Case report: NUT carcinoma in an elderly woman with unique morphology and immunophenotype highlights a diagnostic pitfall

Background: NUT carcinoma (NC) is a rapidly progressing and rare neoplasm that primarily affects younger patients and has a survival time of about 1 year. Most of these neoplasms express epithelial markers with no neuroendocrine markers observed. Retrospective studies have shown that pathologists and clinicians do not have a sufficient understanding of the disease due to the lack of common clinical manifestations, imaging, and morphological features. Case Description: A 60-year-old female presented at Beijing Friendship Hospital, showing repetitive epistaxis, nasal pain, and nasal congestion with obvious masses in the right nasal sinus and frontal sinus. Microscope analysis revealed two unique morphological changes which have not been previously reported in the existing literature: (I) small spindle cells with sparse cytoplasm and densely stained nuclei and (II) large tumor cells with abundant cytoplasm, some cells resembling plasma cells. The sudden appearance of keratinization was also a prominent feature. Immunohistochemical staining showed differences between the two cell morphologies. Small spindle cells simultaneously expressed CK5/6 and P40, and the Ki67 proliferation index was 40%. The large round cells did not express CK5/6 and P40 but were focal positive for synaptophysin and the Ki67 index was 10%. NUT and P63 were strongly expressed in both cell types and fluorescence in situ hybridization (FISH) revealed BRD4-NUTM1 translocation. Following 20 rounds of postoperative radiation treatment, the patient was alive and no recurrence or metastasis was observed during a 5-month follow-up. Conclusions: We present novel information from the oldest known and surviving patient of NC originating in the nasal cavity with unique morphological features and different immunohistochemical results. NUT antibody testing should be performed in undifferentiated or poorly differentiated malignancies, particularly those with either or both cytoplasmic vacuolation of medium-sized cells and abrupt keratinization, irrespective of patient age.