Inclusion of minorities is an important but challenging aspect of epidemiologic studies in the United States. One aspect of this challenge that has received little attention is the actual number of minorities with specific cancers. The authors aimed to understand how population characteristics affect the numbers of minority cancer cases in Surveillance, Epidemiology, and End Results (SEER) regions.
By using SEER data, the authors identified 6 cancers with higher incidence rates in racial and ethnic minorities and reviewed the annual number of cases of those cancers in SEER areas where there are large numbers of blacks, Hispanics, and Asians. The authors examined the age characteristics of the populations in SEER areas using data from the US Census.
Although there are substantial numbers of cases for the most common cancers with higher incidence in blacks, their numbers are quite small for other cancers, <150 cases, and in many areas, <100 per year. Few registries have substantial numbers of Hispanics or Asians. As expected, the proportion of minority populations is lower in older age groups, whereas the proportion of non-Hispanic whites is larger.
Epidemiologists attempt to include racial and ethnic minorities in their studies to obtain a study population that reflects the source population. For a more complete understanding of the sources of health disparities, we are particularly concerned with including minorities in studies of those diseases that have higher incidence rates in minorities. In an earlier study of pancreatic cancer in New York City in which we had relatively few blacks enrolled, we investigated the actual numbers of black cases in the area, which we found to be surprisingly small in light of the large minority population in this area and the higher incidence of pancreatic cancer in blacks. This investigation was expanded to other cancers that have higher age-adjusted incidence rates in blacks and other minorities and to Surveillance, Epidemiology, and End Results (SEER) registry areas to gain an understanding of how age adjustment and age distribution affect the practical aspects of including minority groups in epidemiologic studies. We focused on SEER areas because epidemiologic studies are frequently conducted in these geographically defined areas and because source population data are available.
MATERIALS AND METHODS
By using data from the American Cancer Society,1 we identified the most common cancers in the United States in 2004, choosing this year as the midpoint of the period for which recent data on numbers of cases were available (2002-2006) from the SEER Cancer Statistics Review 1975-2006.2 We limited our analysis to 14 cancers with at least 20,000 cases in total. These cancers (and the numbers diagnosed in 2004, in thousands) were prostate (230), lung/bronchus (173), breast (215), colorectal (147), bladder (60), non-Hodgkin lymphoma (54), melanoma (55), kidney/renal pelvis (35), uterine corpus (40), pancreas (31), oral cavity and pharynx (28), ovary (25), thyroid (23), and stomach (22). We then considered cancers with higher age-adjusted incidence3 in minorities. Compared with non-Hispanic whites, age-adjusted incidence in the 17 SEER registries was higher in blacks for 6 of these 14 cancers; the 6 cancers were prostate, lung/bronchus, colorectal, kidney/renal pelvis, pancreas, and stomach. Stomach cancer was the only one with higher incidence in Hispanics and Asians. For American Indian/Alaska Natives (AI/AN), the incidence of both stomach cancer and kidney/renal pelvis cancer was higher than in non-Hispanic whites.
Inclusion of Specific Minorities in Individual SEER Registries
Because the size of the minority populations in each SEER registry varies considerably, when reporting numbers of cases in minority groups, we included data from any given registry only for those minorities for which the total population of that minority was at least 100,000 in 2000.4 We reasoned that researchers concerned about including a particular minority would be most likely to use registries where there is a substantial number of that minority group in the population.
Incidence Rates, Numbers of Cases, and Median Age at Diagnosis
Age-adjusted incidence rates and numbers of cases for minorities and non-Hispanic whites for the years 2002-2006 for the selected cancers in each registry were obtained from SEER*Stat.3 For AI/AN, data on incidence rates and total numbers of cases came from the Contract Health Service Delivery Areas (CHSDA), as often reported by SEER. We included cases with earlier primary cancers, those without microscopic confirmation, and those diagnosed at autopsy or on a death certificate. The San Francisco/Oakland and San Jose/Monterey registries were combined to compose the Greater Bay area, as defined by SEER. SEER*Stat was also used to estimate the median age at diagnosis3 for each racial and ethnic group by cancer site. We reported data for whites excluding Hispanics, whereas the category “Hispanic” is not mutually exclusive of blacks, Asian/Pacific Islanders, or American Indians/Alaska Natives. Identification of an individual as “Hispanic” is based on the North American Association of Central Cancer Registries (NAACCR) Hispanic Identification Algorithm (NHIA).5
We used intercensal and postcensal population estimates from the National Center for Health Statistics (NCHS) and the Census Bureau6 to describe the number and percentage of the population that is non-Hispanic white, black, Hispanic, Asian, and AI/AN in 5-year age groups from 20-24 years through ≥85 years in 2004 in combined SEER registries. For the SEER registries covering areas other than states, we combined census data from the counties included (shown in footnote to Table 1). To quantify the changes in the proportion of each racial/ethnic group, we calculated the ratio of the proportion in the oldest age group to the proportion in the youngest age group for the SEER registries as a whole and for individual registries with the largest numbers of blacks, Hispanics, and Asians.
