ALK and ROS1 testing on lung cancer cytologic samples: Perspectives.

Cytologic sampling is the mainstay of diagnosing advanced lung cancer. Moreover, to select patients for personalized first-line or second-line treatment, epidermal growth factor receptor (EGFR) mutations and anaplastic lymphoma kinase (ALK) and c-ros oncogene 1 (ROS1) rearrangements are tested on cytologic preparations. Commercially available fluorescence in situ hybridization (FISH) and immunocytochemistry (ICC) assays have primarily been used for the identification of cells harboring ALK or ROS1 gene fusions on histologic rather than cytologic preparations. However, it is now recognized that FISH and ICC also can be applied on cytologic samples provided the cytopathologist is aware that FISH and ICC results are not always concordant and that the performance of ICC largely depends on antibody clones, signal detection systems, and scoring systems. Notably, the routine clinical use of FISH and ICC may be replaced by emerging next-generation sequencing and digital, color-coded barcode technologies, which have the advantage of simultaneously evaluating ALK, ROS1, and EGFR alterations in a single analysis. Although their use in clinical cytologic practice remains to be fully established, it is conceivable that this technology will replace both FISH and ICC analyses in future diagnostic algorithms. Here, the authors review studies devoted to testing ALK and ROS1 on cytology specimens in an attempt to provide an update for the cytopathologist regarding current and evolving practice. Cancer Cytopathol 2017;125:817-30.