Abstract 677: H-score as a diagnostic tool in HER2 equivocal (IHC 2+) breast cancer

Abstract Introduction: Human epidermal growth factor 2 receptor (HER2) overexpression occurs in 15-20% of primary invasive breast cancers, significantly contributing to their aggressive nature and poor prognosis [1]. HER2 status is typically assessed using immunohistochemistry (IHC) and fluorescence in situ hybridization (FISH). IHC results classified as 2+ are considered equivocal, leading to a reliance on FISH for confirmation. However, FISH is costlier and time-consuming, which can delay necessary treatment [2]. This study aims to assess the potential of the histochemical score (H-score), a quantitative measure derived from IHC staining and calculated using ImageJ software, as a reliable tool for classifying HER2 status in cases with equivocal IHC 2+ results. By investigating whether the H-score can accurately predict HER2 status, this study explores the possibility of reducing reliance on FISH for confirmation. Methods: This retrospective study included adult patients (≥18 years) diagnosed with HER2 IHC 2+ breast cancer from January 2014 to December 2023. Demographic and pathologic data were collected from medical records, and diagnostic biopsy slides were reviewed. Images of IHC specimens were captured at ×10 magnification, standardized, and analyzed using ImageJ software. The H-score was calculated using mean staining intensity and stained area percentage. Binary logistic regression evaluated the predictive power of the continuous H-score on HER2 status by FISH. Results: The mean H-score of 182 patients was 2.65, with HER2-positive cases showing a higher mean score (3.06) than HER2-negative cases (2.62). ROC analysis yielded an AUC of 0.668 (95% CI 0.523 to 0.812), indicating poor predictive accuracy. Moreover, when calculating the Youden index, the optimal H-score cutoff score (2.6007) provided moderate sensitivity (0.769) but limited specificity (0.580). These results suggest that while the H-score may provide some predictive value, it has limited accuracy for classifying HER2 status independently. Summary: The H-score lacks accuracy to reliably distinguish HER2 status (positive from negative) in IHC 2+ cases. The moderate AUC and risk of false positives emphasize the need to continue using FISH analysis for confirmation. While moderately correlated with HER2 status, the H-score does not meet clinical precision standards, making FISH essential for accurate classification. Future research should aim to refine the H-score or identify new markers to improve predictive accuracy in equivocal cases. Citation Format: Tasnem Alsebai, Muhammed Yaman Swied, Ricardo Cossyleon, Kathy Robinson, Kristin Delfino, John Gao, Krishna Rao. H-score as a diagnostic tool in HER2 equivocal (IHC 2+) breast cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 677.