This lecture segment addresses the historical and contemporary diagnosis of "fibrosarcoma" in soft tissue pathology. The speaker emphasizes that the classic definition—a sarcoma showing a herringbone fascicular pattern of spindle cells—is now exceedingly rare, as modern techniques (immunohistochemistry and molecular testing) typically reclassify these lesions as other entities like dedifferentiated liposarcoma, synovial sarcoma, or cellular DFSP. The speaker personally avoids the diagnosis of "fibrosarcoma NOS" or "adult type fibrosarcoma" due to its statistical rarity and high probability of misdiagnosis, preferring terms like "undifferentiated spindle cell sarcoma." The only acceptable modern uses of the term fibrosarcoma are when it refers to Dermatofibrosarcoma Protuberans (DFSP), Infantile Fibrosarcoma (a specific ETV6-NTRK3 fusion positive tumor in infants with a good prognosis), or as part of established tumor names like myxofibrosarcoma or sclerosing epithelial fibrosarcoma.
Diagnosis and Reclassification of Fibrosarcoma in Modern Soft Tissue Pathology
00:00:09 Historical Context: Fibrosarcoma was historically a common diagnosis for soft tissue sarcomas exhibiting a herringbone fascicular pattern of spindle cells.
00:00:30 Reclassification: Under modern pathological techniques (IHC, molecular studies), most lesions formerly called fibrosarcoma are now reclassified as other entities (e.g., dedifferentiated liposarcoma, synovial sarcoma, cellular DFSP).
00:01:15 Extreme Rarity: The speaker states that "adult type fibrosarcoma" is vanishingly rare, and they have never personally made this diagnosis in ten years of practice.
00:01:38 Recommendation for Ambiguity: If the diagnosis is unclear and doesn't fit specific criteria, the speaker suggests using the non-specific term "spindle cell sarcoma" or "undifferentiated spindle cell sarcoma" rather than fibrosarcoma NOS.
00:01:49 Acceptable Modern Diagnoses: There are three specific contexts where using the term fibrosarcoma is considered appropriate in modern pathology:
DFSP: Fibrosarcoma arising in a Dermatofibrosarcoma Protuberans (DFSP).
Infantile Fibrosarcoma: A highly specific, rare tumor in infants defined by the $ETV6-NTRK3$ gene fusion, which carries a good prognosis.
Specific Tumor Names: When fibrosarcoma is part of a definitive tumor name, such as myxofibrosarcoma or sclerosing epithelial fibrosarcoma (which may be related to low-grade myxoid sarcoma).
00:02:56 Key Takeaway: The speaker advises against using the general "fibrosarcoma" diagnosis unless the case falls into one of the three defined categories.