Revisiting ALK (D5F3) immunohistochemistry: Insights into focal staining and neuroendocrine differentiation

BackgroundScreening for anaplastic lymphoma kinase (ALK) rearranged non‐small cell lung cancer (NSCLC) is crucial for identifying patients eligible for targeted therapy. The FDA‐approved ALK (D5F3) immunohistochemistry (IHC) assay, used with the OptiView Amplification Kit, demonstrates excellent sensitivity and specificity in detecting these patients. However, the clinical significance of resulting focal positivity remains unclear, and ALK (D5F3) expression unrelated to ALK fusion is observed in some cases of neuroendocrine differentiation. This study aims to validate these findings with molecular testing and contribute to the accurate interpretation of ALK (D5F3) IHC results.MethodsA total of 1619 patients diagnosed with NSCLC and neuroendocrine carcinoma were evaluated using ALK (D5F3) IHC. For cases with strong but focal expression and those with diffuse strong positivity in neuroendocrine differentiation, ALK fluorescence in situ hybridization (FISH) and/or next‐generation sequencing (NGS) tests were performed.ResultsSeven out of 1109 adenocarcinomas (0.6%) and six out of 289 squamous cell carcinomas (2.1%) exhibited strong focal ALK (D5F3) expression. Nine out of 209 neuroendocrine carcinomas (4.3%) showed homogeneously strong ALK (D5F3) expression. All these cases, including adenocarcinoma with neuroendocrine differentiation and combined small cell carcinoma, were negative for ALK fusions by FISH and/or NGS.ConclusionThis study demonstrates that strong but focal ALK (D5F3) immunostaining and strong expression in neuroendocrine differentiation may not indicate ALK fusion. By considering these findings, we can improve the accuracy of patient selection for targeted therapy by minimizing false‐positive interpretations of ALK (D5F3) staining.