Nuclear Protein in Testis (NUT) Carcinoma: A Comprehensive Immunohistochemical Analysis of 57 Cases With Consideration of Interpretation and Pitfall Recognition.

CONTEXT.—: Nuclear protein in testis (NUT) carcinoma is an aggressive carcinoma defined by NUTM1 gene rearrangement. Diagnostic challenges include morphologic overlap with poorly differentiated squamous cell carcinoma, small cell carcinoma, thoracic SMARCA4-deficient undifferentiated tumor, and other small round blue cell tumors.

OBJECTIVE.—: To comprehensively study the immunohistochemistry (IHC) features of a large cohort of NUT carcinomas.

DESIGN.—: Fifty-seven NUT carcinoma cases were identified from 2012-2022, including 38 thoracic/mediastinal, 13 head and neck/sinonasal, and 6 from other sites. Pathology reports and available slides were reviewed. Comprehensive IHC studies were performed on available cases.

RESULTS.—: Keratin stains showed variable positivity and were entirely negative in 15% (8 of 55) of cases. p40 was only positive in 65% (24 of 37) of cases, implying inferior sensitivity when compared to p63 (87% sensitivity, 20 of 23 cases) and other squamous cell markers. Neuroendocrine markers were focally/weakly positive in few cases; however, INSM1 was positive in 54% (7 of 13) of cases, indicating a possible diagnostic pitfall. TTF-1 was mostly negative with focal positivity in 26% (10 of 38) of cases. Occasional CD34 (15%, 3 of 20 cases) and CD99 (21%, 3 of 14 cases) positivity could also cause potential diagnostic confusion. S100, desmin, CD45, and SALL4 were rarely positive. BRG1 and INI1 were retained in all cases. Ki-67 proliferative index was high (median, 60%). PD-L1 was negative in all tested cases.

CONCLUSIONS.—: This comprehensive IHC study demonstrates the immunohistochemical spectrum of NUT carcinoma. The findings can help narrow the differential diagnosis and recognize potential pitfalls.