This video presents the histological analysis of a large, deep-seated leg mass removed from a 42-year-old male. Microscopically, the tumor is strikingly bland, hypocellular, and rich in collagen, lacking the typical features of malignancy such as atypia, pleomorphism, or mitosis. The key diagnostic feature observed is the architectural variability of the tumor, which alternates between hypocellular, sclerotic areas and slightly more cellular areas, sometimes featuring a mixed, mixoid background and Staghorn vessels. The final diagnosis is Low-Grade Fibromyxoid Sarcoma (Evans Tumor)βa rare but clinically important sarcoma that is notoriously deceptive in its benign appearance. Diagnosis is confirmed by the positive MUC4 immunostain and its associated FUS-CREB gene fusion.
0:00 Case Introduction: The case involves a large, deep leg mass excised from a 42-year-old male. The specimen is a huge slab of tumor.
0:26 Histological Appearance: The mass is described as collagenous and very pink. High power reveals spindle cells separated by fine collagen fibers, suggesting a fibroblastic or myofibroblastic origin.
0:58 Deceptively Bland Cells: The cells are very bland and benign-looking, showing almost no atypia, pleomorphism, or mitosis. The tumor is hypocellular.
1:19 Vascular Patterns: The stroma contains branching, "Staghorn vessels," a feature often associated with solitary fibrous tumors (SFTs).
1:31 Architectural Variability: The tumor exhibits significant histological variability: areas range from very pink/hyalinized/sclerotic to slightly more cellular regions. Some areas may show a bluish, mixoid (mucinous) background, which is a key component of this tumor type.
2:09 Diagnosis: The diagnosis is Low-Grade Fibromyxoid Sarcoma (LGFMS), also known as Evans Tumor.
2:21 Key Takeaway (Deceptive Nature): LGFMS is critical to recognize because it is a sarcoma that is "deceptively benign-appearing." It lacks traditional sarcoma features and often mimics benign lesions like neurofibroma, perineurioma, or SFT.
3:07 Immunostaining: The preferred diagnostic marker for this tumor is MUC4, which is highly specific when seen in the proper histological context.
3:23 Molecular Abnormality: LGFMS is associated with a molecular abnormality involving the FUS gene and the CREB3L1 or CREB3L2 genes (FUS-CREB fusion).