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Advances in Anatomic Pathology:
doi: 10.1097/PAP.0b013e318229c004
Review Articles

Solitary Fibrous Tumor of the Central Nervous System: A 15-year Literature Survey of 220 Cases (August 1996–July 2011)

Bisceglia, Michele MD*; Galliani, Carlos MD; Giannatempo, Giuseppe MD; Lauriola, Walter MD; Bianco, Mario MD§; D'Angelo, Vincenzo MD§; Pizzolitto, Stefano MD; Vita, Giulia MD; Pasquinelli, Gianandrea MD#; Magro, Gaetano MD**; Dor, David Ben MD††

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Author Information

*Departments of Pathology

Radiology

§Neurosciences, IRCCS “Casa Sollievo della Sofferenza” Hospital, San Giovanni Rotondo (FG), Italy

Department of Pathology, Cook Children's Medical Center, Fort Worth, TX

Department of Pathology and Laboratory Medicine, “S. Maria della Misericordia” General Hospital, Udine, Italy

Unit of Anatomic Pathology, IRCCS-CROB, Referral Cancer Center of Basilicata, Rionero in Vulture (PZ), Italy

#Department of Hematology, Oncology and Clinical Pathology, Policlinico S. Orsola, University of Bologna, Bologna, Italy

**Department of Pathology, University of Catania, Catania, Italy

††Department of Pathology, Barzilai Medical Center, Ashkelon, Israel

The authors have no funding or conflicts of interest to disclose.

Reprints: Michele Bisceglia, MD, Department of Pathology, Division of Anatomic Pathology, IRCSS “Casa Sollievo della Sofferenza” Hospital, V.le Cappuccini, 71013 San Giovanni Rotondo, Italy (e-mail: bismi@libero.it).All figures can be viewed online in color at http://http://www.anatomicpathology.com.

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Abstract

We reviewed the world literature on solitary fibrous tumors of the central nervous system from August 1996 to July 2011, focusing on both clinicopathological features and diagnostic findings. The anatomical distribution of the 220 cases reported so far reveals that most are intracranial and just over one-fifth are intraspinal. In decreasing frequency, intracranial tumors involve the supratentorial and infratentorial compartments, the pontocerebellar angle, the sellar and parasellar regions, and the cranial nerves. Intraspinal tumors are mainly located in the thoracic and cervical segments. Although most solitary fibrous tumors of the central nervous system are dural based, a small subset presents as subpial, intraparenchymal, intraventricular, or as tumors involving the nerve rootlets with no dural connection. Preoperative imaging and intraoperative findings suggest meningioma, schwannoma or neurofibroma, hemangiopericytoma, or pituitary tumors. Immunohistochemistry is critical to establish a definitive histopathological diagnosis. Vimentin, CD34, BCL2, and CD99 are the most consistently positive markers. The usual histologic type generally behaves in a benign manner if complete removal is achieved. Recurrence is anticipated when resection is subtotal or when the tumor exhibits atypical histology. The proliferative index as assessed by MIB1 labeling is of prognostic significance. Occasionally, tumors featuring conventional morphology may recur, perhaps because of minimal residual disease left behind during surgical extirpation. Rare extracranial metastases and tumor-related deaths are on record. Surgery is the treatment of choice. Stereotactic and external beam radiation therapy may be indicated for postsurgical tumor remnants and for unresectable recurrences. Long-term active surveillance of the patients is mandatory.

Solitary fibrous tumor (SFT), a spindle-cell mesenchymal neoplasm of specialized fibroblastic lineage, was first described in 1931 by Klemperer and Rabin, under the term “localized fibrous mesothelioma,” as a distinct pathologic entity involving the pleura.1–3 Gradually, the ubiquitous distribution of this neoplasm4,5 emerged as SFTs were described arising in the mediastinum (with no attachment to the pleura), retroperitoneum, peritoneal and pericardial serosal cavities and visceral organs therein,6–18 somatic soft tissues,14–16,19–26 skin,27,28 breast,29 periostium,30 bone,31 and head and neck,14–16,19–20,23–25,32–43 including specific paracranial sites, such as the infratemporal fossa,44 orbit,45–52 and sinonasal cavities.20,53–59 Immunohistochemistry had a decisive role in unifying the phenotype, confirming equivocal cases, and putting an end to the controversy of SFT in extrapleural locations. The tumors usually immunoreacted positively with CD34,10,44,60,61 BCL2,62,63 or CD99.25 Seven cases of SFT involving the central nervous system (CNS) were published by Carneiro et al in 1996.64 All of them were meningeal based: 5 intracranial and 2 intraspinal. This seminal report engendered a flurry of publications of SFT of the CNS, some listed in series of extrapleural SFT from various sites.14–16,19–20,41 Most cases of SFT of the CNS reported were dural based, but some had no apparent meningeal connection. In this review, we intend to focus on the evolving histogenetic theories, demographic distribution, anatomical locations, preoperative diagnostic methods, histopathology features, biological behavior and prognostic factors, therapeutic strategies, outcome, and design of an appropriate follow-up schedule for patients with SFT of the CNS.

