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Solitary fibrous tumor

Authors: Brenndan Crumley, M.D., M.P.H., Katherine Schwetye, M.D., Ph.D.

Editorial Board Member: Meaghan Morris, M.D., Ph.D.

Deputy Editor-in-Chief: Chunyu Cai, M.D., Ph.D.

Last author update: 27 April 2023

Last staff update: 11 October 2023

Copyright: 2002-2024, PathologyOutlines.com, Inc.

PubMed Search: Solitary fibrous tumor pathology

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Table of Contents

Cite this page: Crumley B, Schwetye K. Solitary fibrous tumor. PathologyOutlines.com website. https://www.pathologyoutlines.com/topic/cnstumorsft.html. Accessed April 26th, 2024.

Definition / general

Solitary fibrous tumors (SFTs) of the CNS are spindle cell neoplasms with a wide range of histopathologic appearances and a pathognomonic NAB2::STAT6 fusion mutation with a characteristic immunophenotype

Essential features

Spindle cell neoplasm that ranges in stromal collagenization and cellularity, often with a patternless pattern or dilated staghorn vessels
Most common in the dura (especially the tentorium) but very rare compared to other primary meningeal tumors (Acta Neuropathol 2019;137:307)
Pathognomonic NAB2::STAT6 fusion mutation that separates it from other tumors with similar microscopic appearances (Acta Neuropathol 2019;137:307)
Positive IHC includes STAT6 (nuclear), CD34 and CD99
Good prognosis but metastasis is possible, even with low grade tumors (Acta Neuropathol 2019;137:307)

Terminology

Recent changes to the CNS WHO classification system have made solitary fibrous tumor the sole name for this entity (Neuro Oncol 2021;23:1231)
Formerly called solitary fibrous tumor / hemangiopericytoma (SFT / HPC) and angioblastic meningioma
Hemangiopericytoma has been removed from the most recent WHO CNS tumor classification

ICD coding

ICD-O: 8815/1 - solitary fibrous tumor
ICD-10 codes no longer listed by the WHO
ICD-11: 2F7C & XH7E62 - neoplasms of uncertain behavior of connective or other soft tissue & solitary fibrous tumor, NOS
ICD-11: 2B5Y & XH1HP3 - other specified malignant mesenchymal neoplasms & solitary fibrous tumor, malignant

Epidemiology

< 5% of extra-axial CNS tumors (meningiomas are the most common) (J Clin Neurosci 2014;21:1310)
Most common in patients in their 40s - 60s (Brain Pathol 2019;29:18)
M:F = 1.08 (Brain Pathol 2019;29:18)

Sites

Affects the dura most frequently (mainly found in the tentorium)
Other sites include falx, other dural sites, skull base, cerebellum and pineal gland

Pathophysiology

Pathognomonic NAB2::STAT6 fusion mutation on 12q13
Exact pathogenesis of this neoplasm has not been fully elucidated apart from its spindle cell / fibroblastic nature

Etiology

No known familial association

Diagrams / tables

Images hosted on other servers:

Intracranial SFT vascular permeability

Clinical features

Patients most commonly present with symptoms related to mass effect

Diagnosis

Patients are given a preliminary, often nonspecific diagnosis from CNS imaging
Found either during a primary workup or incidentally as part of a separate workup
Typically made by (immuno)histologic examination but can be supplemented with molecular testing for the NAB2::STAT6 mutation

Laboratory

Minority of cases present with hypoglycemia due to IGF1 production but this is nonspecific (Hum Pathol 1996;27:858)

Radiology description

Common: dural tail sign (nonspecific) and clear demarcation from surrounding tissue
Some cases will show a lack of obvious demarcation
Isodense on noncontrast T1 weighted images but bright on T1 with contrast and T2 weighted images
Meningiomas may also show a dural tail sign and similar enhancement patterns but are more likely to show intratumoral calcification and spoked wheel vessels (Neuroradiology 2021;63:1215)

Radiology images

Images hosted on other servers:

