Quantitative imaging is a promising tool that is gaining wide use across several areas of pathology. Although there has been increasing adoption of morphologic and immunohistochemical analysis, the adoption for evaluation of fluorescence in situ hybridization (FISH) on formalin-fixed, paraffin-embedded tissue has been limited because of complexity and lack of practice guidelines.


To perform HER2 FISH validation in breast carcinoma in accordance with the American Society of Clinical Oncology/College of American Pathologists (ASCO/CAP) 2018 guideline.


Clinical validation of HER2 FISH was performed using the US Food and Drug Administration–approved, dual-probe HER2 IQFISH (Dako, Carpinteria, California) with digital scanning performed on a PathFusion (Applied Spectral Imaging, Carlsbad, California) system. Validation parameters evaluated included: z-stacking, classifier, accuracy, precision, software, and hardware settings. Finally, we evaluated the performance of digital enumeration on clinical samples in a real-world setting.


The accuracy samples showed a final concordance of 95.3% to 100% across HER2 groups 1 to 5. During clinical implementation for HER2 groups 2, 3, and 4, we achieved a final concordance of 76% (95 of 125). Of these, only 8% (10 of 125) of the cases had discordances with clinical impact that can be identified algorithmically and triaged for manual review.


Digital FISH enumeration is a useful tool to improve the efficacy of HER2 FISH enumeration and capture genetic heterogeneity across HER2 signals. Excluding cases with high background or poor image quality and manual review for cases with ASCO/CAP group discordances can further improve the efficiency of digital HER2 FISH enumeration.

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Author notes

Results from this study were presented in part at the 2022 College of American Pathologists annual meeting; October 10, 2022; New Orleans, Louisiana.

The authors have no relevant financial interest in the products or companies described in this article.