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REVIEW OF THE LITERATURE

A comprehensive PubMed-Medline search was performed using (extrapleural solitary fibrous tumor), (central nervous system and solitary fibrous tumor), (meninges and solitary fibrous tumor), (brain and solitary fibrous tumor), (intraventricular and solitary fibrous tumor), (spinal cord and solitary fibrous tumor), (medullary cord and solitary fibrous tumor), (intramedullary and solitary fibrous tumor), (solitary fibrous tumor and spine), and (solitary fibrous tumor and bone) as search terms for the period between August 1996 and July 2011. The bibliography of all the pertinent articles and previous reviews were surveyed for cases not captured by the PubMed computerized repository.

We found case reports, institutional case reviews, and literature reviews published after the initial paper by Carneiro et al.64 Among 220 cases of SFT of the CNS on record, 170 were intracranial and 50 intraspinal (Table 1). As previously stated, most reports documented single cases, some were series of SFT of the CNS,65–176 and a few SFT of the CNS were included in series of extrapleural SFT from systemic sites.14–16,19–20,41

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CLINICOPATHOLOGICAL ANALYSIS

Frequency, Demographics, Clinical Manifestations, Preoperative Investigations

The nomenclature applied to SFT of the CNS diagnosed before 1996 was probably fibrous meningioma or hemangiopericytoma (HPC). The SFT of the CNS is rare in comparison with other craniospinal tumors as attested by the retrospective institutional reviews of meningeal lesions or by the neurosurgical and neuroradiological reports on this topic, which have been published so far. Only 6 cases of SFT were found by Carneiro et al in a review of 6348 such tumors from the Mayo Clinic Tissue Registry (Rochester, MN) encompassing the period from 1930 to 1995.64 Tihan et al101 retrieved a single instance in their review of 323 cases from the files of The Johns Hopkins Hospital (Baltimore, MD) between 1985 and 1995. The largest series was collected by Metellus et al139 consisting of 16 cases from the combined experience of 2 institutions (Timone Hospital, Marseille and Centre Hospitalo Universitaire, Lyon, France) between 1999 and 2004. Weon et al143 treated 6 patients between 1995 and 2005 at the Samsung Medical Center, Seoul, Korea. Hayashi et al152 evaluated 1018 cases originally diagnosed as meningioma between 1970 and 2006 from 2 institutions (Kanazawa Hospital, Kanazawa and Kumamoto Hospital, Kumamoto, Japan) and retrieved 4 cases of SFT. Suzuki et al84 evaluated their experience at the Kyushu University Hospital in Fukuoka, Japan, between 1967 and 1999. They identified 2 cases of SFT among 46 fibrous meningiomas and 18 HPC (1 from each group of tumors). Vassal et al172 reported on 11 cases of SFT surgically treated between 2000 and 2008 at their University Hospital in Saint-Etienne, France. Clarençon et al reported on the neuroimaging findings of 9 cases of SFT operated at Pitié-Salpêtrière Hospital, Paris, France, from 2002 to 2007, initially imaged at 3 different institutions. Finally, one of the authors of this review (M.B.) and his associates from the Casa Sollievo della Sofferenza Hospital (San Giovanni Rotondo, Italy) retrieved and reclassified 806 primary meningeal-based tumors collected between June 1992 and December 2009 and found 786 meningiomas of all types, 15 HPC, 5 SFT, and 2 meningeal sarcomas not otherwise specified.176

