Thyroid & parathyroid

Other uncommon lesions / neoplasms

Solitary fibrous tumor



Last author update: 12 May 2023
Last staff update: 12 May 2023

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PubMed Search: Solitary fibrous tumor

Truong Phan Xuan Nguyen, M.D.
Huy Gia Vuong, M.D., Ph.D.
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Cite this page: Nguyen TPX, Vuong HG. Solitary fibrous tumor. PathologyOutlines.com website. https://www.pathologyoutlines.com/topic/thyroidsft.html. Accessed April 14th, 2024.
Definition / general
  • Solitary fibrous tumor (SFT) of the thyroid is a fibroblastic mesenchymal neoplasm characterized by NAB2::STAT6 gene fusion
  • Thyroid SFT is indistinguishable from pleural or other extrapleural solitary fibrous tumors
Essential features
  • Fibroblastic tumor composed of collagen producing spindle cells arranged in a characteristic vascular pattern
  • Immunohistochemical nuclear expression of STAT6 is highly sensitive and specific for the presence of the NAB2::STAT6 gene fusion product; STAT6 is a marker for confirming the diagnosis of solitary fibrous tumor
  • < 10% cases of SFT of thyroid may show malignant histological features
Terminology
  • Extrapleural solitary fibrous tumor
  • Not recommended: hemangiopericytoma; giant cell angiofibroma; benign solitary fibrous tumor
ICD coding
  • ICD-O
    • 8815/0 - solitary fibrous tumor, benign
    • 8815/1 - solitary fibrous tumor, NOS
    • 8815/3 - solitary fibrous tumor, malignant
  • ICD-10: D48.7 - neoplasm of uncertain behavior of other specified sites
Epidemiology
Sites
  • Entire thyroid
Etiology
Diagrams / tables

Images hosted on other servers:

4 variable risk stratification

Clinical features
Diagnosis
  • Based on histological findings and NAB2::STAT6 gene fusion (immunohistochemical or molecular tests)
Laboratory
  • Serum thyroid function tests such as thyroid stimulating hormone (TSH), triiodothyronine (T3), free thyroxine (T4) and thyroperoxidase (TPO) antibodies are normal (Endocr Pathol 2011;22:165)
Radiology description
Radiology images

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Ultrasound: heterogeneous, hyperechoic solitary nodule

CT: heterogeneous enhanced lesion

Prognostic factors
  • High mitotic index, tumor cellularity, necrosis, nuclear pleomorphism correlated with metastatic or local recurrence potential
  • Difficult to predict patient's clinical course if based solely on histologic parameters (Ann Surg Oncol 2017;24:3865)
  • Age ≥ 55, tumor size ≥ 10 cm, mitotic count ≥ 4/10 high power fields, tumor necrosis ≥ 10% are unfavorable prognostic factors in SFTs of all anatomic sites (Mod Pathol 2017;30:1433)
  • TERT promoter mutations may be associated with a poor prognosis (Mod Pathol 2016;29:1511, Histopathology 2018;73:843)
Case reports
Treatment
Gross description
  • Well circumscribed mass
  • Relatively large (5 - 10 cm)
  • White to gray-brown color, usually solid, sometimes with cystic spaces (Head Neck Pathol 2019;13:597)
Gross images

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Solid, well circumscribed white nodule

Encapsulated, tan-pink to tan-white mass

Microscopic (histologic) description
  • Similar to SFT of any other organ
  • Usually unencapsulated
  • Proliferation of spindled cells with alternating hypo and hypercellular areas
    • Tumor cells are spindled with elongated, slender nuclei surrounded by scant cytoplasm; nuclear chromatin is delicate, fine to vesicular
    • Patternless architecture interspersed with loosely to densely collagenized or myxoid stroma
  • Highly vascular, with dilated, branching, hyalinized staghorn-like (hemangiopericytoma-like), thin walled vessels
  • Infiltration by inflammatory cells (mainly mast cells) can be seen
  • Mitoses and nuclear atypia are rare; necrosis is absent or rare
  • Uncommon: cysts, myxoid change, lipomatous features (Thompson: Diagnostic Pathology - Head and Neck, 2nd Edition, 2016)
  • High mitotic index (> 4 mitoses per 10 high power fields); hypercellularity, nuclear atypia; hemorrhage and necrosis are suggestive of malignant transformation (Am J Surg Pathol 1998;22:1501)
Microscopic (histologic) images

