Lung nuclear protein in testis carcinoma in an elderly Korean woman: A case report with cytohistological analysis

Nuclear protein in testis (NUT) carcinoma is a rare, aggressive carcinoma that is a diagnostic challenge for pathologists. Here, we report a case of NUT carcinoma in a 63‐year‐old woman with uncommon immunohistochemical results. The initial bronchoscopic biopsy revealed a poorly differentiated carcinoma with p63 immunohistochemical stain positivity. However, the cytomorphological features of the pleural fluid were unusual. Immunohistochemical staining of the pleural fluid revealed diffuse positivity for vimentin and focal positivity for cytokeratin and neuroendocrine markers. Because of chemoresistance, other malignancies, including sarcomatoid carcinoma, combined small cell carcinoma, and an unusual form of NUT carcinoma, were considered as differential diagnoses. The diagnosis of NUT carcinoma was confirmed using NUT‐specific antibodies and fluorescence in situ hybridization. The current case was a diagnostic challenge because of the poorly differentiated cytomorphology and uncommon immunohistochemical results. Pathologists and clinicians should consider NUT carcinoma in the differential diagnosis, as this malignancy has a dismal prognosis and needs to be diagnosed accurately for the most effective treatment. KEY POINTS: Metastatic NUT carcinoma can show diffuse vimentin positivity and focal neuroendocrine marker positivity. NUT carcinoma can be misdiagnosed as basaloid squamous cell carcinoma in routine diagnosis, especially in older‐aged patients. This study was a diagnostic challenge because of the poorly differentiated cytomorphology and uncommon immunohistochemical results for NUT carcinoma. Pathologists should differentially diagnose NUT carcinoma when rare cytohistological features are observed at any age.