Small cell transformation in crizotinib-resistant ROS1-rearranged non-small cell lung cancer with retention of ROS1 fusion: A case report.

C-ros oncogene 1 receptor tyrosine kinase (ROS1) rearrangement has been detected in patients with advanced non-small cell lung cancer (NSCLC). Although ROS1 tyrosine kinase inhibitors (TKIs) provide a survival benefit for patients with ROS1-rearranged advanced NSCLC, subsequent therapy remains limited. Small cell transformation is an important mechanism of drug resistance in epidermal growth factor receptor-mutant NSCLC. However, its significance in mediating ROS1 resistance has not been determined yet. Here, we present the case of a 63-year-old man with ROS1-rearranged advanced NSCLC who had disease progression with small cell transformation of the mediastinal lymph node after 8 months of treatment with crizotinib. More importantly, fluorescence in situ hybridization of post-progression tumor biopsy demonstrated retention of ROS1 rearrangement. Tissue biopsy remains indispensable for patients who acquire resistance to ROS1 TKIs.