Parapharyngeal inflammatory myofibroblastic tumor harboring fibronectin 1- ros protooncogene 1 fusion responded to crizotinib

Inflammatory myofibroblastic tumor (IMT) is a rare tumor type usually arising in the thoracic or abdominal cavity. Despite its rarity, IMT commonly harbors driver gene rearrangements involving anaplastic lymphoma kinase (ALK), ROS proto-oncogene 1 (ROS1), and neurotrophic tropomyosin-related kinase. We present a rare case of the parapharyngeal IMT with convoluted diagnostic test results in determining driver gene rearrangement. The immunohistochemical stains were ALK-negative and ROS1 positive, but the result of ROS1 fluorescence in situ hybridization was equivocal. Amplicon-based targeted next-generation sequencing (NGS) did not detect any ROS1 rearrangement, but hybridization capture-based NGS revealed a rare fibronectin 1 (FN1)-ROS1 fusion. Eventually, the patient started crizotinib and had a tumor response with tolerable toxicity. This case highlights the importance of appropriate molecular testing of IMTs to guide the proper targeted therapy.