Contributed by Truong Phan Xuan Nguyen, M.D.
Well defined border

Well defined border

Entrapped thyroid follicles

Entrapped thyroid follicles

Patternless proliferation of spindled cells Patternless proliferation of spindled cells

Patternless proliferation of spindled cells


Staghorn-like vessels Staghorn-like vessels

Staghorn-like vessels

Collagenous stroma

Collagenous stroma

STAT6

STAT6

CD34

CD34

Virtual slides

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Solitary fibrous tumor of thyroid

Cytology description
Cytology images

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Scattered spindle-like cells with fusiform nuclei

Scattered spindle-like cells with fusiform nuclei

Positive stains
Negative stains
Molecular / cytogenetics description
  • Multiple NAB2::STAT6 gene fusion variants detected by whole exome sequencing and RT-PCR (Am J Pathol 2014;184:1209)
  • Due to the proximity of the 2 genes on chromosome 12, NAB2::STAT6 gene fusion may be missed by fluorescence in situ (J Clin Pathol 2017;70:508)
  • Moderate to strong and diffuse immunohistochemistry expression of STAT6 correlated with the NAB2::STAT6 gene fusion (Pathol Res Pract 2017;213:1404)
Molecular / cytogenetics images

Images hosted on other servers:

NAB2::STAT6 gene fusion variants

Sample pathology report
  • Thyroid, completion thyroidectomy:
    • Solitary fibrous tumor of thyroid, 35 mm, margins of resection are uninvolved (see comment)
    • Comment: The histological sections show a well circumscribed mass made of moderately cellular spindle cells in a collagenous stroma with prominent dilated staghorn type vasculature. No mitosis or necrosis is seen. Benign thyroid parenchyma is uninvolved and compressed at periphery of the tumor. Immunohistochemistry studies show that the tumor cells are positive for STAT6, CD34 and BCL2 and negative for TTF1, thyroglobulin, calcitonin, cytokeratins, desmin and S100. The overall findings favor a solitary fibrous tumor, likely thyroid primary. According to the 2017 risk stratification criteria by Demicco et. al., this should be classified as a low risk solitary fibrous tumor.
Differential diagnosis
Board review style question #1

A 50 year old woman presented with a 4.5 cm, well circumscribed, firm mass in the right lobe of thyroid. Histologic sections showed ovoid to spindled cells arranged haphazardly around prominent thin walled, hyalinized, dilated staghorn type vasculature (see image above). Which of the following is the diagnosis of this thyroid tumor?

  1. Medullary carcinoma
  2. Riedel thyroiditis
  3. Solitary fibrous tumor
  4. Spindle epithelial tumor with thymus-like differentiation
  5. Anaplastic thyroid carcinoma, paucicellular variant
Board review style answer #1
C. Solitary fibrous tumor. This case has gross appearance and histological features consistent with solitary fibrous tumor of thyroid. Differential diagnosis includes other thyroid tumors with the spindle cell pattern.

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Reference: Solitary fibrous tumor
Board review style question #2
Which of the following is the most sensitive and specific immunohistochemical marker for solitary fibrous tumor of the thyroid?

  1. CD34
  2. Cytokeratin CAM 5.2
  3. STAT6
  4. Thyroglobulin
  5. TTF1
Board review style answer #2
C. STAT6. STAT6 is a highly sensitive and specific marker for the presence of the NAB2::STAT6 gene fusion product, which is identified to be the pathognomonic molecular aberration in solitary fibrous tumor.

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Reference: Solitary fibrous tumor
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