MRI of spinal SFT

MRI of parieto-occipital SFT

MRI of expansive SFT

PET / CT after SFT resection

Vividly enhancing mass, various images

Extra-axial mass, various images

Tumor mass in the left cerebellum

Resection cavity with residual tumor tissue

Prognostic factors

> 75% of patients survive 10 years after diagnosis, with shorter survival time associated with higher tumor grade (Neurosurg Rev 2021;44:1299)
< 50% of tumors recur; the exact rate is cohort dependent (Neurosurg Rev 2021;44:1299)
Literature is inconclusive as to whether spinal or brain tumors have a better prognosis (Neurosurg Rev 2021;44:1299, J Neurooncol 2019;143:457)
SFTs can metastasize outside the CNS or to other locations within the CNS (13.3% according to one case review), with metastasis more likely in higher grade and spinal tumors (Neurosurg Rev 2021;44:1299)
Differences in gene expression associated with increased vascular permeability are thought to increase metastasis risk compared to SFTs outside the CNS (Cancers (Basel) 2021;13:1142)

Case reports

10 year old boy with an intradural extramedullary tumor of the cervical spine (J Neurosurg Pediatr 2017;19:339)
26 year old woman with an extra-axial mass found during pregnancy, causing brain herniation (Case Rep Womens Health 2021;29:e00285)
45 year old woman with a tentorial mass in the right parieto-occipital region (Int J Surg Case Rep 2019;54:10)
53 year old woman with a spindle shaped extradural lesion in the cervical spine (BMC Surg 2021;21:405)

Treatment

All tumors are treated with total resection
Higher grade tumor treatment is often supplemented with adjuvant radiation
Patients may be monitored for many years and even decades to evaluate for recurrence and metastasis

Clinical images

Images hosted on other servers:

Spinal SFT during resection

Gross description

Discrete mass, often contiguous with dura, white to brown
Gross appearance varies based on the relative amount of stromal collagen and tumor cellularity
Some tumors can lose their dural attachment and infiltrate surrounding tissue (Ann Diagn Pathol 2003;7:169)

Gross images

Images hosted on other servers:

SFT metastatic to liver

Frozen section description

Characteristic feature: spindled cells without a single / discrete architecture (patternless pattern)
Enlarged vascular spaces (staghorn vessels) can suggest SFT
Typical frozen diagnosis: spindle cell neoplasm, with differential to include SFT, meningioma and possibly other (schwannoma, etc.) depending on location

Intraoperative frozen / smear cytology images

Contributed by Brenndan Crumley, M.D., M.P.H. and Katherine Schwetye, M.D., Ph.D.

Typical findings

Spindle cells

Spindled neoplastic cell aggregate

Images hosted on other servers:

Squash prep

Microscopic (histologic) description

Essential: patternless pattern of spindle cells, often solid but can also have papillary and other architectures, arranged around branching hyalinized vessels
Varying amounts of stromal collagen and overall cellularity
Nuclei are bland, without features seen in mimicking neoplasms
WHO grading criteria:
- < 5 mitoses/10 high power fields (HPF) = grade 1
- ≥ 5 mitoses/10 HPF = grade 2
- ≥ 5 mitoses/10 HPF with necrosis = grade 3
Often has staghorn vasculature

Microscopic (histologic) images

Contributed by Brenndan Crumley, M.D., M.P.H. and Katherine Schwetye, M.D., Ph.D.

