ALK+ lung adenocarcinoma in never smokers and long-term ex-smokers: prevalence and detection by immunohistochemistry and fluorescence in situ hybridization.

ALK gene rearrangements are identified in 2-5 % of all non-small cell lung cancer and are more common in lifetime non-smokers with adenocarcinoma, but the prevalence of ALK rearrangements is not as well characterized in long-term ex-smokers (quit >10 years prior to diagnosis). Accurate and timely diagnosis of ALK-rearranged tumors is of clinical importance given the remarkable response to targeted inhibitors. ALK gene rearrangement may be detected by fluorescence in situ hybridization (FISH), and abnormal expression of ALK protein may be detected by immunohistochemistry (IHC), the latter of which is faster and less expensive. The aim of this study is to evaluate the prevalence of ALK rearrangement in non-smokers and long-term ex-smokers with lung adenocarcinoma and to assess the performance of IHC for the detection of ALK+ tumors when compared to FISH. Two hundred fifty-one cases of resected lung adenocarcinoma were retrospectively reviewed, including non-smokers (n = 79) or long-term ex-smokers (n = 172). ALK IHC and ALK FISH were performed on each case. Four cases demonstrated ALK rearrangement by FISH (4/251; 1.6 %). All cases were non-smokers (4/79; 5.1 %), and all were positive for ALK by IHC. No additional cases were considered positive by IHC, and only 26 (10.4 %) cases were considered equivocal using a conservative approach to interpretation, resulting in a sensitivity of 100 % and specificity of 89.5 %. ALK rearrangement was not observed in lung adenocarcinoma arising in long-term ex-smokers, whereas it is seen in up to 5.1 % of lifetime non-smokers. ALK IHC using the 5A4 antibody demonstrates high sensitivity, supporting its use as a screening test.