Abstract P5-02-04: Tumor heterogeneity impairs robustness of Ki67 scoring in breast cancer

Abstract

Abstract Background A study by Cheang et al. reported that for ER-positive, HER2-negative breast cancer a Ki67 score ≥14% distinguishes Luminal B from Luminal A tumors. The (neo)adjuvant treatment of these Luminal B tumors consists of endocrine treatment and chemotherapy while chemotherapy can be omitted for Luminal A tumors. For the determination of these subtypes, resection specimens and particularly core biopsies in the neoadjuvant setting are used. Since Ki67 scoring is advocated by some to be a marker in deciding if systemic chemotherapy will be given or not, its assessment should be highly reproducible and reliable. I.e, Ki67 assessment should not vary significantly between different laboratories, but also not within the tumor. To test this, we analyzed (1) interlaboratory agreement using whole slides and (2) agreement within a particular tumor using tissue microarrays (TMA) as well as whole slides from surgical specimens. Material and methods The first 100 patients from the microarRAy prognoSTics-in-breast-cancER (RASTER) study (n=427) were selected. From each patient clinicopathological characteristics and tumor blocks were available. From each tumor block 2 whole slides were cut for staining in laboratory 1 and 2. Also six single cores were taken to construct 2 tissue microarrays (core 1-3 and core 4-6). Intrinsic subtypes were defined as follows: Luminal A, Luminal B/HER2-, Luminal B/HER2+, HER2-overexpressing, and Basal-like. Experienced pathologists (EG, JS, VTHMBS, JW) performed the scoring. Results There were 99 whole slides suitable for analysis. Substantial agreement (κ = 0.723, P<0.001) was demonstrated in discrimination for Ki67 low and high between whole slides stained in laboratory 1 and laboratory 2 with a discordance rate of 13.1% (Table 1). Moderate agreement (κ = 0.595, κ = 0.584, P<0.001) was demonstrated in discrimination of Ki67 low and high between whole slide and core 1-3 and core 4-6 with a discordance rate of 20.3% and 20.8% respectively (Table 2). Substantial agreement (κ = 0.666) was demonstrated for Ki67 expression between core 1-3 and core 4-6. Table 1. Discordance between laboratories for whole slide Ki67 result Laboratory 2 <14%≥14%Discordance rate %KappaSign.Laboratory 1<14%56013.10.723<0.001 ≥14%1330 Table 2. Discordance between whole slide and core biopsies 1-3 and 4-6 for Ki67 result Whole slide <14%≥14%Discordance rate %KappaSign.Core 1-3<14%33920.30.595<0.001 ≥14%730 Core 4-6<14%31820.80.584<0.001 ≥14%830 Conclusion Tumor heterogeneity results in substantial variation of Ki67 scores between TMA cores and whole slides that may result in considerable differences in distinguishing luminal A from luminal B ER-positive, HER2-negative tumors. This may have far reaching consequences for the choice for (neo)adjuvant treatment. Citation Format: Matthijs V Nijenhuis, Emilie Groen, Tim J Dekker, Caroline A Drukker, J Sanders, V T Smit, S Linn, E J Rutgers, J Wesseling. Tumor heterogeneity impairs robustness of Ki67 scoring in breast cancer [abstract]. In: Proceedings of the Thirty-Seventh Annual CTRC-AACR San Antonio Breast Cancer Symposium: 2014 Dec 9-13; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2015;75(9 Suppl):Abstract nr P5-02-04.