Pathology

As a genetically defined cancer, NC cannot be diagnosed by morphology alone. The diagnosis of NUT carcinoma is made definitively by demonstration of NUT rearrangement either by molecular analysis (i.e. FISH, RT-PCR, or NGS demonstrating NUTM1-rearrangement), or positive staining (>50% of tumor nuclei) for NUT protein using the rabbit monoclonal antibody, clone C52B1, available from Cell Signaling Technologies (Danvers, MA). We developed this monoclonal rabbit antibody in collaboration with CST for the purpose of diagnosing NC by immunohistochemistry (IHC), based on the knowledge that NUT expression should not be seen normally outside the testis. IHC with the NUT antibody demonstrates speckled nuclear staining in nearly all NC tumor cells (Fig. 1), except for within differentiated cells, whereas no staining is seen in non-NC cancers, with the exception of faint staining of germ cell tumor nuclei, and hepatocytes. Our findings were that the antibody has an 87% sensitivity, and 100% specificity (Haack H, et al., 2009), thus strongly positive staining is diagnostic of NC.

Figure 1. Diagnosis of NUT carcinoma using a monoclonal antibody to NUT. Left, typical absence of staining of a non-NC, in this case a sinonasal undifferentiated carcinoma. By contrast, right, known NCs reveal diffuse (>90%), speckled, nuclear staining. Below, table demonstrating accuracy of C52 antibody in diagnosis of NC.

The challenge is no longer the diagnosis of NC, but to determine when to perform the test, the reagents for which, though commercially available, are not widely available in pathology laboratories. If NC is not to be missed, any poorly differentiated, monomorphic, midline neoplasm that does not stain for lineage-specific markers should be considered for NUT-rearrangement testing. This group includes sinonasal undifferentiated carcinomas (SNUC) and any poorly differentiated carcinoma, with or without squamous differentiation, whose cells lack the pronounced atypia and pleomorphism characteristic of garden variety poorly differentiated carcinomas. Also included in the differential diagnosis are undifferentiated malignant tumors, such as small round blue cell tumors. Amongst this group are Ewing sarcoma and acute leukemia, both of which have been closely mimicked by NC, resulting in misdiagnosis (Fig. 2). Squamous differentiation, evidence morphologically or by positive p63 IHC, is common in NCs, and may occasionally be extensive. A curious, characteristic finding is focal abrupt squamous differentiation, where stratification and gradual differentiation are absent, resembling Hassel’s corpuscles. To our knowledge, abrupt squamous differentiation, while common in NCs, is rarely seen in other carcinomas.

Figure 2. NCs are monomorphic, poorly differentiated carcinomas with varying degrees of squamous differentiation. (A-B) NCs can mimic other undifferentiated neoplasms such as germ cell tumors, Ewing’s sarcoma, or lymphoma. (C-D) Abrupt squamous differentiation is characteristic of some NCs. (E-F) NUT-variant NCs look similar to BRD4-NUT tumors. (G) Rarely, there can be extensive squamous differentiation. (H-I) Poorly differentiated, non-NC carcinomas often display pleomorphism and atypia that is uncharacteristic of NCs.

Pathology Consultation

Consultation to review/confirm the diagnosis of NUT carcinoma is provided by the Registry pathologist. If the diagnosis has already been made by the above criteria, confirmation by central review is free-of-charge. If not, then review will subject to regular consultation billing by the Brigham and Women’s Hospital Dept. of Pathology. This process also fulfills central review if the patient is eventually enrolled in the Registry.

In addition to diagnostic assistance and Registry enrollment, the Registry asks pathologists to consider sending tissue to the NC Registry Tissue Bank. For details, click HERE.