Expanding the Anatomical Distribution of PRRX1::KMT2D Fusion Mesenchymal Neoplasms: A Rare Mediastinal Case Report

BACKGROUND: PRRX1-rearranged mesenchymal neoplasms are rare soft tissue tumors with a predilection for the superficial subcutaneous tissue. The PRRX1::KMT2D fusion variant is exceptionally rare, with only three previously reported cases, all of which were located in the intermuscular regions. However, its occurrence in deep visceral sites has not been documented. CASE: A 62-year-old woman was admitted after a routine physical examination revealed a space-occupying lesion in the left thoracic cavity. Contrast-enhanced CT showed a mixed-density mass (10.4 × 8.1 × 3.8 cm) at the left cardiophrenic angle. The patient underwent complete thoracoscopic resection. Intraoperative frozen sections suggested a spindle cell tumor. Postoperative pathology, immunohistochemistry, and targeted RNA sequencing identified a PRRX1::KMT2D fusion mesenchymal neoplasm. At 18-month follow-up, no recurrence or progression was observed. CONCLUSION: This is the first reported case of a PRRX1::KMT2D fusion mesenchymal neoplasm arising in the mediastinum, which expands the anatomical spectrum of this emerging entity. Our findings underscore the importance of integrating morphological, immunohistochemical, and molecular approaches for accurate diagnosis, particularly in deep-seated and unusual locations.