Purpose: No population-based study has investigated breast cancer (BC) subtypes defined by including Ki67. The aim of this study was to evaluate the relative proportions of immunohistochemical subtypes and differences in relative and disease-free survival between subtypes, in relation to patient and other cancer characteristics in Italian BC patient.
Methods: Information on estrogen, progesterone, human epidermal growth factor (HER2), Ki67, and relapses was obtained for 3,381 cases, sampled randomly and anonymously from cases diagnosed in 2003-2005 in nine Italian cancer registries. Relative excess risks (RERs) of death and risks of relapse 5 years after diagnosis were estimated.
Results: Luminal A cancers were 42 % of the total, luminal B 27 %, luminal-HER2 14 %, triple-negative 11 %, and HER2-enriched 7 %. For non-metastatic (3,302) cases, 4 and 7 % developed locoregional and distant metastases, respectively. RERs of death and risks of relapse were significantly greater for all cancer subtypes than luminal A, particularly for triple-negative and HER2-enriched cancers, which were more frequent in women <40 years.
Conclusions: Our population-based findings confirm that subtype is an independent prognostic factor for BC. Triple-negative and HER2-enriched subtypes would benefit from the development and wide application, respectively, of targeted treatments, which would also improve survival for younger patients.