STBT5:Solitary fibrous tumour

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Primary Author(s)*

Reba Daniel and Shashi Shetty

WHO Classification of Disease

Structure	Disease
Book	WHO Classification of Soft Tissue and Bone Tumours (5th Edition)
Category	Soft tissue tumours
Family	Fibroblastic and myofibroblastic tumours
Type	Solitary fibrous tumour
Subtype(s)	None

Definition / Description of Disease

Solitary fibrous tumor (SFT) is a mesenchymal spindle cell neoplasm that can develop at any site in the body, including soft tissue, visceral organs, bone, etc.  It was first described by Klemperer and Rabin in 1992 as a tumor of the pleura. However, since then this entity has been increasingly described from extrapleural sites. A morphologic clue to the diagnosis is the distinctive branching “staghorn” vessels. SFT is characterized by NAB2::STAT6 fusion resulting from a paracentric inversion at chromosome 12q13q13.

Synonyms / Terminology

Formerly SFTs were categorized as hemangiopericytomas.

Epidemiology / Prevalence

SFT occurs most commonly in adults with no gender predilection. Incidence of the tumor is highest in the age group of 40-70 years.

Clinical Features

Signs and Symptoms	SFTs present as slow-growing, painless neoplasms. Clinical symptoms can be due to mass effect in the site of involvement. e.g. Abdomen/Pelvis: abdominal distention, constipation, urinary retention Head/Neck: Dysphonia, nasal obstruction, dysphagia
Laboratory Findings	EXAMPLE: Cytopenias EXAMPLE: Lymphocytosis (low level)

Sites of Involvement

SFTs may occur at any site of the body. Involvement of head and neck, deep soft tissues, abdominal cavity, retroperitoneum, pelvis, bone, and visceral organs have been reported.

Morphologic Features

Morphologically, biopsy or resection specimens will show a bland spindle cell proliferation arranged haphazardly in a background of collagenous stroma. Cells contain ovoid nuclei within eosinophilic cytoplasm with indistinct cell borders. Tumors can vary in cellularity and stromal collagen. On low power, branching, “staghorn” shaped vessels can often be appreciated. Mitotic counts are usually low. Myxoid and lipomatous change have been described. SFT with adipocytic component are referred to as fat-forming (lipomatous) SFTs. There is also a variant known as giant cell rich SFT, which has the classic patternless spindle cell proliferation admixed with multinucleated giant cells. Dedifferentiated SFTs will show conventional SFT transitioning to a high-grade pleomorphic variant. Heterologous elements may be present. Standard immunophenotypic expression of CD34 and STAT6 is often lost in the dedifferentiated component.

Immunophenotype

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Finding	Marker
Positive (universal)	CD34
Positive (universal)	STAT6 (nuclear)
Negative (universal)
Negative (subset)

Chromosomal Rearrangements (Gene Fusions)

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Chromosomal Rearrangement	Genes in Fusion (5’ or 3’ Segments)	Pathogenic Derivative	Prevalence	Diagnostic Significance (Yes, No or Unknown)	Prognostic Significance (Yes, No or Unknown)	Therapeutic Significance (Yes, No or Unknown)	Notes
inv(12)(q13q13)	3'STAT6 / 5'NAB2	NA	55-100%	Yes	Unknown	No	Many different breakpoints in the exons and introns are associated with this fusion. Ex: NAB2ex4-STAT6ex2; NAB2ex6-STAT6ex16/17

Individual Region Genomic Gain / Loss / LOH

Not Applicable

Chr #	Gain / Loss / Amp / LOH	Minimal Region Genomic Coordinates [Genome Build]	Minimal Region Cytoband	Diagnostic Significance (Yes, No or Unknown)	Prognostic Significance (Yes, No or Unknown)	Therapeutic Significance (Yes, No or Unknown)	Notes

Characteristic Chromosomal Patterns

Not Applicable

Chromosomal Pattern	Diagnostic Significance (Yes, No or Unknown)	Prognostic Significance (Yes, No or Unknown)	Therapeutic Significance (Yes, No or Unknown)	Notes

Gene Mutations (SNV / INDEL)

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Gene; Genetic Alteration	Presumed Mechanism (Tumor Suppressor Gene [TSG] / Oncogene / Other)	Prevalence (COSMIC / TCGA / Other)	Concomitant Mutations	Mutually Exclusive Mutations	Diagnostic Significance (Yes, No or Unknown)	Prognostic Significance (Yes, No or Unknown)	Therapeutic Significance (Yes, No or Unknown)	Notes
	EXAMPLE: TSG	EXAMPLE: 20% (COSMIC) EXAMPLE: 30% (add Reference)	EXAMPLE: IDH1 R123H	EXAMPLE: EGFR amplification	EXAMPLE: Yes	EXAMPLE: No	EXAMPLE: No	EXAMPLE: Excludes hairy cell leukemia (HCL) (add reference).

Note: A more extensive list of mutations can be found in cBioportal (https://www.cbioportal.org/), COSMIC (https://cancer.sanger.ac.uk/cosmic), ICGC (https://dcc.icgc.org/) and/or other databases. When applicable, gene-specific pages within the CCGA site directly link to pertinent external content.

Epigenomic Alterations

Not Applicable

Genes and Main Pathways Involved

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Gene; Genetic Alteration	Pathway	Pathophysiologic Outcome
NAB2::STAT6; Activating mutation	EGR Pathway	Increased activation of EGR1
EXAMPLE: CDKN2A; Inactivating mutations	EXAMPLE: Cell cycle regulation	EXAMPLE: Unregulated cell division
EXAMPLE: KMT2C and ARID1A; Inactivating mutations	EXAMPLE: Histone modification, chromatin remodeling	EXAMPLE: Abnormal gene expression program

Genetic Diagnostic Testing Methods

Ancillary studies such as immunohistochemistry and molecular tests are useful in differentiating soft tissue tumors. SFTs have historically been diagnosed by morphology and strong diffuse CD34 positivity. Additional immunohistochemical phenotype previously used for identification included expression of Bcl2, CD99, and vimentin and absence of expression of epithelial, muscle, and neural markers. However, the introduction of STAT6 (signal transducer and activator of transcription 6) immunostain now dominates due to its high sensitivity and specificity. STAT6 expression is demonstrated by nuclear staining. Molecular testing by next generation sequencing is also highly useful in detecting the NAB2::STAT6 fusion. Breakapart fluorescence in situ hybridization (FISH) probe for STAT6 can detect rearrangement of this gene.  In the context of SFT, the rearrangement of the STAT6 gene is highly suggestive for the presence of NAB2-STAT6 fusion. A dual color dual fusion probe targeting both genes would be a direct confirmation for NAB2-STAT6 fusion.

Familial Forms

Not Applicable

Additional Information

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Links

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References

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Notes

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