Pleural-Based GLI1-Altered Mesenchymal Tumor With ACTB-GLI1 Fusion: A Rare Pulmonary Entity Mimicking Synovial Sarcoma.

GLI1-altered mesenchymal tumors are exceptionally rare mesenchymal tumors, with recent molecular studies implicating GLI1 gene fusion or amplification. We report a 39-year-old nonsmoking woman who presented with an incidentally detected right pleural-based mass. Cross-sectional imaging identified a paraspinal pleural lesion at the T5-T6 level without intraspinal extension. Video-assisted thoracoscopic resection revealed a biphasic tumor and next-generation sequencing confirmed an ACTB-GLI1 fusion. The diagnosis of GLI1-altered mesenchymal tumor was established. After multidisciplinary discussion, the patient opted for radiologic surveillance instead of adjuvant therapy. She remains disease-free at 1-year follow-up. This case expands the anatomic spectrum of GLI1-altered tumors and underscores the value of molecular testing in distinguishing them from other pleural neoplasms.