Intracranial or intraspinal SFT of the CNS generally affects adults, with 172 of the 192 patients (89.58%) for whom the age was stated being ≥30 years of age. However, SFT may also arise in young adults, adolescents, and in children,101,108,130 with at least 20 patients being in the first 3 decades of life (10.41%), and 7 being ≤18 years of age (Table 1). In a single case the tumor was prenatally diagnosed, at 22 weeks of intrauterine life.171 Of 217 patients for whom the sex was known, 114 were female patients and 103 were male patients. The clinical presentation varied according to the specific localization and size. Headaches, disturbances of memory, loss of consciousness, impairment of parieto-cortical sensations, partial or generalized tonic-clonic seizures, side weakness, nausea, vomiting, anosmia, loss of visual acuity or impairment of visual fields, aphasia, dysarthria, neurovegetative dystonia, dizziness, tinnitus, vertigo, unstable gait or ataxia up to the full blown cerebello-vestibular syndrome, intracranial hypertension or hemiparesis were among the most common manifestations for intracranial tumors. Occasionally, cranial nerve palsies may be observed. For intraspinal tumors, the most common complaints were neck or back pain, dysesthesia, paresthesia, hypo-hyperalgesia, stiffness, limb weakness, motor deficits, progressive myelopathy, and spastic paraparesis depending on the level of spinal cord compression, with bilaterally deep tendon hypereflexia of the knees and ankles, and bilateral Babinski sign. The bulbocavernous reflex and bowel and bladder function were usually preserved, but occasionally impaired.114 Tumor-associated hypoglycemia, a rare paraneoplastic syndrome (so-called non-islet-cell-tumor hypoglycemia or Doege-Potter syndrome) due to tumor secretion of insulin-like growth factor 2,177 observed in approximately 4% of pleural SFT2,178,179 as an isolated manifestation or associated with clubbing of the fingers (Pierre-Marie-Bamberg syndrome),180,181 was reported only once in a 32-year-old man with a sellar SFT, who had a preoperative diagnosis of pituitary adenoma.162

The preoperative diagnosis of SFT is challenging, if not impossible. Computerized tomography (CT), magnetic resonance imaging (MRI), and angiography are essential for operative planning. There is no reliable neuroradiologic sign; only awareness of the entity and subtle neuroimaging clues might suggest the preoperative diagnosis of SFT to an astute observer.143,163 CT scan without contrast may show heterogeneous, isodense, and hyperdense tumors. MRI often shows an isointense signal on T1-weighted images and low-intensity signal on T2-weighted images; however not infrequently, different findings have been reported, possibly reflecting the heterogeneous histological paterns (SFT of the pleura also exhibits variable appearance on MRI, but most frequently shows low-signal intensity on T1-weighted images.182). Calcifications are rarely documented.108,132,156,163,176 Contrast-enhanced CT and MRI reveal heterogeneous or homogeneous enhancement, best appreciated on T1-weighted MR images (Figs. 1, 2). Cystic lesions show peripheral enhancement. According to some authors, precontrast low T2-signal intensity as opposed to markedly enhanced postcontrast T1 MR images may assist in the differential diagnosis of dural-based tumors and suggest the possibility of SFT.163 Another MRI observation that has been proposed as useful in suggesting SFT is the so-called “ying-yang” or a patchy “black and white” pattern consisting of alternating areas of hyperintensity and hypointensity in postcontrast T2-weighted scans.108,143 The “ying-yang” sign probably reflects the staghorn vascularity and variations in cellularity of different areas of SFT.81,118 The usual absence of the so-called dural tail, namely the dural enhancement adjacent to the mass, which is consistently seen in neuroimaging studies of meningiomas, by far the more frequent tumor of CNS coverings, might also alert the neuroradiologist to the possibility of SFT in the differential diagnosis. Still, a dural tail sign was seen in the postcontrast MRI in a few cases of SFT.41,78,122,143,153,163 Regarding intracranial SFT, angiography most often displays a moderately or highly vascularized lesion, fed by either the external or internal or both carotid arteries and/or vertebrobasilar arterial system, with a usual late and persistent tumor blush,96,122,129,131,141,176 although hypovascularized tumors have also been seen89,163,176 (Figs. 3–5). In selected spinal tumors, myelography, used in 2 cases, may be useful as an adjunct to the preoperative evaluation because it may provide complementary information with regard to cord deviation and compression.67,113 In practice, due to the rarity of intracranial SFT, most imaging studies of dural-based lesions are interpreted preoperatively as meningiomas163 or alternatively as HPC, and occasionally—in very rare special locations such as the cerebellopontine angle, the proximal segments of cranial nerves, the sellar region or within the ventricles—as schwannoma or a (nonfunctioning) pituitary tumors or even as choroid plexus papilloma, subependymoma, neurocytoma, and subependymal giant cell astrocytoma, respectively. In contrast, most intraspinal SFT present as intradural, extramedullary tumors and are usually diagnosed as meningioma, schwannoma, neurofibroma, HPC, lipoma, and non-neoplastic (dermoid, epidermoid) cysts. Tumors with intramedullary growth and extradural lesions, which also occur in the spine, can create diagnostic confusion with either parenchymal tumors (ependymoma, hemangioblastoma) or epidural processes (discal hernia, lipoma, metastasis, or tuberculosis), respectively. Awareness that SFT can present both in the brain and spinal cord and grow, as (mostly) an extra-axial and (occasionally) an intra-axial tumor may facilitate radiographic or clinical recognition in both locations.

