Abstract S4-6: An international Ki67 reproducibility study
Authors: Nielsen TO , Polley M-YC , Leung SCY , Mastropasqua MG , Zabaglo LA , Bartlett JMS , Viale G , McShane LM , Hayes DF , Dowsett M ,   the International Ki67 in Breast Cancer Working Group of the BIG-NABCG collaboration
Published: August 2014

Introduction: In breast cancer, immunohistochemical assessment of the cell proliferation marker Ki67 is of interest for potential use in clinical management (e.g. prognosis, prediction of treatment response, monitoring the effect of neoadjuvant therapy). However, lack of consistency across labs has limited the value of Ki67. A working group was assembled to devise a strategy to harmonize Ki67 analysis and scoring and to identify procedures to improve concordance (Dowsett et al. JNCI,'11). As a result, we have conducted an international Ki67 reproducibility study.

Methods: Step 1: 100 breast cancer cases arranged into 1 mm core tissue microarrays (TMAs) were centrally stained using MIB-1 antibody. Eight labs scored Ki67 as the percentage of positively stained invasive tumor cells using their own local method. Six labs repeated scoring of 50 cases on the same TMA 3 times. Ki67 data were log2-transformed to approximate a normal distribution. Pairwise intra-lab and inter-lab concordance was displayed using Bland-Altman plots. Sources of variation (e.g. patient and lab) were analyzed using two-way crossed random effects models with quantification of reproducibility via intraclass correlation coefficient (ICC; range of 0–1, 1 = highest agreement).

Step 2: A web-based scoring calibration interface was created to mitigate systematic differences in Ki67 interpretation observed in Step 1. Digital images of TMA cores from 9 “training” and 9 “test” cases representing a range of Ki67 values were assembled onto the web-based interface. An initial set of observers yielding consistent scores using a standardized scoring method served as reference labs. Detailed instructions for the standardized method were provided to 8 other labs, who were asked to score the cases and learn from discrepancies observed via the web tool. Statistical criteria for passing were pre-specified.

Results: In Step 1, intra-lab reproducibility was high (ICC = 0.94). Inter-lab reproducibility was only moderate (ICC = 0.71). Absolute mean Ki67 values across the series ranged from 7% to 24%. Contributing to inter-lab discordance were tumor region selection, formal counting of nuclei versus visual estimation, and subjective assessment of staining positivity. Labs using formal counting methods gave more consistent results than those using visual estimation. The calibration exercise (Step 2) helped to improve consistency: all 8 labs passed the training exercise, 4 at first attempt and 4 after 1–2 rounds of retraining. When the same 8 labs scored the 9 test calibration cases, 3 passed on first attempt. Although overall concordance improved, discrepancies for cases with low Ki67 were largely responsible for the failure of the remaining 5 labs to pass the testing.

Conclusions: Absolute values and cutpoints for Ki67, and associated clinical decisions, cannot be transferred between laboratories without careful standardization of scoring methodology. Our calibration training tool offers an initial process to improve inter-lab concordance, but further studies are required to establish what concordance can be achieved in practice. A large subsequent study will assess whether a pre-specified target of success (ICC = 0.9) can be achieved on glass slides after training with the web-based calibration tools developed as part of this study

Citation Information: Cancer Res 2012;72(24 Suppl):Abstract nr S4-6.