RESUMO
BACKGROUND: Young breast cancer patients have a poorer prognosis, especially when their tumors are hormone receptor positive. We analyzed the association between Ki67 and age and the impact of these factors on outcomes in hormone receptor-positive breast cancer. METHODS: The records of 9,321 hormone receptor-positive invasive breast cancer patients from three large centers were retrospectively reviewed. Each institution separately assayed Ki67 level immunohistochemically. Univariate and multivariate analysis for recurrence-free survival (RFS) was performed on 4,738 patients from a single center. RESULTS: Ki67 level was inversely proportional to age in all three data sets and was significantly higher for younger patients (p < 0.001, 0.03, and <0.001, respectively). This correlation was seen only in the human epidermal growth factor receptor 2 (HER2)-negative population. Survival analysis showed that both very young age (<35 years) and high Ki67 level (≥10 %) were independent prognostic factors. Although young age was a worse prognostic indicator regardless of HER2 status, Ki67 index was associated with worse prognosis only in HER2-negative patients. When patients were stratified into those with low and high Ki67, young age remained a significant factor for RFS, with hazard ratios in these two Ki67 groups of 2.15 and 2.57, respectively (p < 0.001). Also, the young age/low Ki67 group had significantly poorer RFS than the older age/high Ki67 group (p < 0.001). CONCLUSIONS: Ki67 level was higher in younger patients. However, very young patients had a poorer prognosis regardless of Ki67 level. Unknown biologic factors other than high cell proliferation might play a role in the aggressiveness of hormone receptor-positive breast cancer in very young patients.