Comparative Effectiveness of Chemotherapy Regimens in Prolonging Survival for Two Large Population-Based Cohorts of Elderly Adults with Breast and Colon Cancer in 1992-2009

Abstract

Objectives: To compare the effectiveness of chemotherapy in prolonging survival according to age in breast and colon cancer.

Design: Retrospective cohort study with a matched cohort analysis based on the conditional probability of receiving chemotherapy.

Setting: The 16 Surveillance, Epidemiology, and End Results (SEER) areas from the SEER-Medicare linked database.

Participants: Women diagnosed with Stage I to IIIa hormone receptor-negative breast cancer (n = 14,440) and 26,893 men and women with Stage III colon cancer (n = 26,893) aged 65 and older in 1992 to 2009.

Measurements: The main exposure was the receipt of chemotherapy, and the main outcome was mortality.

Results: In women with breast cancer aged 65 to 69, the risk of all-cause mortality was statistically significantly lower in those who received chemotherapy than in those who did not in the entire cohort (hazard ratio (HR) = 0.70, 95% confidence interval (CI) = 0.57-0.88) and in a propensity-matched cohort (HR = 0.82, 95% CI = 0.70-0.96) after adjusting for measured confounders. These patterns were similar in participants aged 70 to 74 and 75 to 79, but in women aged 80 to 84 and 85 to 89, risk of all-cause mortality was no longer significantly lower in those receiving chemotherapy in the entire and matched cohorts, except that, in a small number of women who received doxorubicin (Adriamycin) and cyclophosphamide (Cytoxan), risk of mortality was significantly lower for those aged 80 to 84. Chemotherapy appeared to be effective in all ages from 65 through 84 in participants with Stage III colon cancer. For example, in those aged 85 to 89, chemotherapy was significantly associated with lower risk of mortality in the entire cohort (HR = 0.79, 95% CI = 0.67-0.92) and the matched cohort (HR = 0.79, 95% CI = 0.66-0.95).

Conclusion: The effectiveness of chemotherapy decreased with age in participants with breast cancer, in whom chemotherapy appears to be effective until age 79 except for the doxorubicin-cyclophosphamide combination, which was effective in participants aged 80 to 84. In individuals with Stage III colon cancer, chemotherapy appears to be effective to age 89. These findings were consistent with those of randomized clinical trials.

Keywords: breast cancer; chemotherapy; colon cancer; effectiveness; survival.