Synovial Sarcoma

Definition

Keratin positivity
- Epithelial component either keratin of EMA positive in essentially 100% of cases
- Spindle cell component 50-80% positive
No more than mild pleomorphism
- Moderate pleomorphism may be seen following radiation
May be circumscribed or infiltrative
t(X;18;p11;q11) is definitional

All soft tissue tumors with this translocation are considered synovial sarcomas
Because of this new definitional criterion, the histologic features described may in the future be expanded
- See poorly differentiated pattern description

Contains both epithelial and spindle components
- Components may merge or be distinct
- Metastases may show different predominance of components
Epithelial component usually large pale or columnar cells
- Occasionally cuboidal, flat or spindled
- May form glands, tubules or papillae (rare)
  - May contain mucin, rarely mucin rich
- Rare focal squamous differentiation
- Round vesicular nuclei
Spindle component usually uniform small elongate plump cells
- Dark, even stippled chromatin
- Scant cytoplasm with indistinct cell margins
- Usually sheets or fascicles
  - Occasionally with nodules or myxoid foci
Characteristic stromal features usually present
- Thick ropy collagen bundles
  - Often surrounding cellular nodules
- Hemangiopericytoma-like vessels
- Calcification

Pure epithelial pattern rare to nonexistent
Pure spindle pattern contains spindle cells with above features
Requires at least one of the following:
- t(X;18;p11;q11)
- Keratin reactivity
- One of the three characteristic stromal features above
Frequently seen nonspecific features
- Palisading
- Pseudorosettes
- Herringbone pattern
- Retiform or microcystic pattern
- Metaplastic bone or cartilage
- Areas with slightly larger cells

Richard L Kempson MD
Robert V Rouse MD
Department of Pathology
Stanford University School of Medicine
Stanford CA 94305-5342

Original posting/updates: 7/31/07, 1/20/09, 8/22/09, 8/21/10, 3/8/12