Soft Tissue Sarcoma Classification
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Direct answer
Soft tissue sarcoma classification changed architecture in the WHO 5th edition (2020): tumours are arranged by lineage of differentiation — adipocytic, fibroblastic-myofibroblastic, vascular, smooth and skeletal muscle, nerve sheath, and a molecularly defined group for translocation-driven tumours. The old "fibrohistiocytic" family was dismantled, and malignant fibrous histiocytoma survives only as history — what was called MFH is now undifferentiated pleomorphic sarcoma, a diagnosis of exclusion. Grading for most adult-type sarcomas uses the French Federation (FNCLCC) system scoring differentiation, mitotic count and necrosis into grades 1-3, and reporting is checklist-driven: size, depth, grade, mitoses per 10 high-power fields, necrosis percentage and margin status (R0 versus R1) drive both staging and adjuvant decisions.
What you must remember
- FNCLCC grading: tumour differentiation score 1-3, mitotic count (1: 0-3, 2: 4-9, 3: ≥10 per 10 HPF) and necrosis (0: none, 1: <50%, 2: ≥50%); total 2-3 = grade 1, 4-5 = grade 2, 6-8 = grade 3.
- Diagnoses of exclusion: undifferentiated pleomorphic sarcoma, undifferentiated round cell sarcoma and dedifferentiated liposarcoma are reached only after a panel — pleomorphic sarcoma in a retroperitoneal or spermatic cord location demands MDM2 testing before the label is applied.
- Translocation sarcomas: synovial sarcoma t(X;18) SS18-SSX with TLE1 positivity; myxoid liposarcoma t(12;16) FUS-DDIT3; Ewing and Ewing-like sarcomas (EWSR1 with FLI1 or non-ETS partners); alveolar rhabdomyosarcoma FOXO1 fusions; desmoplastic small round cell tumour EWSR1-WT1 with polyphenotypic dot-like keratin and desmin.
- Lineage panels first: S100/SOX10 (nerve sheath and melanocytic), desmin plus myogenin (skeletal muscle), SMA/h-caldesmon (smooth muscle), CD34 (fibroblastic and vascular), ERG/CD31 (endothelial), STAT6 (solitary fibrous tumour), MDM2-CDK4 (adipocytic amplification).
- GIST is not staged like sarcoma: risk stratification uses site-specific mitotic-count tables — gastric GIST over 5 mitoses per 50 HPF and non-gastrical GIST over 5 (small bowel) behave very differently at identical size; KIT exon 11 mutations respond to imatinib, exon 9 needs higher dose, PDGFRA D842V is resistant.
- Margin language: R0 clear, R1 microscopic residual at ink, R2 gross disease — a 1 mm clear rim in sarcoma does not demand re-excision by itself; margin quality (fibrous versus fatty tissue) matters.
- Ancient terminology to unlearn: haemangiopericytoma (now solitary fibrous tumour), MFH, and "malignant triton" (MPNST with rhabdomyoblasts — still used descriptively).
Reading a limb sarcoma report line by line
Take an 8 cm deep thigh tumour in a 54-year-old. The report opens with site, size, depth and plane; a deep-fascia breach changes T staging in the AJCC 8th edition (size thresholds T1 ≤5 cm, T2 >5 cm, with nodal spread upstaging more aggressively than in the 7th). Histology next: a myxofibrosarcoma shows curvilinear vessels, atypical spindle cells and a mucin-rich tail infiltrating fat — its infiltrating edge explains why local recurrence exceeds that of equally-graded tumours, and why the surgeon resects with a 2 cm cuff rather than shelling it out. FNCLCC scoring then runs in front of you: poorly differentiated myxofibrosarcoma scores 2, six mitoses per 10 HPF scores 2, 20% necrosis scores 1 — total 5, grade 2.
The checklist ends with the decisions the numbers feed: grade 2-3 deep sarcomas over 5 cm justify neoadjuvant radiotherapy discussion; a positive margin after preoperative radiation means re-excision if anatomically possible; synovial or myxoid liposarcoma histology would push toward a molecular confirmation request, because fusion status is both diagnostic and, in myxoid liposarcoma, predictive of radiosensitivity.
How the exam frames it
Two patterns dominate. One is the definition question: asked to grade a sarcoma, candidates recite NCI (National Cancer Institute) criteria instead of FNCLCC — both exist, but FNCLCC is the international default and the only one examiners expect number-by-number. The second is the negative-exclusion stem: a pleomorphic sarcoma in the retroperitoneum is dedifferentiated liposarcoma until MDM2 FISH says otherwise — examiners love this because calling it "UPS" there forfeits the MDM2-amplified biology, including CDK4-inhibitor trial eligibility. Indian practice note: most district hospitals lack FNCLCC-discipline panels, so referral centres re-grade outside slides before adjuvant planning — a frequent viva question on reporting discrepancies.
Frequently asked questions
What three components make up the FNCLCC grade?
Tumour differentiation (1-3), mitotic count per 10 high-power fields (1-3) and necrosis extent (0-2), summed to grades 1-3 with totals 2-3, 4-5 and 6-8 respectively.
What replaced malignant fibrous histiocytoma?
Undifferentiated pleomorphic sarcoma — a diagnosis of exclusion made only after lineage and MDM2 testing exclude differentiated and dedifferentiated tumours.
Which molecular finding defines synovial sarcoma?
The t(X;18) SS18-SSX fusion, supported by diffuse TLE1 immunostaining and often cytokeratin and EMA dot positivity.
Why must GIST risk use site-specific tables?
Mitotic thresholds for aggressive behaviour differ by organ — 5 mitoses per 50 HPF is high risk in a gastric GIST at smaller sizes than in small-bowel tumours — so size, site and mitoses are read together.
What is an R1 margin?
Microscopic tumour at the inked resection margin; management depends on grade, site and whether radiotherapy was already given, not on automatic re-excision.