Endometrial Cancer Survival Among U.S. Black and White Women by Birth Cohort
Cook, Linda S.*†; Kmet, Leanne M.*; Magliocco, Anthony M.‡§; Weiss, Noel S.¶∥
From the *Department of Community Health Sciences, University of Calgary, Calgary, Alberta, Canada; †Division of Population Health and Information, Alberta Cancer Board, Calgary, Alberta, Canada; ‡Department of Pathology and Laboratory Medicine, University of Calgary, Calgary, Alberta, Canada; §Department of Oncology, University of Calgary, Calgary, Alberta, Canada; ¶Division of Public Health Services, Fred Hutchinson Cancer Research Center, Seattle, Washington; and ∥Department of Epidemiology, University of Washington, Seattle, Washington.
Submitted 27 October 2005; accepted 3 February 2006.
Supported by the Peter Lougheed New Investigator Research Award (awarded to LSC), Canadian Institutes of Health Research.
Correspondence: Linda S. Cook, Department of Community Health Sciences, University of Calgary, 3330 Hospital Drive N. W., Calgary, AB T2N 4N1 Canada. E-mail: firstname.lastname@example.org.
Background: Endometrial cancer incidence is lower but mortality is higher among black relative to white women. This disparity results from a relatively higher level of case-fatality in African Americans. We investigated whether the inter-racial difference in survival has diminished in more recent birth cohorts.
Methods: Women diagnosed with endometrial malignancies during 1977–1996 were identified from the U.S. Surveillance, Epidemiology and End Results program (45,261 white and 1986 black women). We calculated cumulative 5-year relative survival for 6 birth cohorts.
Results: Survival was relatively higher in younger women than older women and in women diagnosed in earlier years compared with more recent years. Higher survival was evident in more recent birth cohorts for groups of women 60+ years of age and in all time periods, particularly for black women.
Conclusions: Although black women with endometrial cancer have a poorer prognosis than white women, the inter-racial difference in survival has narrowed in more recent birth cohorts.
Age-adjusted endometrial cancer incidence among white women in the United States is twice that of black women, yet age-adjusted mortality from uterine cancer among black women is twice that of white women.1–21 Mortality rates have decreased for both black and white women over the last 25 years,22 but inferences regarding possible trends in inter-racial survival are hindered by racial differences in incidence trends during this period. We sought to determine whether there has been an improvement in relative survival for blacks diagnosed with endometrial cancer in more recent birth cohorts beyond that experienced by whites.
Data were from 9 registries in the Surveillance, Epidemiology and End Results (SEER) Program.23 (See Table 1 footnote for complete list.) We identified women diagnosed with a first primary cancer of the uterine corpus or the uterus, not otherwise specified, between 1977 and 1996.23 Because follow-up data were available through the end of 2001, 1996 was the last diagnosis year to ensure a minimum 5-year follow-up period. Of the 53,225 white and 3182 black women identified, women were excluded based on the following criteria: diagnosis via autopsy/death certificate (n = 305 and n = 24, respectively); no microscopic confirmation (376 and 41); International Classification of Diseases for Oncology (ICD-O)24,25 site codes of C54.2 (myometrium) and C54.8 (overlapping lesion of the corpus uteri; 200 and 27); nonglandular histologies (4641 and 822); clear cell or serous adenocarcinoma histologies (because survival among women with these tumors is known to be lower than survival among women with more common endometrial adenocarcinomas; 366 and 196)19; age less than 40 years at diagnosis (1076 and 86).
Thus, the final analysis included 45,261 white and 1986 black women whose tumors were classified by ICD-O-3 codes of 8140–8143, 8200, 8210, 8260–8263, 8323, 8340, 8380, 8440, 8470, 8480, 8481, 8560, 8570, 8571, or 8573. Vital status 5 years after diagnosis was available for 99.2% of white and 98.9% of black women. Women lost to follow-up before 5 years were censored at the last known follow-up year.
Cumulative 5-year relative survival was calculated by comparing the ratio of the observed overall survival of the cases to the expected survival of women in the general population who were similar to the cases with respect to age, race and time period of diagnosis.22,23 All deaths, irrespective of cause, were included. The major advantage of relative survival in population-based studies is that cause of death information is not required. Relative survival provides a measure of excess mortality in endometrial cancer cases beyond that experienced by their counterparts in the general population.26 Relative survival by birth cohort also was examined by age group and by time period of diagnosis. The relatively small number of black women precluded simultaneous stratifications by birth cohort, age group at diagnosis and time period. Results for strata with fewer than 25 cases available for follow-up were not reported due to unstable survival estimates, nor were corresponding estimates in the other racial group.
During 1977–1996, 5-year relative survival was greater in younger than in older women (Table 1; Fig. 1), in more recent years than in earlier ones (Table 1; Figs. 2 and 3), and particularly in more recent birth cohorts (Table 1; Figs. 1 and 3). Relative survival improved in both black and white women over successive birth cohorts (29% to 79% for black and 63% to 95% for white women, P for both trends <0.001), but absolute improvement was more pronounced for black women (Table 1). The association of survival with birth cohort was evident in women 60 years of age and older (Fig. 1) and in all time periods (Fig. 3). The disparity in five-year relative survival between black and white women was substantially smaller in more recent birth cohorts than in earlier ones (Table 1; Figs. 1 and 3).
Although black women consistently experienced poorer survival relative to white women after a diagnosis of the most common forms of endometrial cancer, substantial absolute improvements in 5-year relative survival were noted for black women over successive birth cohorts beyond those experienced by white women.
Relative survival was better in younger women than in older women (Table 1). The earliest cohort (1880–1899) included a high proportion of women who were older at diagnosis, whereas the most recent cohort (1940–1956) included a high proportion who were younger at diagnosis. Thus, the higher relative survival among women in recent birth cohorts partially reflects the differing ages at diagnosis across birth cohorts. However, when we stratified the data by age at diagnosis (Fig. 1), we continued to see increases in survival for similarly aged women in more recent cohorts, at least among women 60 years of age and older.
Earlier detection, even without changes in the natural history of the disease, can lead to a calculated increase in survival despite no real improvement in prognosis.27 Although the SEER program does collect information on stage of disease at diagnosis, the likely incomparability of the assessment of stage between races especially over time, limits the usefulness of this information for identifying changes in stage at diagnosis that could influence prognosis. However, age-adjusted mortality from endometrial cancer has decreased for both black and white women over the time period of our study,22 suggesting that the increases in survival that we observed among women in recent birth cohorts are indicative of some additional lives having been saved in these women.
Prior studies have noted poorer survival after endometrial cancer among black women relative to white women, whether this was assessed by overall survival,5,7,11,18,19 relative survival,1,19,21 or cause-specific survival.3,17 In most studies that adjusted for various other factors such as age, disease stage, tumor grade/histology, treatment and income, survival differences diminished but were not eliminated,6,9,11,18,28 but in others race was no longer predictive of survival after such adjustments.12,15,16 To our knowledge, no other study has investigated endometrial cancer survival by birth cohort for white and black women.
In conclusion, our results indicate that relative survival is increasing over successive birth cohorts in black women diagnosed with the most common types of endometrial cancer. Nonetheless, a disparity in survival between white and black women remains, and the basis for this disparity continues to be elusive.
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© 2006 Lippincott Williams & Wilkins, Inc.
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