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Histogenesis, Anatomical Locations, Gross Features

Once the derivation of pleural and extrapleural SFTs3,10,22–24,44,60,183,184 was attributed to the almost ubiquitous CD34-positive fibroblast (“dendritic interstitial cell”),5,183,184 proof of the same origin was showed for SFT of the CNS. CD34-positive fibroblasts were found in the dura,90 subpial interstice, perineurium, and endoneurium of normal nerves, and in the intraparenchymal perivascular connective tissue.90,108,131,183 For those of intraventricular location—at least in some cases—the presumptive origin is from the mesenchymal constituent of the choroid plexus.108,135,172

According to our literature review, the majority of SFT of the CNS are intracranial: 170 cases (77.27%) from a total of 220 (Table 1). Of the 160 cases for which the exact location was indicated, 82 were supratentorial, 55 infratentorial, 3 supratentorial and infratentorial,137,148,175,176 and 20 involved a structure adjacent or closely related to the base of the skull, the latter including 6 in the sellar region,41,102,104,125,151,162 6 involving the proximal intradural segments of cranial nerves,65,97,99,125,131,167 and 1 in the ganglion of Gasser.115 Of the infratentorial tumors, 11 (20.00%) arose from the cerebello-pontine angle, a fact that needs to be considered during preoperative imaging evaluation. Although most intracranial SFT (148 of 170) were extra-axial dural-based tumors, 22 (accounting for 12.94%) had no apparent connection with the dura. Of the latter, 13 were intraventricular,90,103,105,108,132,133,135,142,146,156,157,169,172 occasionally found attached to the choroid plexus,108,135,172 3 were parenchymal lesions deeply located in the brain substance,91,94,108 5 arose from the proximal (intradural) segment of the cranial nerves,65,97,99,131,167,168 and 1 at the cerebello-pontine angle139 (Table 1). [We did not include in our tabulation the following cases: 7 cases of meningeal SFT without clinical data, which were included as controls in an immunohistochemical study on claudin-1 expression in meningiomas185; two cases, 1 intracranial and 1 intraspinal, reported by Ginat et al186 in a review of radiological images of SFT from various sites in the absence of sufficient clinicopathological information; the case of “myofibroblastic neoplasm related to SFT associated with a malignant neuroblastic element” reported by Bracey et al187; the case of intracranial (frontal) meningeal fibroma described by Reyes-Mugica et al in 1992188; the case of primary intracranial CD34 positive meningeal myxoma189; and the 2 orbital cases of Metellus et al139 included in a large series of SFT of the CNS. Finally, we also excluded all cases of intrasellar/suprasellar HPC, a dozen of which have been described to date,190,191 despite the contention that these tumors are SFTs.162 and ref. 7 therein The chief criterion for inclusion was the original diagnosis of SFT made by the reporting authors.]