Hypercellular neoplasm, patternless pattern

Neoplasm, haphazard spindle cells

Hypercellularity and staghorn vessels

CD34 positivity can be focal

STAT6 nuclear positivity

Virtual slides

Images hosted on other servers:

Fibrous dural mass

Occipital dural mass

Occipital dural mass, CD34

Occipital dural mass, reticulin

Cytology description

Lower grade tumors will show the characteristic patternless pattern but there is not a standardized list of cytology criteria for SFT diagnosis (J Pathol Transl Med 2019;53:192)
Higher grade tumors can show sheets of monotonous cells with heterochromatic nuclei, often with rhabdoid cells and eosinophilic cytoplasm (J Pathol Transl Med 2019;53:192)

Positive stains

CD34 (lost in some high grade tumors) (Am J Surg Pathol 1997;21:1354)
STAT6 (nuclear, lost in some dedifferentiated tumors) (Int J Surg Pathol 2017;25:58)
Vimentin
CD99
SMA (tumor periphery)
BCL2

Negative stains

SSTR2A
S100 (up to 26% some S100 positivity in the CNS) (Iran J Pathol 2016;11:195)
EMA (up to 29% some EMA positivity in the CNS) (Iran J Pathol 2016;11:195)

Molecular / cytogenetics description

NAB2::STAT6 fusion mutation can be detected by sequencing, RT PCR or a proximity ligation assay
NAB2 and STAT6 are close to each other on chromosome 12q, which can make genetic analysis difficult and the WHO considers a positive STAT6 immunostain to be sufficient for diagnosis in most cases (Nat Genet 2013;45:131)

Molecular / cytogenetics images

Images hosted on other servers:

NAB2::STAT6 mutation analysis results

NAB2::STAT6 RT PCR and sequencing

Videos

General overview of solitary fibrous tumors (not specific to CNS tumors) by Dr. Jerad Gardner

Sample pathology report

Brain, tentorium, resection:
- Solitary fibrous tumor, CNS WHO grade 1 (see comment)
- Comment: Sections show a neoplasm composed of bland spindle cells within a heavily collagenized stroma, with 3 mitoses in 10 high power fields. A panel of immunohistochemical stains is strongly positive for CD34 and STAT6 (nuclear), with focal positivity for SMA and is negative for EMA and SSTR2A. These findings are consistent with a solitary fibrous tumor.

Differential diagnosis

(Fibrous) meningioma (closest histologic mimic):
- STAT6-, EMA+, SSTR2A+
- Often shows psammoma bodies / calcifications, which are not seen in solitary fibrous tumor
Dural Ewing sarcoma:
- STAT6-, evidence of EWSR1 rearrangement
Synovial sarcoma:
- EMA+, TLE1+, evidence of SS18 rearrangement
Malignant peripheral nerve sheath tumor:
- Focal SOX10 and S100 positivity, STAT6-
Mesenchymal chondrosarcoma:
- Nests of STAT6- small round cells with rare cartilage islands
Non-SFT metastasis:
- Immunophenotype of the primary tumor, STAT6-

Additional references

WHO Classification of Tumours Editorial Board: Central Nervous System Tumours, 5th Edition, 2022

Board review style question #1

This STAT6 stained slide is taken from the excision of an intracranial neoplasm. Which of the following stains should be negative in this neoplasm?

ALDH1
CD34
SMA
SSTR2A

Board review style answer #1

D. SSTR2A. A is incorrect because ALDH1 stains are often positive in solitary fibrous tumors, with ALDH1 acting as a CD34 surrogate. B is incorrect because CD34 stains are positive in the vast majority of solitary fibrous tumors, with loss of expression seen in some higher grade tumors. C is incorrect because SMA can be expressed on the periphery of solitary fibrous tumors. SSTR2A is correct because this marker's positive staining in some meningiomas and complete lack of staining in solitary fibrous tumors can help distinguish between the two diagnostic entities.

Comment Here

Reference: Solitary fibrous tumor

Board review style question #2

What is the most common location of the extra-axial, STAT6 positive CNS neoplasm seen in this image?

Cerebellum
Pineal gland
Spinal cord
Tentorium

Board review style answer #2

D. Tentorium. A, B and C are incorrect because while solitary fibrous tumors can be found in the cerebellum, pineal gland and spinal cord, it is not their most common location. D is correct because solitary fibrous tumors are most common in the dura, and the dural site they are most common in is the tentorium.

Comment Here

Reference: Solitary fibrous tumor

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