Persistent area socioeconomic disparities in U.S. incidence of cervical cancer, mortality, stage, and survival, 1975–2000†
Version of Record online: 26 JUL 2004
Published 2004 by the American Cancer Society
Volume 101, Issue 5, pages 1051–1057, 1 September 2004
How to Cite
Singh, G. K., Miller, B. A., Hankey, B. F. and Edwards, B. K. (2004), Persistent area socioeconomic disparities in U.S. incidence of cervical cancer, mortality, stage, and survival, 1975–2000. Cancer, 101: 1051–1057. doi: 10.1002/cncr.20467
This article is a U.S. government work and, as such, is in the public domain in the United States of America.
- Issue online: 18 AUG 2004
- Version of Record online: 26 JUL 2004
- Manuscript Accepted: 1 JUN 2004
- Manuscript Revised: 6 APR 2004
- Manuscript Received: 26 FEB 2004
- Epidemiology and End Results program;
- cervical cancer;
- socioeconomic status;
Temporal cervical cancer incidence and mortality patterns and ethnic disparities in patient survival and stage at diagnosis in relation to socioeconomic deprivation measures have not been well studied in the United States. The current article analyzed temporal area socioeconomic inequalities in U.S. cervical cancer incidence, mortality, stage, and survival.
County and census tract poverty and education variables from the 1990 census were linked to U.S. mortality and Surveillance, Epidemiology, and End Results cancer incidence data from 1975 to 2000. Age-adjusted incidence and mortality rates and 5-year cause-specific survival rates were calculated for each socioeconomic group and differences in rates were tested for statistical significance at the 0.05 level.
Substantial area socioeconomic gradients in both incidence and mortality were observed, with inequalities in cervical cancer persisting against a backdrop of declining rates. Cervical cancer incidence and mortality rates increased with increasing poverty and decreasing education levels for the total population as well as for non-Hispanic white, black, American Indian, Asian/Pacific Islander, and Hispanic women. Patients in lower socioeconomic census tracts had significantly higher rates of late-stage cancer diagnosis and lower rates of cancer survival. Even after controlling for stage, significant differences in survival remained. The 5-year survival rate among women diagnosed with distant-stage cervical cancer was approximately 30% lower in low than in high socioeconomic census tracts.
Census-based socioeconomic measures such as area poverty and education levels could serve as important surveillance tools for monitoring temporal trends in cancer-related health inequalities and targeting interventions. Cancer 2004. Published 2004 by the American Cancer Society.
Contemporary data indicate that higher cervical cancer incidence and mortality rates are associated with lower socioeconomic status (SES).1, 2 Although several studies have examined cross-sectional patterns,3–10 temporal cervical cancer incidence and mortality patterns in relation to socioeconomic deprivation measures, especially at the national level, have been less well studied.11 The analysis of socioeconomic and ethnic patterns in stage-specific survival for cervical cancer at the national level has also not received much attention.12–14 Documenting such patterns is an important surveillance activity, both in terms of quantifying cancer-related health disparities between the least and most advantaged socioeconomic groups and for identifying population groups or areas that are at greatest risk of cancer morbidity, mortality, and poor survival and that may therefore benefit from focused social and medical interventions. Such analyses may also provide insights into the impact of cancer control interventions, such as screening for cervical carcinoma.11
Individual-level socioeconomic data are not available for patients with cancer in the National Cancer Institute's Surveillance, Epidemiology, and End Results (SEER) database.15 Reliable socioeconomic data are also lacking in national mortality databases.16, 17 Therefore, in the current study, we link two census-based area measures, the poverty rate and the percentage of population with at least a high school diploma, to the U.S. mortality data using the county of residence of the decedent and to the incidence data from 11 population-based SEER cancer registries using the county and census tract residence of the patient with cancer at the time of diagnosis.15–18 This linkage allows us to examine temporal socioeconomic patterns in cervical cancer incidence and mortality and recent cross-sectional socioeconomic patterns in incidence, mortality, stage, and survival for the total population and for major racial/ethnic groups.
MATERIALS AND METHODS
The poverty rate is defined as the percentage of population in a county or census tract below the poverty level, a threshold that varies by family size and composition ($12,674 for a family of 4 in 1990).18 Poverty rate is a measure of economic deprivation and correlates highly with other SES measures, such as educational attainment, unemployment rate, and occupational composition.11 Three categories of poverty rate in 1990 were used to classify areas: < 10% (low poverty), 10–19.99% (middle poverty), ≥ 20% (high poverty). The other SES measure used was the percentage of county or census tract population aged ≥ 25 years with at least a high school diploma in 1990.18 Three categories of education (based on quintile distribution of the U.S. population) were used to classify areas: < 69.69% (first quintile), 69.69–82.02% (second through fourth quintiles), and > 82.02% (fifth quintile). The first and fifth education quintiles represent low and high SES groups, respectively.
