Racial disparities in cancer survival among randomized clinical trials patients of the Southwest Oncology Group

Abstract

Background: Racial disparities in cancer outcomes have been observed in several malignancies. However, it is unclear if survival differences persist after adjusting for clinical, demographic, and treatment variables. Our objective was to determine whether racial disparities in survival exist among patients enrolled in consecutive trials conducted by the Southwest Oncology Group (SWOG).

Methods: We identified 19 457 adult cancer patients (6676 with breast, 2699 with lung, 1244 with colon, 1429 with ovarian, and 1843 with prostate cancers; 1291 with lymphoma; 2067 with leukemia; and 2208 with multiple myeloma) who were treated on 35 SWOG randomized phase III clinical trials from October 1, 1974, through November 29, 2001. Patients were grouped according to studies of diseases with similar histology and stage. Cox regression was used to evaluate the association between race and overall survival within each disease site grouping, controlling for available prognostic factors plus education and income, which are surrogates for socioeconomic status. Median and ten-year overall survival estimates were derived by the Kaplan-Meier method. All statistical tests were two-sided.

Results: Of 19 457 patients registered, 2308 (11.9%, range = 3.9%-21.6%) were African American. After adjustment for prognostic factors, African American race was associated with increased mortality in patients with early-stage premenopausal breast cancer (hazard ratio [HR] for death = 1.41, 95% confidence interval [CI] = 1.10 to 1.82; P = .007), early-stage postmenopausal breast cancer (HR for death = 1.49, 95% CI = 1.28 to 1.73; P < .001), advanced-stage ovarian cancer (HR for death = 1.61, 95% CI = 1.18 to 2.18; P = .002), and advanced-stage prostate cancer (HR for death = 1.21, 95% CI = 1.08 to 1.37; P = .001). No statistically significant association between race and survival for lung cancer, colon cancer, lymphoma, leukemia, or myeloma was observed. Additional adjustments for socioeconomic status did not substantially change these observations. Ten-year (and median) overall survival rates for African American vs all other patients were 68% (not reached) vs 77% (not reached), respectively, for early-stage, premenopausal breast cancer; 52% (10.2 years) vs 62% (13.5 years) for early-stage, postmenopausal breast cancer; 13% (1.3 years) vs 17% (2.3 years) for advanced ovarian cancer; and 6% (2.2 years) vs 9% (2.7 years) for advanced prostate cancer.

Conclusions: African American patients with sex-specific cancers had worse survival than white patients, despite enrollment on phase III SWOG trials with uniform stage, treatment, and follow-up.

Figure 1

Kaplan–Meier overall survival plots for African American patients vs patients of all other races or ethnic groups. A) Premenopausal adjuvant breast cancer. B) Postmenopausal adjuvant breast cancer. C) Advanced ovarian cancer. D) Advanced prostate cancer. The number of patients at risk, the number of deaths, the 10-year survival estimate, and median survival estimate are presented. The Wald χ² test statistic for the effect of race on survival in the multivariable setting was also included. All statistical tests were two-sided. The 10-year overall survival estimates and the corresponding 95% confidence intervals (CIs) by race (all other patients vs African American patients, respectively) for each disease group are as follows: 77% (95% CI = 76% to 79%) vs 68% (95% CI = 62% to 75%) for premenopausal adjuvant breast cancer, 62% (95% CI = 60% to 64%) vs 52% (95% CI = 47% to 58%) for postmenopausal adjuvant breast cancer, 17% (95% CI = 14% to 19%) vs 13% (95% CI = 3% to 22%) for advanced ovarian cancer, and 9% (95% CI = 8% to 11%) vs 6% (95% CI = 3% to 8%) for advanced prostate cancer. The median overall survival estimates and the corresponding 95% confidence intervals by race (all other patients vs African American patients, respectively) for each disease group are as follows: not reached for premenopausal adjuvant breast cancer, 13.5 years (95% CI = 12.9 to 14.2 years) vs 10.2 years (95% CI = 9.1 to 12.0 years) for postmenopausal adjuvant breast cancer, 2.3 years (95% CI = 2.1 to 2.5 years) vs 1.3 years (95% CI = 1.1 to 1.8 years) for advanced ovarian cancer, and 2.7 years (95% CI = 2.5 to 2.9 years) vs 2.2 years (95% CI = 2.0 to 2.4 years) for advanced prostate cancer. Follow-up survival was truncated at 20 years for consistent presentation between panels. AA = African American.

Figure 2

Forest plot of the associations between survival and race among premenopausal and postmenopausal breast cancer patients by estrogen receptor (ER) status. A vertical line representing no effect (HR = 1) is shown. The area of each square is proportional to the sample size for the subgroup, and the length of the whiskers indicate the 95% confidence intervals. P values for interaction are shown. The Wald χ² test statistic for the effect of race on survival in the multivariate setting is also included. All statistical tests were two-sided. HR = hazard ratio; AA = African American.