Case report of a pan-cytokeratin negative NUT midline carcinoma of pulmonary origin, a BRD3-NUT variant: Challenges in cytomorphologic presentation.

NUT carcinoma is an aggressive malignancy defined genetically by a balanced translocation of the NUT gene on chromosome 15q14, most commonly associated with the bromodomain-containing protein 4 (BRD4) gene on 19p13.1 but less frequently with variant genes, including BRD3 and NSD-3. We present a case report of a metastatic pulmonary NUT carcinoma found to have a BRD3-NUT fusion and to have only focal pan-cytokeratin staining. Biopsy of the pulmonary mass revealed dyscohesive cells with enlarged nuclei, prominent nucleoli and high nuclear to cytoplasmic ratio without areas of squamous differentiation. Initial immunohistochemical stains were positive for NUT, p63 and retained SMARCA4, while negative for Lu-5 (pan-cytokeratin), TTF-1, p40, S100 protein, OCT-4, HMB-45, SMA, and PAX-8. Tempus ×T assay revealed a BRD3-NUTM1 fusion gene. Post-mortem analysis revealed an ill-defined mass abutting the trachea and superior vena cava, as well as a perirenal mass.