The Ki67 labelling index (LI - proportion of staining cells) is widely used to reflect proliferation in breast carcinomas. Several cut-off values have been suggested to distinguish between tumours with low and high proliferative activity. The aim of the current study was to evaluate the distribution of Ki67 LIs in breast carcinomas diagnosed at different institutions by different pathologists using the method reflecting their daily practice. Pathologists using Ki67 were asked to provide data (including the LI, type of the specimen, receptor status, grade) on 100 consecutively stained cases, as well as details of their evaluation. A full dataset of 1709 carcinomas was collected from 19 departments. The median Ki67 LI was 17% for all tumours and 14% for oestrogen receptor-positive and HER2-negative carcinomas. Tumours with higher mitotic counts were associated with higher Ki67 LIs. Ki67 LIs tended to cluster around values ending with 5 or 0 both in cases where the values were obtained by counting the proportion of stained tumour cell nuclei and those where the values were obtained by estimation. On the basis of the distribution pattern described, some currently used Ki67 LI cut off values are not realistic, and it is proposed to select more realistic values ending with 0 or 5.
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