NUT carcinoma is considered by some authors to represent a subtype of squamous cell carcinoma (SCC). However, the BRD4-NUT fusion and the lack of SCC molecular signature in NUT carcinoma makes this relationship somewhat controversial [2, 10]. NUT carcinoma has overlapping features with other poorly differentiated carcinomas, making the diagnosis challenging, and extensive IHC and molecular testing are often required for definitive diagnosis. Although the number of reported salivary gland origin NUT carcinomas is low, it is thought that the prevalence of this disease remains under-reported due to misdiagnosis [7, 11,12,13].
Characteristic histopathological features of NUT carcinoma include diffuse sheets of undifferentiated oval-to-polygonal cells with prominent nucleoli showing eosinophilic-to-clear cytoplasm. This is accompanied by necrosis, mitoses and abrupt keratinisation or squamous change. Recognizing these features can be helpful to exclude some other neoplasms, but these features are not entirely diagnostic. NUT carcinomas arising from the salivary glands are known to demonstrate a similar immunoprofile to those seen in other anatomic sites. Moreover, diagnosis is likely to be challenging on core biopsy material, which may not be representative of the complete morphology and architecture of the neoplasm. The tumour commonly expresses cytokeratins and squamous markers such as p63. The morphology of NUT carcinoma can often mimic neuroendocrine carcinoma; however, neuroendocrine markers are predominantly negative, with variable patchy positivity for synaptophysin and CD56 being described [8, 10, 14]. This variable expression of synaptophysin is a major pitfall, in particular on biopsies.
This case was challenging because the largely undifferentiated morphological appearance was difficult to distinguish from a high-grade salivary neoplasm (e.g. neuroendocrine carcinoma, high grade MEC, acinic cell carcinoma with high-grade transformation, high grade myoepithelial carcinoma) or a metastatic neoplasm [15, 16]. Salivary gland neoplasms that commonly show a clear cell population such as mucoepidermoid carcinoma, epithelial-myoepithelial carcinoma and myoepithelial carcinoma should be considered in the differential diagnosis, although it is uncommon to see undifferentiated or very high-grade features in these tumours. The presence of ‘squamous’ or ‘epidermoid’ areas and clear cells mimicking mucoepidermoid carcinoma can be potentially misleading; however, the absence of mucin, cytokeratin 7 expression and lack of MAML2 rearrangement can help exclude this possibility. Clear cell change can easily be mistaken for sebaceous differentiation and can be ruled out using androgen receptor. Staining for p16 is positive in approximately 40% of NUT carcinoma, which can suggest a diagnosis of metastatic oropharyngeal SCC. Crucially, however, tests for HPV will be negative, and clinical examination and imaging will not show an oropharyngeal lesion. The absence of convincing neuroendocrine marker expression is not sufficient for the diagnosis of neuroendocrine carcinomas, and the absence of p40 and CD99 positivity helps rule out Ewing Sarcoma whereas molecular evidence of EWSR1 gene rearrangement aids in the exclusion of adamantinoma like Ewing sarcoma as these can show consistent cytokeratin and p40 expression [10, 11, 17].
Confirming the diagnosis of NUT carcinoma is not possible without the use of NUT IHC or molecular testing. IHC for NUT is performed using commercially available NUT (C52B1) antibody, which shows 100% specificity and 87% sensitivity. This IHC has been shown to be more sensitive than other methods, with diffuse and strong expression in more than 50% of tumour nuclei confirming the diagnosis . Alternative methods to confirm NUT rearrangement include FISH, reverse transcription-polymerase chain reaction (RT-PCR), cytogenetics or next-generation sequencing-based approaches. These methods can help determine the fusion partner (which can include BRD3, and more rarely ZNF532, ZNF592 or CIC) and NSD3 may aid treatment involving targeted inhibitors [7, 19,20,21,22]. However, aberrant NUT protein expression can be seen in germ cell neoplasia and must be interpreted cautiously.
NUT carcinoma has an aggressive behaviour, with a poor prognosis. Patients normally present with a rapidly enlarged mass and lymphadenopathy [6, 18]. Currently, there are no gold standards or guidelines for the management of NUT carcinoma. Surgical resection of the primary tumour remains the mainstay of treatment for local disease control. Chemo-radiotherapy can show mixed results, and a limited response to chemotherapy has also been reported. Post-treatment metastatic disease remains a common complication, despite multimodality treatment approaches being adopted in many cases. The development of BET inhibitors aimed to inhibit the binding of BRD-NUT and chromatin acetylation with histone deacetylase may provide targeted therapy and potentially improve prognosis for this highly aggressive malignancy [4, 6, 7, 18,19,20].