Q1. B
Q2. A
Q3. A
Q4. True

NUT carcinoma

EBUS-TBNA aspirate revealed singly lying tumor cells with scant cytoplasm, round to oval nuclei with finely granular chromatin and single prominent nucleolus (Figure 1). Occasional cells with dense cytoplasm and prominent cell border suggestive of squamous differentiation were admixed (Figure 2). Endobronchial biopsy showed a malignant small round cell tumor composed of sheets of tumor cells with very scant cytoplasm and hyperchromatic nucleus. Nuclear molding, crush artefacts, infiltrating neutrophils, and karyorrhectic debris were seen (Figures 3 and 4). Tumor cells were immunopositive for pan-cytokeratin, p40 (Figure 5) and NUT (Figure 6) and negative for TTF-1, diagnostic of NUT carcinoma. No other possible primary site was identified on whole body PET-CT. A final diagnosis of primary lung NUT carcinoma with metastases to mediastinal, left hilar and peripancreatic lymph nodes was made. The patient is currently on chemotherapy.

NUT (nuclear antigen in testis) carcinoma, originally termed NUT midline carcinoma, is a highly aggressive type of squamous cell carcinoma that predominantly originates in the midline structures of the upper aerodigestive tract and mediastinum. With increasing reports of its occurrence in non-midline locations such as abdomen, lacrimal gland and iliac bone, the nomenclature has been changed to NUT carcinoma. Primary pulmonary NUT carcinoma accounts for less than 1% of lung cancer with less than 20 cases reported to date. These cases present at an advanced stage with frequent bony metastases and are associated with extremely poor outcome.

NUT carcinomas are defined by rearrangements involving the NUTM1 gene located at 15q14 that result in fusion with a member of the bromodomain-containing protein (BRD) family, usually BRD4 located on chromosome 19. These tumors are morphologically and immunohistochemically undifferentiated and are composed of primitive tumor cells with high mitotic indices. Focal and abrupt squamous differentiation adjacent to undifferentiated tumor cells is characteristic but is usually focal. Due to their undifferentiated morphology, these tumors have a high likelihood of being misdiagnosed. NUT carcinomas are diffusely positive for p63 or p40 (Fig 4) and cytokeratin, but nearly always negative for neuroendocrine markers and TTF-1. Immunohistochemistry using monoclonal NUT antibody is highly specific and sensitive for the diagnosis of NUT carcinoma (at a cut-off of >50% tumor cell staining) and shows a speckled staining pattern in tumor cell nuclei. Germ cell tumors may show focal NUT expression; however, NUT carcinomas are always negative for other germ cell markers such as SALL4 and OCT4.

In aspirates, the tumor cells are usually singly dispersed as naked nuclei or with a thin rim of cytoplasm, frequently associated with karyorrhectic debris and neutrophilic infiltration. 3-dimensional clusters with nuclear molding can also be seen. Nuclear chromatin is fine to coarse granular and usually shows a single conspicuous nucleolus. Presence of cells with dense cytoplasm representing tumor cells showing squamous differentiation, and frankly dyskeratotic cells are rare findings. Tumor cells with cytoplasmic vacuolation and multinucleation have also been described. The cytomorphological features of NUT carcinoma show considerable overlap with other small round cell tumors, and immunohistochemistry for NUT or molecular testing is invariably required for confirmation of diagnosis.

Take home message for trainees: NUT carcinomas are rare but highly aggressive malignancies that are frequently misdiagnosed due to overlapping morphological features. A high index of suspicion is necessary for accurate diagnosis, the latter being essential for prognostication and treatment decisions.

Bishop JA, French CA, Ali SZ. Cytopathologic features of NUT midline carcinoma: A series of 26 specimens from 13 patients. Cancer Cytopathol 2016;124:901-8.

Kakkar A, Antony VM, Irugu DVK, et al. NUT midline carcinoma: A series of five cases, including one with unusual clinical course. Head Neck Pathol 2018;12(2):230-6.

Sholl LM, Nishino M, Pokharel S, et al. Primary pulmonary NUT midline carcinoma: clinical, radiographic, and pathologic characterizations. J Thorac Oncol 2015;10:951-9.

Aruna Nambirajan, MD, DNB
Senior Research Associate
Department of Pathology
All India Institute of Medical Sciences
New Delhi, India

Deepali Jain, MD, FIAC
Associate Professor
Department of Pathology
All India Institute of Medical Sciences
New Delhi, India