NUT carcinoma in a nutshell: A diagnosis to be considered more frequently.

NUT carcinoma is a rarely diagnosed, poorly differentiated subtype of squamous cell carcinoma, defined by chromosomal rearrangements of the gene encoding nuclear protein of the testis (NUT). It is characterized by extremely aggressive clinical behavior resulting in a dismal prognosis, with a median survival of 6.7 months. Though most frequently detected along the body midline, NUT carcinoma can arise in any organ. Fewer than 100 cases have been reported in medical literature with the majority of patients being children or young adults. Here we present a case of sinonasal NUT in a 48-year-old male who came to our hospital due to progressive cephalalgia. Radiographically, an irregular mass in the left sphenoidal sinus suspicious for a malignant process was detected, and biopsies were taken. Histopathologically, a tumor of highly mitotic, predominantly small to middle-sized cells with a focal abrupt transition to mature-appearing, squamous epithelium was noted. Of critical importance for the diagnosis, the undifferentiated tumor cell population robustly expressed NUT. The diagnosis of NUT carcinoma was confirmed by the identification of BRD4-NUT fusion. This case integrates typical morphological, immunohistochemical and molecular characteristics of NUT carcinoma and highlights the need to consider this entity in cases of poorly differentiated squamous carcinoma.