p63-negative thoracic NUT carcinoma - A potential diagnostic pitfall.

NUT carcinoma is a rare, aggressive, undifferentiated carcinoma characterized by NUTM1 gene rearrangement. It was formerly termed NUT midline carcinoma due to its propensity to occur in midline anatomical structures. While initially believed to affect children and young adults predominantly, subsequent research has revealed its capacity to affect individuals of any age, without gender predilection. NUT carcinoma has been included in the 2015 WHO classification of thoracic tumors. NUT carcinoma can be difficult to distinguish from poorly differentiated tumors, often requiring immunohistochemistry (IHC) or molecular studies for diagnosis. The diagnostic immunohistochemical staining for NUT carcinoma is the NUT antibody. NUT carcinoma is usually positive for p63 IHC. Rare instances of p63-negative NUT carcinoma have been documented, particularly in the thoracic region. Here, we present a unique case of p63-negative thoracic NUT carcinoma, emphasizing the potential diagnostic pitfall.