Interpathologists discrepancies in Ki67 assessment in the PACS01 trial: An independent prognosis factor.

543 Background: Several reports suggest an inter-observer variability in Ki67 assessment. Nevertheless, there is no large study that evaluates the rate of discrepancies, together with their impact. Methods: Ki67 expression was assessed on 663 samples from patients with ER-positive breast cancers included in the PACS01 trial (Roche, J Clin Oncol, 2006). Ki67 staining was done using MiB1 antibody (Dako, Copenhagen, Denmark, Dilution 1:250). Prognostic and predictive values have been reported previously (Penault-Llorca, J Clin Oncol, 2009). A second central review was done by a senior breast pathologist from a French Cancer center. A discrepancy was defined as either a false positive or false negative result. Cut-off for positivity was defined at 15% according to data from Cheang et al (JNCI, 2009). Results: The rate of discrepancy was correlated with the percentage of stained tumor cells. A 10% discrepancy rate between the 2 pathologists was observed when the first pathologist reported <10% tumor cells stai...