ALK ambiguous-positive non-small cell lung cancers are tumors challenged by diagnostic and therapeutic issues.

Searching for ALK rearrangements using the approved fluorescent in situ hybridization (FISH) test and complementary immunohistochemistry (IHC) has become the rule to treat patients with advanced non‑small cell lung cancer (NSCLC) with anti‑ALK targeted therapy. The concordance between the two techniques is reported to be strong but imperfect. We report our experience with cases of ALK‑rearranged lung adenocarcinomas pointing out particularly ambiguous cases. FISH and IHC data on ALK but also c‑MET IHC as well as EGFR and KRAS mutation screening are considered, together with response to crizotinib treatment. We classified the 55 FISH ALK‑rearranged tumors into two groups according to the FISH and IHC results: a concordant FISH+IHC+ group (31 tumors) and an ambiguous group (24 tumors). These tumors were considered as 'ambiguous' ALK‑positive due to negative (21 tumors) or non‑contributive (3 tumors) IHC. In addition, the percentage of FISH-positive nuclei was between 15 and 20% in 17 tumors belonging to one or the other group (now called borderline tumors). We discuss the accuracy of the different tests with intent to determine whether ambiguous and borderline tumors are real positive ALK‑rearranged tumors. To conclude, ambiguous ALK‑positive lung cancers are challenging tumors with diagnosis and therapeutic issues that can justify parallel FISH, IHC and molecular screening strategy.