Low correlation between Ki67 assessed by qRT-PCR in Oncotype Dx score and Ki67 assessed by Immunohistochemistry

Breast cancers expressing high levels of Ki67 are associated with poor outcomes. Oncotype DX test was designed for ER+/HER2− early-stage breast cancers to help adjuvant chemotherapy decision by providing a Recurrent Score (RS). RS measures the expression of 21 specific genes from tumor tissue, including Ki67. The primary aim of this study was to assess the agreement between Ki67(RNA) obtained with Oncotype DX RS and Ki67(IHC). Other objectives were to analyze the association between the event free survival (EFS) and the expression level of Ki67(RNA); and association between RS and Ki67(RNA). Herein, we report a low agreement of 0.288 by Pearson correlation coefficient test between Ki67(IHC) and Ki67(RNA) in a cohort of 98 patients with early ER+/HER2− breast cancers. Moreover, Ki67(RNA)(high) tumors were significantly associated with the occurrence of events (p = 0.03). On the other hand, we did not find any association between Ki67(IHC) and EFS (p = 0.26). We observed a low agreement between expression level of Ki67(RNA) and Ki67 protein labelling by IHC. Unlike Ki67(IHC) and independently of the RS, Ki67(RNA) could have a prognostic value. It would be interesting to better assess the prognosis and predictive value of Ki67(RNA) measured by qRT-PCR. The Ki67(RNA) in medical routine could be a good support in countries where Oncotype DX is not accessible.