Soft tissue tumors of uncertain origin

1. Synovial Sarcoma

Overview

• Malignant tumor of uncertain origin (NOT derived from synovium despite name).
• Can occur anywhere in the body, usually near large joints or in extremities (knee, thigh).
• Young adults (20–40 yr).

Pathogenesis

• Chromosomal translocation t(X;18) → fusion of SS18 (SYT) gene on chr 18 with SSX1, SSX2, or SSX4 genes on chr X.
• → Aberrant transcription factor activity.

Histology

• Three patterns:
  • Monophasic: spindle cells (mesenchymal) arranged in fascicles — most common.
  • Biphasic: spindle cells + gland-like epithelial structures.
  • Undifferentiated: highly cellular round cells.
• Mast cells commonly present.
• IHC: cytokeratin⁺, EMA⁺, TLE1⁺.

Clinical Features

• Usually asymptomatic — may present as a slowly growing lump or with arthritis-like symptoms when near a joint.
• Often misdiagnosed as soft tissue mass or synovitis.
• Metastasizes to lungs + bones.
• Treatment: surgical removal + radiation ± chemo.

2. Alveolar Soft Part Sarcoma (ASPS)

Overview

• Rare malignancy; usually adults.
• Slowly growing mass of deep soft tissues (thigh, buttock, head and neck in kids).

Pathogenesis

• t(X;17) translocation → ASPSCR1-TFE3 fusion gene.

Clinical Features

• Early metastases to brain — often the first manifestation of disease.
• Also mets to lung, bone.
• Indolent local growth but poor long-term prognosis.

Morphology

• Distinctive nesting (alveolar) pattern of cells separated by thin fibrovascular septa.
• Vesicular nucleus + prominent nucleoli.
• Clear / granular eosinophilic cytoplasm.
• PAS⁺ crystalline inclusions in cytoplasm (rod-shaped/rhomboid).
• IHC: TFE3⁺ (nuclear).

3. Comparison Table

Feature	Synovial sarcoma	Alveolar soft part sarcoma
Origin	Uncertain (not synovium)	Uncertain
Age	20–40 yr	Young adults
Site	Near large joints + extremities (knee/thigh)	Deep soft tissue (thigh, H&N kids)
Genetics	t(X;18) SS18-SSX	t(X;17) ASPSCR1-TFE3
Histology	Monophasic / biphasic spindle ± glands; mast cells	Alveolar nests, clear cytoplasm, PAS⁺ crystals
Metastases	Lungs + bones	Brain (early), lung, bone
Tx	Surgery + radiation	Surgery; chemoresistant; tyrosine kinase inhibitors

💡 High-yield: Synovial sarcoma = uncertain origin (NOT synovium!); young adults (20–40), near large joints + extremities; t(X;18) → SS18-SSX fusion; monophasic (spindle) vs biphasic (spindle + glandular) patterns; cytokeratin⁺ + TLE1⁺; mast cells common; lung/bone mets. Alveolar soft part sarcoma = rare; deep soft tissue; t(X;17) → ASPSCR1-TFE3 fusion; alveolar nesting pattern with clear cytoplasm + PAS⁺ crystals; early brain metastasis — often first sign; TFE3⁺.