Table 1. Age-Adjusted Incidence Rates per 100,000 for Selected Cancers in Total and Individual SEER Registries by Race-Ethnicity (2002-2006)
Lung & Bronchus
Kidney & Renal Pelvis
Statistic could not be calculated.
County definitions for SEER areas other than states are Los Angeles: Los Angeles County; Greater Bay Area: Alameda, Contra Costa, Marin, San Francisco, San Mateo, Monterey, San Benito, Santa Clara, and Santa Cruz counties; Greater California: remaining counties minus Los Angeles County, and the Greater Bay Area; Seattle-Puget Sound: Clallam, Grays Harbor, Island, Jefferson, King, Kitsap, Mason, Pierce, San Juan, Skagit, Snohomish, Thurston, and Whatcom counties; Detroit Metro Area: Macomb, Oakland, and Wayne counties; Atlanta Metro Area: Clayton, Cobb, DeKalb, Fulton, and Gwinnett counties; Rural Georgia: Glascock, Greene, Hancock, Jasper, Jefferson, Morgan, Putnam, Taliaferro, Warren, and Washington counties.
17 SEER sites include Alaska Native Registry, Atlanta, California (excluding San Francisco/San Jose-Monterey and Los Angeles), Connecticut, Detroit, Hawaii, Iowa, Kentucky, Los Angeles, Louisiana, New Jersey, New Mexico, Rural Georgia, San Francisco/San Jose-Monterey, Seattle-Puget Sound, and Utah. Data for American Indian/Alaska Natives come from CHSDA.
Table 1 shows age-adjusted incidence rates for the 6 cancers with higher overall incidence rates in blacks, Hispanics, Asians, or AI/AN than in non-Hispanic whites in the 17 SEER areas in total and, except for AI/AN, in the individual SEER areas. Individual areas are ordered by population size from largest to smallest. For several registries, particularly the smaller ones, statistics are based on <16 cases or cannot be calculated. Among blacks, the overall pattern of higher incidence compared with non-Hispanic whites is consistent for most of the cancers, with some exceptions, particularly in the smaller registries and for lung cancer and kidney/renal pelvis cancer. Among Hispanics and Asians, incidence of stomach cancer is higher than in non-Hispanic whites in all registries with adequate statistics.
Table 2 shows the average annual number of patients with the 6 cancers for which the age-adjusted incidence is higher in blacks in each of the 10 registries with black population ≥100,000. For non-Hispanic whites, there are ≥150 cases of all cancers studied in these 10 registries, with the exception of pancreatic and stomach cancers in Atlanta. Among blacks, for the more common cancers (prostate, lung/bronchus, colorectal), there are ≥150 cases in most of the registries studied, with the exception of Seattle, and for colorectal cancer, Connecticut. For the less common cancers (kidney/renal pelvis, pancreas, and stomach), there are <150 black cases in each of the 10 registries, with the exception of kidney/renal pelvis and pancreas cancers in Louisiana. In 7 of the 10 registries, there are <100 cases per year for some or all of these less common cancers. Table 3 shows the number of non-Hispanic whites, Hispanics, and Asians with stomach cancer in each registry studied. Only Greater California and Los Angeles have ≥150 Hispanics with stomach cancer, and only the 3 California registries have ≥150 Asians with stomach cancer per year. For AI/AN, the total number of cases in CHSDA with these cancers is 600 prostate, 788 lung, 789 colorectal, 349 kidney, 170 pancreas, and 209 stomach (not shown in tables).
Table 2. Average Annual Number of Incident Cases of Selected Cancers for Blacks and Non-Hispanic Whites in Individual SEER Areas with Black Population ≥100,000 (2002-2006)
Lung & Bronchus
Kidney & Renal Pelvis
Greater Bay Area
Detroit Metro Area
Atlanta Metro Area
Table 3. Average Number of Incident Cases of Stomach Cancer by Race/Ethnicity in Individual SEER Areas with ≥100,000 Hispanics or Asians (2002-2006)
NA indicates not applicable because there were <100,000 of that race/ethnic group in the registry area.
Greater Bay Area
Detroit Metro Area
Atlanta Metro Area
Table 4 contains data on the median age at diagnosis across the racial/ethnic groups studied. Among non-Hispanic whites, the median age at diagnosis ranges from 66 years (for kidney/renal pelvis cancer) to 73 years (for pancreas and stomach cancers), whereas blacks are diagnosed at younger ages (range, 61-68 years) for each of these cancers. For stomach cancer, Hispanics are diagnosed at a median age of 66 years, whereas Asians are diagnosed at a median age of 70 years. AI/AN are diagnosed with stomach and kidney cancer at younger ages than the other groups, at 64 years and 60 years, respectively.
