Combined prognostic value of age at onset and socioeconomic status for long-term survival in metastatic breast cancer

We investigated the combined long-term prognostic value of age at onset and socioeconomic status (SES) on breast cancer overall survival (OS) and metastasis-free survival (MFS) in a cohort of women with metastatic breast cancer. Age at onset was defined as the age at primary diagnosis, with a 40-year cutoff to distinguish early- and late-onset. We estimated individual SES from patients’ reported household income, occupational categories, and education levels. We estimated Kaplan‒Meier survival curves and performed regression analyses using the Cox model, including an interaction term of age at onset and SES. In 109 women included in the study, those with early-onset breast cancer had a non-significantly higher mortality rate than those with late-onset breast cancer (rate difference 0.03/PY). The hazard of metastasis was significantly lower for the medium SES (HR 0.37, 95% CI 0.19–0.69) and the high SES (HR 0.34, 95% CI 0.18–0.63) than for the low SES group. Compared to late-onset, early-onset cases had no difference in OS or MFS within the first 10 and 2 years following primary diagnosis, respectively. However, early-onset cases had a worse prognosis thereafter. Similarly, high SES was associated with better survival but only after 7.5 and 3 years following primary diagnosis for OS and MFS, respectively. We found a significant effect modification: higher SES mitigates the effect of early-onset status while it offered minimal survival benefit when age at onset was over 65 years. The combination of age at onset and SES may provide long-term prognostic information. As breast cancer survival has improved in recent decades, addressing SES-related social determinants in clinical settings has become increasingly important for optimizing long-term outcomes.