Abstract:

This lecture segment focuses on the pathological diagnosis of "fibrosarcoma," emphasizing that the historic term, characterized by a herringbone fascicular pattern of spindle cells, is now exceedingly rare and largely obsolete due to modern diagnostic techniques. The speaker explains that most historical cases previously labeled as fibrosarcoma are now reclassified as other entities like dedifferentiated liposarcoma or solitary fibrous tumor (SFT). The speaker advises against using the term "fibrosarcoma NOS" or "adult type fibrosarcoma" unless the diagnosis is absolutely certain, suggesting "spindle cell sarcoma" as a safer alternative when features are ambiguous. The lecture outlines the three specific, modern contexts where the term "fibrosarcoma" is still considered acceptable: as a descriptor for a fibrosarcoma arising in a Dermatofibrosarcoma Protuberans (DFSP), for Infantile Fibrosarcoma (a prognostically favorable, molecularly defined tumor in infants with an ETV6-NTRK3 fusion), or when it is part of a proper tumor name like Mixed Fibrosarcoma or Sclerosing Epithelial Fibrosarcoma.

Fibrosarcoma Diagnosis in Modern Pathology

• 00:00:05 Historic Terminology: "Fibrosarcoma" historically described sarcomas with a herringbone fascicular pattern of spindle cells, once common but now "vanishingly rare."
• 00:00:30 Reclassification: Modern techniques (IHC, molecular testing) reclassify most historical fibrosarcomas as other tumors, such as dedifferentiated liposarcoma, synovial sarcoma (monophasic type), or cellular schwannoma/CFS.
• 00:01:15 Rarity in Practice: The speaker, a soft tissue pathologist, states they have never diagnosed "fibrosarcoma NOS" or "adult type fibrosarcoma" in ten years, suggesting a diagnosis of "spindle cell sarcoma" is safer if unsure.
• 00:01:47 Acceptable Modern Diagnoses: The term fibrosarcoma is still appropriate only in three specific contexts:
  • DFSP-related: As a descriptor for fibrosarcoma arising in a Dermatofibrosarcoma Protuberans (DFSP).
  • Infantile Fibrosarcoma: A distinct, rare sarcoma in infants defined by the $ETV6-NTRK3$ gene fusion, which carries a good prognosis.
  • Specific Tumor Names: When the term is part of a proper tumor name (e.g., Mixed Fibrosarcoma or Sclerosing Epithelial Fibrosarcoma), which are distinct entities.