Analytical validity of CD20 as a biomarker in breast cancer patients using tissue microarrays and whole slide sections.

BACKGROUND: Clinical utility of biomarker-based treatment depends on both analytical and clinical validity. CD20+ B-lymphocytes often form nodular lymphoid structures within tumors, presenting potential analytical issues if using tissue microarrays (TMA). We compared CD20 expression in TMAs and whole-slide sections (WS) using digital image analysis. MATERIALS AND METHODS: Immunohistochemical CD20+ expression was assessed on 1 TMA core/patient and corresponding WS from 221 breast cancer patients. CD20+ area fractions were quantified within four spatial tissue compartments: (1) tumor epithelium; (2) tumor margin; (3) tumor-related stroma; (4) total tumor area. Bland-Altman analysis and t-test compared CD20+ area fractions of WS and TMA. RESULTS: CD20+ cells were predominantly located in the stroma and often formed nodules. Across all compartments, the absolute area fraction of CD20+ was significantly lower in TMAs than in WS. Mean ratios of CD20+ area fractions (WS vs. TMA) ranged from 2.5 (95% confidence interval: 1.7-3.6; p < 0.001) to 6.4 (4.7-8.9; p < 0.001) within the four spatial regions. CONCLUSIONS: These findings indicate that TMAs may underestimate the presence of CD20+ cells in breast cancer, presumably due to the formation of nodules. This heterogeneity in distribution is important to consider when exploring the use of CD20 as a potential clinical biomarker.