NUT Midline Carcinoma: A Series of Five Cases, Including One with Unusual Clinical Course.

NUT midline carcinomas (NMCs) are rare, poorly differentiated tumors with aggressive biological behavior and a characteristic molecular signature. Availability of NUT antibody has facilitated diagnosis of NMC without molecular testing. We report a series of head and neck NMCs diagnosed using NUT IHC at our institute, including one case with an unusual course. Immunohistochemistry for NUT was performed in nasal and sinonasal tumors with diagnoses of undifferentiated carcinoma, poorly differentiated squamous cell carcinoma and malignant neoplasm, not otherwise specified, to identify cases of NMC. Clinicopathological features were reviewed. Five cases of NMC were identified, accounting for 9.6% of poorly differentiated/undifferentiated carcinomas of the sinonasal region. These patients had a sex ratio of 2:3, and ranged in age from of 10 to 31 years (mean: 25.2 years). Patient 4 had previously been diagnosed with basal cell carcinoma arising in left nasolacrimal duct, and inverted papilloma of nasal cavity. She presented to us with a left lacrimal fossa mass extending into nasal cavity, which was diagnosed as NMC. NMC is a rare neoplasm, the awareness of which is imperative for pathologists to identify cases in which NUT IHC should be ordered. NUT IHC should be performed in all cases of a poorly differentiated carcinoma, particularly those with foci of squamous differentiation, irrespective of patient age and unusual tumor location, as seen in one of our cases. Although considered a highly aggressive and lethal neoplasm, NMC can have a more prolonged clinical course on occasion.