LAUSR

e12512 Background: HER2 status dictates treatment decisions in breast cancer making the precision and accuracy of laboratory testing essential. While most HER2 testing is straightforward using ASCO-CAP interpretation guidelines, 5% of cases fall into the less common categories (groups 2-4) and can present unusual diagnostic dilemmas. 6 high volume HER2 FISH laboratories evaluated 10 challenging HER2 cases. The results highlight the challenges and potential for variability in interpreting rare cases with unusual results. Methods: Ten diagnostically challenging HER2 FISH cases were selected from our breast cancer archives (41 months/3863 cases). Unstained slides were evaluated by 6 high volume laboratories. Laboratories performed HER2 FISH testing (along with HER2 IHC as needed) and rendered an interpretation. Participants discussed results during a conference call. Cases included the following situations: low-level amplification (ratio ≥ 2.0, mean HER2 signals/cell between 4.0-5.9), HER2 mean copy number or ratio near a clinically actionable threshold, complex heterogeneity or clones, and a technical challenge (signal clustering). Results: All laboratories elected to perform HER2 IHC in conjunction with FISH in all cases. Only 3/10 cases resulted in 100% agreement between the 6 institutions, all of which were in the category addressing complex heterogeneity. During the study, we identified a problematic subset (low amplified FISH cases) which may have discordant (negative) IHC results. Another problematic scenario is the finding of a small, highly amplified HER2 clone. Lastly, we identified cases with high cell to cell variability, and we saw discordant values resulting in disparate overall HER2 interpretations. Conclusions: Oncologists and pathologists should be aware of these challenges and the potential for variability in interpretation of these rare case scenarios. Evaluation for concordance between IHC and FISH results, correlation with other clinico-pathologic features and additional steps to ensure the accuracy of test results can help resolve unusual results, but grey zones may still remain and should be discussed when making clinical decisions based on HER2 test results.