Solitary Fibrous Tumour

 

Epidemiology:

    • Middle aged adults, 20-70 years

 

Common sites:

    • Ubiquitous
    • Pleura (visceral pleura most common)
    • lung
    • subcutaneous

 

Gross features:

    • pleura:
      • attached to pleura by a pedicle often
      • confined to surface of the lung usually
      • no pleural effusion usually
      • imaging: sharply demarcated with no chest wall abnormality
    • dense fibrous tissue
    • occasional cysts filled with viscous fluid
    • well-delineated
    • multinodular
    • whorled
    • whitish grey
    • firm
    • myxoid and hemorrhagic changes occasionally
    • necrosis and infiltrative borders in malignant tumours
    • solitary usually (may be multiple)
    • size variable (often > 10 cm)

 

Histologic features:

    • “patternless” architecture
      • Varying cellularity hypocellular and hypercellular areas separated by thick bands of collagen (hyalinized, somewhat keloidal)
      • whorls of reticulin and collagen fibres
      • interspersed spindle cells resembling fibroblasts
    • thin-walled “staghorn” branching vessels
      • varying sizes and numbers
      • perivascular hyalinization common
    • myxoid areas (may be predominant rarely)
    • fibrotic areas
      • stromal hyalinization variable
      • keloidal-type collagen deposition frequent
    • interstitial mast cells
    • tumour cell features:
      • non-atypical / bland
      • round to spindle-shaped
      • little cytoplasm
      • indistinct borders
      • vesicular nuclei
        • dispersed chromatin
    • scarce mitoses (<3/10HPF) (2 mm2)
    • mature adipocytes in some
    • giant multinucleated stromal cells in some
    • malignant features:
      • high mitotic rate (>=4/10HPF) (2 mm2) (most reliable indicator)
      • hypercellular (usually)
      • mod to marked cytologic atypia (often this is limited or focal) (not reliable)
      • necrosis (infrequent)
      • infiltrative margins
      • large size (>10cm) (not reliable)
      • dedifferentiation rarely
      • isolated cases with benign morphology may give rise to metastasis
    • hemangiopericytoma (not quite fitting into SFT):
      • resembles cellular areas of SFT
      • numerous, variably ecstatic or compressed, thin-walled branching vessels often with a staghorn configuration
      • closely packed cells
      • no hyalinization
      • variable mitotic rate
      • some with a prominent adipocytic component
    • DDx:
      • Synovial sarcoma
      • Sarcomatoid mesothelioma
      • Nerve sheath tumours
      • Thymoma (type A)

 

Immunophenotype:

Marker:

Sensitivity:

Specificity:

STAT6

> 95%

good

CD34 

90-95% 

Nonspecific

CD99

70%

nonspecific

Reticulin

 

 

SMA (neg)

Some are positive

 

EMA

Some

 

Keratin

Some

 

S100

Some

 

Desmin

Some

 

 

Molecular features:

    • NAB2-STAT6 gene fusion (characteristic and specific)
      • inv(12)(q13q13)
      • not detectable by conventional karyotype
      • leads to STAT6 overexpression
    • IGF2 overexpression (some cases)
      • Loss of IGF2 imprinting

 

Other features:

    • most are benign
      • up to 10%  malignant
      •  
    • Hemangiopericytoma is an old term referring to lesions that mostly fit into solitary fibrous tumour, and others.  Most SFTs were called hemangiopericytomas in the past.

 

References:

    • WHO book 2002