Whether or not PSA testing affects prostate cancer mortality, the widespread use of PSA testing has resulted not only in the early detection of prostate cancer but also in the discovery of a high proportion of latent and non-life-threatening tumors (34). Screening has been an important means of controlling other cancers (e.g., breast cancer and cervical cancer). However, the specific benefits of screening for prostate cancer are uncertain (35,36). The relationships between the benefits of PSA testing and the potentials for overdiagnosis and overtreatment resulting from PSA testing need to be carefully evaluated.
To our knowledge, this is the first study to examine the relationship between SES and prostate cancer incidence with respect to the time periods before and after PSA testing became widely available. Our measurement of SES, in contrast with measurements used in previous studies (523) on the relationship between SES and prostate cancer incidence, was based on a composite index that considered the combined impact of education and incometwo important indicators of SES. The capability of capturing the changing SES gradient in the incidence of prostate cancer among diverse racial/ethnic populations demonstrates the usefulness and the sensitivity of our SES measurement.
In conclusion, our data suggest that SES is an important factor that has a substantial impact on prostate cancer diagnosis and secular trends in prostate cancer incidence.
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NOTES |
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Cancer incidence data were collected under a subcontract with the Public Health Institute, Berkeley, CA, which manages the regional contracts for the California Cancer Registry. The subcontract is supported by the California Department of Health Services as part of its statewide cancer-reporting program, as mandated by Health and Safety Code Sections 103875 and 103885. Support was also provided by Public Health Service contract N01CN67010 from the NCI, National Institutes of Health, Department of Health and Human Services.
The ideas and opinions expressed herein are those of the authors, and no endorsement by the State of California, Department of Health Services, or the Public Health Institute is intended or should be inferred.
We thank Frances Wang for programming assistance.
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Manuscript received August 16, 2000; revised February 13, 2001; accepted February 26, 2001.
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