A rare primary mediastinal mesenchymal tumor diagnosed by endobronchial ultrasound (EBUS).

Primary mesenchymal tumors of the mediastinum are extremely rare and often pose considerable diagnostic challenges owing to their histological diversity and anatomical location. We report the case of a 51-year-old man with a 45 pack-year smoking history who was referred for evaluation of a mediastinal mass initially detected by chest radiography. Thoracic computed tomography (CT) and positron emission tomography-CT revealed a 3 × 3 cm lesion extending from the right upper and lower paratracheal regions into the right upper lobe parenchyma, demonstrating intense FDG uptake. Fiber-optic bronchoscopy revealed no endobronchial lesions. A CT-guided transthoracic needle biopsy was non-diagnostic. Subsequently, linear endobronchial ultrasound (L-EBUS) was performed, and a transbronchial needle biopsy sample was obtained for histopathological examination. Histopathological examination revealed an undifferentiated malignant mesenchymal tumor, positive for vimentin, with a Ki-67 proliferation index of approximately 30%. Despite an extensive immunohistochemical panel, no specific line of differentiation was established. Following the diagnosis, the patient was discussed at a multidisciplinary tumor board and subsequently referred to medical oncology for systemic treatment. This case highlights the role of L-EBUS in obtaining adequate tissue for the diagnosis of a rare malignant mesenchymal mediastinal tumor, particularly after inconclusive conventional biopsy attempts. Accurate diagnosis of such rare tumors requires a multidisciplinary strategy incorporating imaging, histopathology, and immunohistochemistry to guide appropriate treatment planning and prognostication of the disease.