Malignant solitary fibrous tumor with high-grade nuclear atypia: An alternate entity for the undetermined tumor group

Recently, a novel fusion transcript, NAB2–STAT6, and its variants have also been reported to be specific diagnostic markers for solitary fibrous tumors (SFTs). In this study, we validated the existence of the NAB2–STAT6 fusion gene in SFTs and examined its relation with the pathological features. Frozen samples from 9 tumors were assessed for fusion gene. The detected fusion genes exhibited large intron sequences and the insertion of unknown and previously unreported sequences. The fusion genes were not detected in the 2 malignant cases with high-grade nuclear atypia, nuclear pleomorphism and necrosis, that was confirmed by multiplex PCR method. In addition, 1 of the 2 NAB2–STAT6 fusion gene-negative tumors showed amplification of the MDM2 and CDK4 genes. It was suggested that a certain proportion of tumors previously diagnosed as malignant SFTs with high-grade nuclear atypia lacking NAB2–STAT6 should be categorized into a special subtype of SFT, which is genetically different from conventional SFTs, and which cannot be apparently distinguished from dedifferentiated liposarcoma or undifferentiated pleomorphic sarcoma.