BACKGROUND:Pulmonary NUT carcinoma is rare, but lethal, thus, must not be overlooked. The definitive diagnosis is made by a NUT monoclonal antibody or gene analysis, but these are not always routinely available. Therefore, the diagnosis depends on this rare disease being suspected from the clinical and pathological findings. Generally, NUT carcinoma of the lung occurs near the hilum in younger adults with severe subjective symptoms. Histologically, it is characterized by the monomorphic growth of small cells which showed positivity of p63 immunohistochemistry.CASE PRESENTATION:An 82-year-old man was referred for an incidental finding of an abnormal shadow at the peripheral apex of the right lung on computed tomography for a regular follow-up examination of renal cancer. Microscopically, small cell carcinoma was initially suspected; however, immunohistochemistry was not typical. NUT carcinoma with BRD4-NUT fusion was ultimately diagnosed using a NUT monoclonal antibody, fluorescence in situ hybridization, and RNA-seq. p63 and p40 protein expression was not detected.CONCLUSIONS:This is the first case of pulmonary NUT carcinoma to show negativity for p63 and is the oldest among previously reported cases. The present case suggests that NUT carcinoma should be suspected when the morphology of monomorphic growth of small cells without lineage-specific differentiation, regardless of age, clinical symptoms, the tumor location, or p63 expression.
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