Table 4. Median Age at Diagnosis for Selected Cancers by Race-Ethnicity (2002-2006)
American Indian/ Alaska Native
Lung & bronchus
Kidney & renal pelvis
For the combined SEER registries, Figure 1 shows the number of individuals in each of the racial/ethnic groups in 5-year age categories from ages 20-24 years through ages >85 years. It can be seen that the number of individuals is generally smaller in each successive age group, particularly at ages ≥50 years. The increase in non-Hispanic whites between the ages of 25 and 49 years reflects births in the United States from the late 1940s to the 1970s,7 the period of the post-World War II baby boom. Figure 2 shows the percentage of each racial/ethnic group in the age categories 20-24 years through >85 years in the SEER areas. Across age groups, there is a clear increase in the proportion of the population that is non-Hispanic white, whereas Hispanics show a decline. Blacks and Asians have similar proportions in the population and similar changes in relation to age group; the proportion who are AI/AN is small overall. To quantify the changes between age groups, we calculated the ratio of the proportion in the oldest age group to that in the youngest age group for each racial/ethnic group (Table 5). For non-Hispanic whites, the proportion in the oldest age group is considerably higher than in the youngest group, with a ratio of 1.59:1. For the other groups, the ratios of oldest age group to youngest are lower with 0.67:1 for Asians, 0.46:1 for blacks, 0.26:1 for Hispanics, and 0.25:1 for AI/AN. Examples of different population structures among registries are also illustrated in Table 5. For example, for blacks in Louisiana and Atlanta, the ratios of oldest age group to youngest are somewhat higher (0.62:1 and 0.53:1) than in New Jersey (0.37:1). For Hispanics, the ratio is slightly higher in Los Angeles than in Greater California and the Greater Bay Area. For Asians, the results from Los Angeles show relatively little change across age groups (ratio of oldest to youngest, 0.86:1), whereas in Hawaii, there is a higher proportion of Asians in the oldest group than in the youngest, with a ratio of 1.33:1 (data not shown in table).
Table 5. Proportion of Each Racial/Ethnic Group, Overall and in Selected Registries, in the Youngest (20-24 Years) and Oldest (≥85 Years) Age Groups
Percentage of Total Population Aged 20-24 y in SEER Areas
Percentage of Total Population Aged ≥85 y in SEER Areas
Our examination of SEER and census data shows that, even with higher incidence rates in minorities, the actual number of minority patients in SEER registry areas for all but the most common cancers is usually quite small. This is related to the population structure of the United States, reflected in the SEER registry areas, with minority groups having a relatively small proportion of the total population in older age groups likely to be affected by cancer. Comparing ratios of the proportions in oldest to youngest age groups, we noted the largest differences in population composition for Hispanics and AI/AN, intermediate changes for blacks, and smaller changes for Asian. We also noted differences among SEER areas for blacks and Asians. The reasons for differences in population structure vary for the ethnic groups; the relatively smaller numbers of blacks and AI/AN in older age groups are related at least in part to shorter life expectancies,8 whereas those noted for Hispanics are more likely related to their recent immigration to the United States.9 The smaller differences for Asians, compared with blacks and Hispanics, may represent longer life expectancy for Asians.8 Age standardization is necessary for comparison of incidence rates across populations with different age structures; however, because minorities represent relatively small proportions of the population, particularly in older age groups, age adjustment can distort expectations of the actual number of patients. Because we did not exclude cases with earlier primary cancers, without microscopic confirmation, or diagnosed at autopsy or by death certificate, the incidence rates and numbers of cases reported are maximized, and studies with these exclusions would have even smaller numbers of eligible minority cases. With the exception of kidney/renal cell cancer, incidence of the cancers we studied has declined or remained stable in most racial/ethnic groups in recent years.10
Population-based approaches using cancer registries for case ascertainment can readily identify minority cases, although locating Hispanics may not be straightforward.11 Minority cases may be more difficult both to contact and to recruit into studies.11-13 Hospital-based studies at large centers allow for ready identification of and contact with cases, but there may be small minority-patient populations because of hospitals' locations, insurance policies, or factors related to patients' socioeconomic status.
It continues to be important to recruit minorities into epidemiologic studies to provide generalizable results and to explore differences in risk factors and outcomes. Investigators should make use of successful strategies that have been identified12-19 and continue to develop and refine these strategies, while recognizing the limitations imposed, for some cancers, by absolute numbers of cases.