Solitary fibrous tumor of soft tissues with giant hyalinized rosettes

Solitary fibrous tumor is a rare mesenchymal tumor characterized by CD34-positive dendritic stromal cells, slow growth, and aggressiveness. Solitary fibrous tumors have many histological features. We detected a tumor with distinct histological characteristics in the form of giant hyalinized rosettes and made the initial diagnosis of a low-grade fibromyxoid sarcoma. On IHC, the tumor was positive for STAT6 and CD34 and negative for MUC4. Given that the NAB2–STAT6 gene fusion is a highly sensitive and specific molecular marker for solitary fibrous tumor, and that its surrogate IHC marker—transcription activator 6 (STAT6)—has significant diagnostic value, a diagnosis of solitary fibrous tumor was considered. We did not detect any rearrangements in the FUS and EWSR1 genes in molecular genetic testing. Due to a rare occurrence of this condition in a daily pathological practice, we use this case to discuss the diagnostic features and differential diagnosis of solitary fibrous tumor and low-grade fibromyxoid sarcoma. To the best of our knowledge, a solitary fibrous soft tissue tumor with giant hyalinized rosettes has not yet been described. Keywords: soft tissue tumor, fibromyxoid sarcoma, solitary fibrous tumor, anatomic pathology, histological and molecular genetic diagnosis