A Case Report of Undifferentiated NUT Carcinoma of Lung Diagnosed by EBUS-FNA: If You Hear Hoofbeats Do Not Assume They Are Horses.

NUT carcinoma (NC) is a rare and highly aggressive type of poorly differentiated carcinoma that originates from undefined anatomical sites, primarily in midline structures of the upper respiratory tract. We report the case of a male patient in his fifth decade of life who is a non-smoker. He presented with symptoms of dyspnea and cough, and had previously been treated for an atypical infectious process. A PET scan revealed a right pulmonary hilar mass alongside pathological lymphadenopathy. An endobronchial ultrasound-guided fine needle aspiration (EBUS-FNA) of the subcarinal lymph node was performed, followed by a cytomorphological evaluation in situ (Rapid On-Site Evaluation-ROSE) for sample management. The smear showed high hypercellularity with loosely cohesive cells arranged mostly in a single-cell pattern, exhibiting a poorly differentiated appearance. Immunohistochemical studies on the cell block indicated nuclear positivity for NUT expression, confirming the diagnosis of pulmonary NC. Poorly differentiated high-grade neoplasms in middle-aged patients present a significant challenge when the tumor sample is limited. The cytopathologist's role is to optimize the available material for a complete and accurate diagnosis, which is essential for appropriate treatment.