Equivocal ALK fluorescence in-situ hybridization (FISH) cases may benefit from ancillary ALK FISH probe testing

Aims: Accurate assessment of anaplastic lymphoma kinase (ALK) gene rearrangement in non-small-cell lung cancers (NSCLCs) is critical to identify patients who are likely to respond to crizotinib. The aim of this study was to evaluate the ALK/EML4 TriCheck FISH probe in a series of NSCLCs enriched for tumours with equivocal ALK status. Methods and results: ALK FISH was prospectively performed on 45 NSCLCs with the ALK/EML4 TriCheck probe (ZytoVision) and the Vysis ALK break-apart probe (Abbott Molecular). ALK immunohistochemistry was performed with 5A4 and D5F3 antibodies. Fourteen cases had equivocal ALK status, based on borderline or focal FISH positivity, an atypical FISH pattern, or discrepancy between ALK FISH and immunohistochemistry. Four of the 14 equivocal cases showed discordance between the two FISH probes. All other cases were concordant. The TriCheck probe showed that, of 31 unequivocal cases, 15 were ALK-rearranged, and 60% of these had EML4 as the translocation partner. Within the group of 14 equivocal cases, 12 showed rearrangement with the Tricheck probe; only one of these showed EML4 rearrangement. Of the six equivocal cases that received crizotinib, four showed clinical benefit. Conclusions: The ALK/EML4 TriCheck FISH probe may be useful for the detection of ALK rearrangements, especially in borderline or atypical cases, where an additional unique ALK FISH probe may provide further confirmation of rearrangement.