Unveiling Diagnostic Clues of <scp><i>NTRK</i></scp>‐Rearranged Spindle Cell Neoplasms in Fine‐Needle Aspiration Specimens

NTRK (neurotropic tropomyosin receptor kinase)-rearranged spindle cell tumours represent a rare group of molecularly defined soft tissue neoplasms. These tumours, excluding infantile fibrosarcomas, are characterised by NTRK gene rearrangements and exhibit a range of histomorphologies, including spindle, epithelioid or rhabdoid cells with invasive growth. Their prognosis correlates with histological grade, and surgical resection is the primary treatment. The abnormally expressed oncogenic receptor tyrosine kinases TRK-A, TRK-B and TRK-C in this tumour have been shown to be therapeutical targetable, which may improve patient prognosis. We report a 17-year-old male patient presenting with a left axillary mass. Both fine-needle aspiration cytology (FNAC) and surgical resection specimens were submitted for examination. Comprehensive analysis of cytomorphology, immunohistochemical staining results and next-generation sequencing (NGS) data led to a final diagnosis of NTRK-rearranged spindle cell tumour. By comparing cytological and histological morphologies, we identified diagnostic clues in cytological specimens. NTRK-rearranged spindle cell tumours' definitive diagnosis enables targeted therapy. Fine-needle aspiration cytology, being minimally invasive, offers the potential for earlier and more definitive diagnoses, thereby improving patient treatment options.