This video provides an in-depth microscopic examination of Low-Grade Fibromyxoid Sarcoma (LGFMS), also known as Evans tumor, emphasizing its deceptive, often bland, benign-like appearance which frequently leads to misdiagnosis despite its malignant potential. LGFMS is a translocation-associated sarcoma characterized by a distinctive biphasic pattern of alternating pink/fibrous and blue/myxoid zones. The tumor cells are bland, uniform spindle cells with low mitotic activity and lack the pleomorphism seen in its namesake mimic, Myxofibrosarcoma. Crucially, LGFMS exhibits unusually delayed metastatic behavior, sometimes metastasizing decades after initial excision. The diagnosis relies on recognizing this unique histologic pattern, distinguishing it from mimics like Neurofibroma and Perineurioma, and confirming via ancillary tests, particularly MUC4 immunohistochemistry and the defining FUS-CREB3L2 or FUS-CREB3L1 gene translocations.
0:03 Deceptive Malignancy: LGFMS (Evans Tumor) is frequently confused in pathology, partly due to its name being similar to Myxofibrosarcoma, but primarily because it often looks deceptively benign (0:50).
0:58 Delayed Metastasis: The tumor behaves malignantly, but often in a very delayed manner, with metastasis occurring decades after initial presentation (up to 30 or 40 years later). This delayed behavior also contributed to the initial difficulty in recognizing it as a sarcoma (10:19).
1:49 Low Power Appearance – The Key Clue: LGFMS typically appears predominantly pink (fibrous/fibroblastic component) with lesser pale/bluish (myxoid component). This biphasic pattern, where pink and blue areas mingle, is the most important low-power clue (1:49, 7:28).
3:19 Differential Diagnosis (DDx): LGFMS is often misdiagnosed as benign lesions like Neurofibroma, Fibroma, or Desmoid Fibromatosis (Fibromatosis). Pathologists should avoid calling bland spindle cell lesions "fibroma, NOS" (3:35).
4:06 Bland Cytology (The Pitfall): The tumor cells are thin, bland, spindle-shaped, and lack significant atypia, pleomorphism, or high mitotic activity, which is the main reason for misdiagnosis (4:27). Recognizing the pattern is key to diagnosis, not the cytological features (5:37).
6:56 Distinguishing from Myxofibrosarcoma: Low-grade Myxofibrosarcoma must contain pleomorphism, whereas LGFMS typically does not, making them distinct entities despite the similar names (7:09).
11:36 Distinctive Histology: The fibrous background features very fine, delicate, thread-like collagen (the "Ramen Noodle sign") (32:00).
19:18 Vascular Pattern: LGFMS often exhibits characteristic arcuate (curved arch) or hairpin-turn vessels (19:28, 20:25), frequently surrounded by dense perivascular hyalinization or sclerosis (20:08, 27:03).
12:40 Distinguishing from Mimics (IHC): LGFMS is typically negative for CD34 (unlike DFSP) and S100 (unlike Neurofibroma/Schwannoma). It can be difficult to distinguish from Perineurioma (which can share EMA and Claudin-1 positivity).
14:43 MUC4 – The Key IHC Marker: MUC4 (EMUC4) is a highly sensitive and specific immunohistochemical marker for LGFMS (14:59).
15:46 Molecular Basis (Translocations): LGFMS is defined by the FUS-CREB3L2 translocation (t(7;16)) or, less commonly, the FUS-CREB3L1 gene fusion. Molecular testing (e.g., FUS FISH) can confirm the diagnosis (16:03).
27:50 Hyalinizing Variant: A morphologic variant, previously known as Hyalinizing Spindle Cell Tumor with Giant Rosettes, is now recognized as LGFMS. It is characterized by large, dense, hypocellular collagen rosettes (28:27).