Pulmonary NUT carcinoma mimicking small cell lung cancer: a diagnostic pitfall in high-grade thoracic tumors

Pulmonary nuclear protein in testis (NUT) carcinoma is a rare and aggressive tumor that can closely mimic small cell carcinoma or poorly differentiated squamous cell carcinoma, particularly in limited biopsies complicated by necrosis or crush artifact. We report a 61-year-old man with a centrally located lung mass that was initially misdiagnosed as small cell carcinoma. Bronchoscopic biopsies were inconclusive due to extensive necrosis, prompting an open biopsy. Histopathological examination revealed a poorly differentiated carcinoma with focal abrupt keratinization. Strong nuclear immunoreactivity for NUT protein confirmed the diagnosis of NUT carcinoma. This report highlights the importance of considering NUT carcinoma in undifferentiated lung tumors to avoid diagnostic pitfalls and to enable timely and appropriate management.