Challenges and Pitfalls to Diagnosing NUTM1-Rearranged Neoplasia of the Pancreas by Cytology and Ancillary Studies.

Fine-needle aspiration cytology specimens are frequently utilized for ancillary studies to identify diagnostic and prognostic information. This case highlights diagnostic pitfalls and challenges in diagnosing NUTM1-rearranged neoplasia on pancreatic cytology. NUT carcinomas and sarcomas have highly variable cytomorphology, immunohistochemical staining profiles, and various gene partners involved in the fusion. Detecting these clinically relevant gene fusions in cytologic specimens can be particularly challenging as new diagnostic entities emerge. In cytologic specimens with a wide differential, maximizing the tissue for ancillary studies and selecting appropriate studies are critical. Finally, it highlights the importance of molecular cytopathology for patient care in complex and rare fusion-driven entities and the need for orthogonal testing when results are discordant and unexpected.