Primary pulmonary spindle cell sarcoma with novel MBNL2::NUTM1 fusion and associated langerhans cell hyperplasia.

NUTM1-rearranged sarcomas are rare, genetically diverse tumors with variable histologic features and fusion partners. Their diagnosis remains challenging, especially in low-grade or unusual presentations. We describe the first reported case of a pulmonary spindle cell sarcoma with MBNL2::NUTM1 fusion in a 67-year-old man. The patient presented with multiple lung masses and respiratory symptoms. Histology showed spindle cell proliferation with mild atypia and areas rich in eosinophils, lymphocytes, and Langerhans cells. Immunohistochemistry revealed NUT, CD31, SMA, and Cyclin D1 positivity in tumor cells; CD1a and Langerin were positive in Langerhans cells. RNA-based sequencing confirmed an in-frame fusion between MBNL2 (exon 7) and NUTM1 (exon 3). PET-CT(Positron emission tomography-computed tomography) revealed widespread metastasis despite the tumor's low-grade morphology. This case expands the spectrum of NUTM1-rearranged sarcomas and highlights the potential for misdiagnosis in low-grade cases. NUT immunostaining and molecular testing are essential for accurate classification of undifferentiated or spindle cell tumors.