Abstract:

This presentation clarifies the diagnostic differentiation between two soft tissue sarcomas—Low-Grade Fibromyxoid Sarcoma (LGFMS, or Evans Tumor) and Myxofibrosarcoma (MFS, Grade 1)—which are often confused due to their similar-sounding names but are microscopically and clinically distinct. The video contrasts the tumors based on four key features: histological composition (fibrous vs. myxoid dominance), the presence of nuclear atypia (rare in LGFMS, required in MFS), specific immunohistochemical markers (MUC4 positivity in LGFMS), and underlying molecular genetics. LGFMS is characterized by a deceptively benign appearance and recurrent FUS-CREB3L2 translocations, while MFS exhibits nuclear pleomorphism indicative of its aneuploid nature.

Differentiating Low-Grade Fibromyxoid Sarcoma and Myxofibrosarcoma

• 0:16 Named Alikes vs. Look-Alikes: Low-Grade Fibromyxoid Sarcoma (LGFMS) and Myxofibrosarcoma (MFS) are "named alikes"—their names are similar and confusing, but their microscopic and clinical features are significantly different and easy to distinguish.
• 0:34 Histological Composition (Fibrous vs. Myxoid):
  • LGFMS: More fibrous than myxoid (pink appearance). The name order ("Fibro" first) helps recall this.
  • MFS: More myxoid than fibrous (blue, gelatinous appearance). The name order ("Myxo" first) helps recall this.
• 1:06 Atypia and Pleomorphism:
  • LGFMS: Atypia/nuclear pleomorphism is rare. These tumors look deceptively benign, mimicking conditions like fibromatosis or neurofibroma.
  • MFS: Atypia/nuclear pleomorphism is essentially required for diagnosis, even in Grade 1, making the tumor appear worrisome for malignancy immediately.
• 1:51 Immunohistochemistry (IHC):
  • LGFMS: Relatively sensitive and specific marker: MUC4 positive.
  • MFS: Does not have any specific, routinely stained IHC marker that helps diagnostically.
• 2:08 Molecular Abnormalities:
  • LGFMS: Has recurrent, balanced translocations, most commonly FUS-CREB3L2 (and sometimes CREB3L1).
  • MFS: Is an aneuploid tumor with random chromosomal gains and losses; there is no specific recurrent molecular abnormality to test for.
• 3:02 Key Sarcoma Rule of Thumb: Translocation-associated sarcomas (like LGFMS) typically have uniform, monotonous nuclei. Pleomorphism in sarcomas (like MFS) is usually a surrogate marker for underlying aneuploidy (random chromosomal gains and losses).
• 3:16 Further Resources: The speaker encourages viewers to watch full-length videos on both entities for in-depth details.