Solitary Fibrous Tumor
Key Points
Solitary fibrous tumor (SFT), previously known as hemangiopericytoma, comprises a histologic spectrum of fibroblastic mesenchymal neoplasms
The SFT group includes SFT and related lesions ranging from heterogeneous, multinodular, partially sclerotic tumors to monotonous, highly cellular ones
The etiology remains unknown, but data support the notion that these tumors are rare spindle cell neoplasms that arise from cells of mesenchymal origin
SFTs are associated with NAB2–STAT6 gene fusions that arise from recurrent intrachromosomal rearrangements on chromosome 12q13
They usually present as a painless, hard, immobile, slow-growing palpable subcutaneous mass and eyelid swelling and can cause mechanical obstruction to eyelid function, such as ectropion
Surgical management with wide and negative resection margins and the preservation of critical surrounding organs or tissues has been the mainstay treatment
The clinical behavior of SFT is generally benign, but recurrence can be seen and tumors considered histologically benign may behave more aggressively
Solitary fibrous tumor (SFT) comprises a histologic spectrum of fibroblastic mesenchymal neoplasms that rarely metastasize.1 Although Wagner2 provided the first detailed histologic description of a SFT tumor of the pleura in 1870, the lesion was only recognized as a distinct clinical entity by Klemperer and Rabin in 1931.3 It has been recognized under several different names in the past, including benign mesothelioma, localized mesothelioma, solitary fibrous mesothelioma, pleural fibroma, submesothelial fibroma, subserosal fibroma, and localized fibrous tumor.
Hemangiopericytoma (HPC) was first described in 1942 and initially believed by Stout and Murray4 to be a vascular tumor related to smooth muscle pericytes. However, over the years since its original description, it has become evident that HPC is simply a descriptive term to characterize a heterogeneous group of neoplasms that share a common growth pattern.5 Three categories have been individualized within this heterogeneous group of HPC-like neoplasms. One corresponds to some non-HPC neoplasms that occasionally display HPC-like features, such as synovial sarcoma, infantile liposarcoma, and leiomyosarcoma. Another contains lesions that show evidence of myoid/pericytic differentiation such as glomangiopericytoma/myopericytoma, infantile myofibromatosis, and some sinonasal tumors.5 The final category is the SFT group, which includes SFT and related lesions ranging from heterogeneous, multinodular, partially sclerotic tumors to monotonous, highly cellular ones.6,7,8,9,10 Proposed variants of SFT include the fibrous variant (conventional SFT), cellular variant (conventional HPC), a fat-forming variant (lipomatous HPC), and a giant cell-rich variant (giant cell angiofibroma).5 With the discovery of a unique fusion gene in SFT and other tumors histologically identified as HPC, these two tumor groups are now generally considered to be the same entity with the term “SFT” as the preferred name.1,11
SFT has an equal distribution among men and women12,13 and occurs in adults of all ages, most commonly in the fifth and sixth decades.12,13,14 They may be found in almost any site of the body with intrathoracic being the most common location where the pleura is most often involved, followed by lung parenchyma, mediastinum, and the diaphragm.12,15 The most common extrapleural site is the abdomen,6,12,16 followed by the trunk, extremities, head and neck, and intracranial sites.17,18,19,20,21,22,23,24,25 SFT arising in the head and neck most often arises in the sinonasal tract, oral cavity, orbit, or intracranially from the meninges.26,27,28,29,30 Primary eyelid SFT is uncommon with fewer than 40 cases reported.10,31,32,33,34,35,36,37,38 However, the majority of the lesions involving the eyelids represent anterior extensions or coexisting eyelid lesions associated with orbital tumors.33,39,40,41,42
Most SFTs behave benignly, but a minority of patients will develop metastatic disease to the lungs, bones, and liver.43 In lesions arising from the pleura, 13% to 23% are classified as malignant, whereas most extrapleural SFTs behave in a more benign fashion.7 Larger tumor size (>10 cm), high mitotic rates, increased cellularity, pleomorphism, hemorrhage, and necrosis are predictive of a greater potential for recurrence, local invasion, and metastatic spread.15,44
Etiology and Pathophysiology
The etiology of SFTs remains unknown but data support the notion that these tumors are rare spindle cell neoplasms that arise from cells of mesenchymal origin.7,45,46 In 1931, Klemperer and Rabin3 documented the occurrence of a distinctive localized pleural-based tumor and proposed a submesothelial cell origin. Later, Stout and Murray4 proposed a derivation from mesothelial cells based on tissue culture experiments. Since then, immunohistochemistry studies
have established a fibroblastic origin, occasionally with myofibroblastic differentiation.8
have established a fibroblastic origin, occasionally with myofibroblastic differentiation.8
SFTs are associated with NAB2-STAT6 gene fusions47,48 that arise from recurrent intrachromosomal rearrangements on chromosome 12q13.49 Gene expression analyses show that NAB2-STAT6 fusions lead to altered function of the transcriptional corepressor NAB2, an important regulator of the growth response 1 transcription factor,50 and SFTs have been shown to deregulate the expression of target genes of early growth response 1 and other developmentally important genes. The detection of this gene fusion in most cases of SFT strongly implies that this fusion gene may play a significant role in its pathogenesis.50 Several studies have suggested that different NAB2-STAT6 fusion types may be associated with different clinicopathologic subgroups of SFT.51 The most common fusion variant, NAB2ex4-STAT6ex2/3, corresponds to the classic pleural SFT,52 whereas tumors with NAB2ex6-STAT6ex16/17 are found in younger patients with deep soft tissue tumors that display a more aggressive clinical behavior.52
Steroid hormone receptors, particularly progesterone receptors, rarely are expressed in extrapleural SFTs.32,53 It has been proposed that progesterone may participate as a growth factor in many CD34+ neoplasms, and that progesterone receptor positivity is a feature of SFTs showing a higher proliferative activity and a trend toward recurrence.54
FIGURE 116.2 Solitary fibrous tumor (SFT) of the upper eyelid causing mechanical ptosis. (Courtesy of Dr. Robert A. Goldberg.)
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