Soft Tissue Tumor with PAK2::RAF1 Kinase Fusion: An Emerging Sarcoma Entity

Introduction: Here, we report the first case of a soft tissue tumor with a PAK2::RAF1 fusion. Case Presentation: 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 PAK2::RAF1 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. Conclusion: This is the first case of a soft tissue tumor harboring a PAK2::RAF1 fusion with histological features in keeping with previous cases of RAF1 and other kinase fusion soft tissue tumors.