Because the national mortality database lacks the census tract geocode, temporal and cross-sectional analyses of mortality data involved the use of county poverty and education measures.16 The SEER database contains the county geocode from 1975 to 2000 and the census tract geocode from 1988 to 2000.15 The incidence trend analyses from 1975 to 2000 involved the use of county socioeconomic variables. However, for cross-sectional patterns in incidence during 1988–1992 and for stage and survival analyses, the census tract socioeconomic variables were used.
Incidence and mortality rates for each socioeconomic group were age adjusted by the direct method using the age composition of the 2000 U.S. standard population and 5-year age-specific incidence and mortality rates.11, 15 Five-year cause-specific survival rates were computed by treating patients dying of causes other than cervical cancer as censored observations.11, 19 Differences in rates were tested for statistical significance at the 0.05 level.
Figure 1 presents temporal socioeconomic patterns in SEER incidence and U.S. mortality rates from 1975 to 2000. Although U.S. cervical cancer mortality rates decreased consistently for all area poverty groups between 1975 and 2000, the socioeconomic gradients in mortality did not diminish during this period (Fig. 1B). Rather, the gradient (the ratio of mortality rates for high and low poverty counties) appeared to have increased slightly, from 1.62 (95% confidence interval [CI] = 1.49–1.75) in 1975 to 1.83 (95% CI = 1.66–2.00) in 2000. This is primarily because of a faster rate of mortality decline for low poverty areas than for high poverty areas. The average annual exponential rates of mortality decline for low, medium, and high poverty counties were 2.55% (95% CI = 2.32–2.77%), 2.28% (95% CI = 2.06–2.49%), and 1.88% (95% CI = 1.60–2.16%), respectively. Temporal patterns in mortality by county education were almost identical to those for poverty (Fig. 1D). The average annual rates of mortality decline for low, medium, and high education counties were 2.02% (95% CI = 1.76–2.27%), 2.32% (95% CI = 2.11–2.52%), and 2.61% (95% CI = 2.36–2.86%), respectively.
The SEER cervical cancer incidence rates also showed a downward trend for all county poverty groups during 1975–2000 (Fig. 1A). However, a substantial gradient remained, with women in high poverty counties having at least a one-third higher incidence rate than those in low poverty counties throughout the study period. The socioeconomic gradients in incidence (the ratio of incidence rates for high and low poverty counties) appeared to have decreased somewhat over time, from 1.77 (95% CI = 1.55–1.99) in 1975–1977 to 1.37 (95% CI = 1.18–1.55) in 1998–2000. Although the average annual decreases for high poverty counties were similar for incidence and mortality trends, the rate of decrease for low poverty counties was greater for mortality than for incidence trend. The average annual exponential rates of decline in incidence for low, medium, and high poverty counties were 1.30% (95% CI = 1.06–1.54%), 2.05% (95% CI = 1.70–2.40%), and 2.10% (95% CI = 1.86–2.34%), respectively. Similar temporal patterns can be noted by county education (Fig. 1C), with annual rates of decline in incidence for low, medium, and high education counties being 2.10% (95% CI = 1.88–2.33%), 1.61% (95% CI = 1.35–1.87%), and 1.47% (95% CI = 1.16–1.77%), respectively.
U.S. cervical cancer mortality generally increased with increasing poverty and decreasing education levels for women in all racial/ethnic groups (Fig. 2B,D). During 1996–2000, American Indian women in low SES counties had more than twice the mortality of their counterparts in high SES counties. The rates were, respectively, 46% (95% CI = 37–55%), 49% (95% CI = 34–63%), and 82% (95% CI = 51–113%) higher for non-Hispanic white, black, and Hispanic women in high than in low poverty counties. The mortality rates were, respectively, 62% (95% CI = 53–72%), 74% (95% CI = 53–96%), and 49% (95% CI = 20–77%) higher for non-Hispanic white, black, and Hispanic women in low than in high education counties.
The higher the census tract SES, the lower the cervical cancer incidence during 1988–1992 (Fig. 2A,C). Compared with the rates for their counterparts in low poverty census tracts, the incidence rates for non-Hispanic white, black, American Indian, Asian Pacific Islander, and Hispanic women were, respectively, 97% (95% CI = 80–114%), 30% (95% CI = 12–49%), 292% (95% CI = 31–615%), 44% (95% CI = 17–71%), and 83% (95% CI = 62–105%) higher in high poverty census tracts. Patterns by education were similar, with the incidence rates being almost two times higher for non-Hispanic whites and Hispanics and four times higher for American Indians in low than in high education census tracts.
