Associations between age and chemotherapy dose reductions in women with stage I-IIIA breast cancer.

[BACKGROUND] Older women (>65 years) diagnosed with breast cancer may be at risk for chemotherapy dose reductions. We evaluated associations of age at diagnosis with 2 measures of chemotherapy dose reductions: first cycle dose proportion (FCDP) < 90% and average relative dose intensity (ARDI) < 90%.

[METHODS] From the Optimal Breast Cancer Chemotherapy Dosing study, we included 10 166 women aged 18+ years treated with adjuvant chemotherapy for stage I-IIIA breast cancer at Kaiser Permanente Northern California (KPNC) and Washington (KPWA) between 2004 and 2019. We examined associations between age at diagnosis with FCDP < 90% (reflecting clinician intent at chemotherapy initiation) and ARDI < 90% (reflecting average dose across the chemotherapy course). We used generalized linear models of the Poisson family with a log-link function and robust standard errors to calculate prevalence ratios (PR) for FCDP < 90% and ARDI < 90% with 95% confidence intervals (CI) adjusted for patient and tumor characteristics, with and without adjusting for pre-existing comorbidities. All tests for statistical significance were 2-sided.

[RESULTS] The proportion of women with FCDP < 90% ranged from 2.9% among women aged 18-39 years to 18.6% among women aged 75+ years. Before adjusting for comorbidities, women aged 75+ years were more likely to have FCDP < 90% (PR = 4.88; 95% CI = 3.58 to 6.66) and ARDI < 90% (PR = 1.91; 95% CI = 1.58 to 2.32) versus women aged 40-49 years. Results were similar after adjusting for comorbidities as a composite comorbidity score or individual comorbidities.

[CONCLUSION] Older age at diagnosis was strongly associated with chemotherapy dose reductions in this population-based cohort, particularly at chemotherapy initiation but also across the course of treatment.