Histopathologic characteristics of advanced-stage ROS1-rearranged non-small cell lung cancers.

ROS1 rearrangement accounts for 1%-2% of non-small cell lung cancer (NSCLC) with a remarkable response to crizotinib. Although ROS1-rearranged tumors are known to have characteristic histologic features, only a few studies have investigated the histologic features of advanced-stage ROS1-rearranged tumors. We analyzed the histopathologic features of ROS1-rearranged tumors in advanced-stage NSCLC patients and assessed the ROS1 immunohistochemistry (IHC) staining patterns of ROS1-rearranged cases. A total of 37 ROS1 fluorescence in situ hybridization (FISH)-positive cases and 64 ROS1 FISH-negative cases were analyzed, and all tumors were EGFR-, ALK-, and RET-negative. Solid pattern, round nuclei with macronucleoli, solid signet-ring cells, extracellular mucin, and a close relation with adjacent bronchioles were significantly associated with ROS1 rearrangement, and the solid signet-ring cell feature was exclusively identified in ROS1-rearranged tumors. ROS1 IHC showed a 97.3% sensitivity when weak to strong protein expression was considered positive. Our findings highlight distinct histologic features of ROS1-rearranged tumors, including their nuclear features. A thorough understanding of ROS1 rearrangement-related histologic features would be helpful to identify ROS1-rearranged tumors in advanced-stage NSCLC.