Challenging Diagnosis in NUT Carcinoma.

Nuclear protein in testis (NUT) carcinoma represents a highly aggressive, poorly differentiated carcinoma that is genetically defined by rearrangement of NUT gene. The histomorphological appearance ranges from entirely undifferentiated carcinoma to carcinoma with prominent squamous differentiation. NUT carcinoma can display neuroendocrine features. Although it is typically distributed along the midline axis, it may manifest in nonmidline locations. The majority of patients develop rapidly disseminated disease. We illustrate 2 cases of NUT carcinoma, one located in the lung, which closely resembled a neuroendocrine carcinoma, and the other one with assumed lung origin demonstrating metastatic dissemination with diffuse bone involvement, which was clinically first suspected to be a hematological malignancy. Due to its undifferentiated nature, NUT carcinoma may be confused with many entities. NUT immunohistochemistry is considered to be sufficient for the diagnosis. Fluorescence in-situ hybridization analysis and next-generation sequencing are currently used to confirm the diagnosis.