LAUSR

ROS1‐rearranged lung adenocarcinoma has been recently identified. We report a case of ROS1‐rearranged lung adenocarcinoma with special emphasis on cytological findings. Here, we report a case of young woman with ROS1‐rearranged lung adenocarcinoma diagnosed by cytology and discuss the clinical, cytological, and molecular findings. Cytologically, the tumor consisted of small tight clusters of cells with high nuclear/cytoplasmic ratio. Nuclei were enlarged and small nucleoli were occasionally observed. Signet‐ring cells were focally identified. Neoplastic cells were positive for ROS1 immunocytochemistry. Subsequently, the translocation of ROS1 gene was confirmed in a histological specimen. In conclusion, the specific histology of adenocarcinoma on cytological materials should promote testing for ROS1 immunohistochemistry. Immunocytochemical detection of ROS1 protein helps identify patients suitable for molecular targeted therapy.