Women were 20% (95% CI = 1.00–1.40%) more likely to be diagnosed with a distant-stage cervical cancer in high than in low poverty census tracts, with the impact of poverty on late-stage diagnosis being most pronounced for Hispanics (Fig. 3A). Educational gradients in distant-stage cervical cancer diagnoses were less consistent, although educational patterns in localized and regional-stage cancers were similar to those for poverty (Fig. 3B). Among women diagnosed with invasive cervical cancer between 1988 and 1994, the 5-year survival rates were 79.1% (95% CI = 78.0–80.2%), 75.6% (95% CI = 73.9–77.2%), and 72.6% (95% CI = 70.9–74.3%) in low, medium, and high poverty census tracts, respectively. The 5-year survival rates were 73.1% (95% CI = 70.2–76.0%), 76.6% (95% CI = 73.5–79.6%), and 80.3% (95% CI = 77.2–82.2%) in low, medium, and high education census tracts, respectively. Even after controlling for stage, significant differences remained (Fig. 4). For example, among women diagnosed with distant-stage cervical cancer, the 5-year survival rate was 31% (95% CI = 18.1–43.4%) lower in high than in low poverty census tracts and 30% (95% CI = 17.2–42.2%) lower in low than in high education census tracts.
By analyzing population-based SEER incidence and U.S. mortality data, the current study has shown substantial socioeconomic disparities in cervical cancer, which have persisted over time against a backdrop of declining incidence and mortality rates. The significant association between lower SES and higher incidence and mortality rates and lower likelihoods of survival and early-stage diagnoses was generally observed for each racial/ethnic group, although the magnitude of the association varied by ethnicity. These patterns are consistent with the literature.4–8, 11, 20–24
We used only three broad socioeconomic categories for a simpler presentation of data and for minimizing potential misclassification of areas over time, but the impact of poverty and education on cervical cancer may be graded across the entire range of the social hierarchy.7, 11 The 1980 poverty and education variables are more likely than the corresponding 1990 variables to accurately characterize the socioeconomic characteristics of counties during 1975–1984 due to its temporal proximity. However, because the 1980 and 1990 county socioeconomic variables were highly correlated (γ = 0.91 [poverty] and 0.94 [education] for the United States; γ = 0.90 [poverty] and 0.82 [education] for the SEER region), the cervical cancer incidence and mortality trends based on the 1980 SES variables were essentially similar to those based on the 1990 variables (data not shown), suggesting very little, if any, misclassification of areas over time.
Our findings may be affected by possible ethnic misclassification in patient medical records and death certificates and by any incorrect geocoding of patients to specific census tracts.25–29 Results could also be biased if the socioeconomic category associated with residence at the time of diagnosis or death differed from that at exposure. Despite these potential limitations, census-based area measures, e.g., poverty and education levels, can serve as important surveillance tools for monitoring temporal trends in cancer-related health inequalities, as well as in cancer control planning and health resource allocation.
- 1Socioeconomic differentials in cancer incidence and mortality. IARC Sci Publ. 1997; 138: 65–176., , , .
- 11Area socioeconomic variations in U. S. cancer incidence, mortality, stage, treatment, and survival, 1975-2000. NCI Cancer Surveillance Monograph Series, No.4. Bethesda, MD: National Cancer Institute, 2003, NIH Publ. No. 03-5417., , , .
- 12Socioeconomic differences in cancer survival: a review of the evidence. IARC Sci Publ. 1997; 138: 177–206., .
- 13Possible explanations for social class differences in cancer patient survival. IARC Sci Publ. 1997; 138: 377–397., .
- 15RiesLAG, EisnerMP, KosaryCL, et al., editors. SEER cancer statistics review, 1975-2000. Bethesda, MD: National Cancer Institute; 2003. Available from URL: http://seer.cancer.gov/csr/1975_2000 [accessed 2004].
- 16National Center for Health Statistics. Vital statistics of the United States, 1950-2000, vol. II: mortality, parts A and B. Washington, DC: Public Health Service, 2002.
- 17Deaths: final data for 2000. National Vital Stat Rep. 2002; 50: 1–120., , , , .
- 18U.S. Bureau of the Census. Census of population and housing, 1990: summary tape file 3A on CD-ROM. Washington, DC: U.S. Bureau of the Census, 1992.
- 22Implementing recommendations for early detection of breast and cervical cancer among low-income women. MMWR Morb Mortal Wkly Rep. 1997; 46: 1–31., , , .
- 28Quality of death rates by race and Hispanic origin: a summary of current research. Vital Health Stat. 1999; 2: 1–13., , , et al.