Membranous and cytoplasmic staining of Ki67 is associated with HER2 and ER status in invasive breast carcinoma

Membranous and cytoplasmic Ki67 immunoreactivity has recently been observed in a number of histopathological entities, but frequency of occurrence and relationship to prognosis in more common cancers have not been described. The aim was to describe the pattern and frequency of membranous/cytoplasmic Ki67 in a cohort of invasive breast carcinomas, and their associations with grade, HER2 amplification and oestrogen receptor (ER) expression. Three hundred and twenty-two cases of invasive ductal carcinoma were assessed for histological grade, Ki67 (MIB-1 clone) proliferation index and pattern of immunoreactivity, ER expression by immunohistochemistry, and HER2 amplification status by fluorescence in situ hybridization. Overall, 26/322 (8%) breast carcinomas showed membranous/cytoplasmic Ki67, and expression was significantly associated with grade 3, HER2-amplified and ER- tumours. Membranous/cytoplasmic Ki67 was not, however, an independent prognostic factor on multivariate analysis. Membranous/cytoplasmic Ki67 identifies a group of breast carcinomas that may be important to consider separately in prognostic and predictive studies. The mechanism of subcellular Ki67 relocalization remains elusive and further studies are required to establish both the cause and effect of this unusual pattern of Ki67 immunoreactivity.