Soft Tissue Tumor with <italic>PAK2::RAF1</italic> Kinase Fusion: An Emerging Sarcoma Entity.

<bold><italic>Introduction:</italic></bold> Here, we report the first case of a soft tissue tumor with a <italic>PAK2::RAF1</italic> fusion. <bold><italic>Case Presentation:</italic></bold> The patient was an 11-year-old female presenting with a 5 cm intramuscular mass in the lower leg. Microscopic examination revealed a mitotically active spindle cell lesion with monomorphic and moderately atypical cells growing in a patternless pattern with the presence of stromal and perivascular keloidal collagen with focal immunoreactivity for smooth muscle actin and S100; negative stains included cytokeratins, CD34, and caldesmon. Whole genome and RNA sequencing detected a chromosome 3 inversion and a resultant <italic>PAK2::RAF1</italic> fusion as well as a CDKN2A homozygous deletion. The patient was treated with neoadjuvant chemoradiotherapy with only minimal response and the tumor was excised surgically. There was no evidence of disease progression at 4 months. <bold><italic>Conclusion:</italic></bold> This is the first case of a soft tissue tumor harboring a <italic>PAK2::RAF1</italic> fusion with histological features in keeping with previous cases of <italic>RAF1</italic> and other kinase fusion soft tissue tumors. [ABSTRACT FROM AUTHOR]