Of all SFTs of the CNS, 50 cases (22.72%) were intraspinal. Some intraspinal SFTs extended from the paravertebral region as dumbbell lesions.76,100,107,145,154 Of the 42 cases for which the exact location was stated, in decreasing order of frequency, 20 were thoracic (including 1 epidural lesion metastatic from an intracranial primary initially diagnosed as fibrous meningioma),108 14 cervical, 4 lumbar, 3 sacral, and 1 lumbosacral. Approximately half of the spinal SFTs reported in the literature (23 of 50; 46.00%) had no dural attachment: of these 11 were intraparenchymal and found within the medullary portion of the cord,66,69,82,101,155,165 6 were attached to the pia mater or pia-arachnoid layer,64,65,123,155,160 4 to the nerve rootlets of the spinal nerves,100,107,124,176 and 2 were adherent to the surface of the cord with no clear attachment109,113 (Table 1 intraspinal cases marked with an asterisk). [We included in our tabulation 1 case of (para)vertebral SFT for which the precise location (likely extradural) was not established,150 and a case of spinal thoracic SFT of Cincu et al,168 despite the fact that the authors did not use CD34. We excluded a sclerosing fibrous tumor of the cauda equine reported by Hisaoka et al,192 in 1993 probably the first SFT (sclerosing type) ever reported, which showed fibroblastic differentiation by ultrastructural examination but was not analyzed immunohistochemically, and a case by Kashiwazaki et al,193 an intraspinal subpial HPC with marked extramedullary growth and histopathological features that would fit the diagnostic criteria of SFT (with HPC-like pattern).]

Tumor size, according to the neuroimaging literature, ranged between 1 and 13 cm. Of 95 tumors for which the maximum dimension was specified, 50 (52.63%) were ≥5 cm, including 8 which were described as “large” or “huge” and assumed to be greater than 5 cm, although precise measurements were not given. The largest intracranial tumor was 13 cm,106 whereas the largest intraspinal lesion measured 8 cm.98 (Table 1). Almost all SFTs were well circumscribed, smoothly surfaced, partly or completely (pseudo-)encapsulated, with sharply defined margins as assessed radiologically and intraoperatively, except for a rare clinically malignant basicranial SFT that invaded bone104 and perhaps other rare exceptions.

All tumors were single, except for 1 intraspinal case with 2 separate intramedullary masses, 1 in each of 2 adjacent medullary segments123 that was interpreted as a multifocal SFT (to our knowledge, there is only 1 other case of multifocal SFT that was reported in the pleura).194 On sectioning, these tumors were described as firm, mostly solid, occasionally with cystic areas, having a gray-white to tan, whorled, cut surface. Surgical specimens were often received for pathological examination as either large or small nodular tissue fragments according to the tumor site and the technique used for resection.

An anecdotal case of an SFT associated with salivary gland ectopia,106 in a recurrence of a tumor previously diagnosed as fibrous meningioma, and an SFT embedded in a meningothelial meningioma,90 analogous to a collision tumor, is on record. No case of tumor to tumor metastasis was found insofar as SFT of the CNS is concerned, even though 1 instance of urothelial carcinoma of the urinary bladder metastasizing to an SFT of the pleura and one case of breast carcinoma metastatic to an SFT of the mediastinum have been recently reported.195,196 [Collision tumors (ie, juxtaposition of 2 tumors of different lineages) and tumor to tumor metastasis, with carcinomas of the lung, breast, and prostate as donor tumors, are 2 phenomena that occasionally can occur with meningioma.197–199] No case of SFT directly metastatic to the CNS from a distant extracranial site has been described so far, although 1 case of direct invasion of the brain from an adjacent paracranial site45 and 1 intra-abdominal SFT that metastasized to the skull-base200 are on record.

All SFT were sporadic, and no tumor arose in the setting of an established or suspected clinical syndrome predisposing to cancer. However, nonspecific tumor associations were seen in a few cases. Of 4 patients with a previous histories of tumors, 1 each had been operated on for breast cancer,64 breast fibroadenoma, and uterine leiomyoma,19 grade 2 astrocytoma,120 and germ cell tumor of the pineal gland.70 In addition, a patient had synchronous occurrence of an occipital SFT with 2 parasagittal frontal tumors, 1 of which was a histologically proven meningioma,96 another patient had café-au-lait spots not associated with neurofibromatosis type 1.125 Finally, in 2 cases,70,173 SFT was suspected to be radiation induced.

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Microscopical Features and Variants, Immunohistochemical Findings, and Differential Diagnosis
Conventional (“Benign”) or Usual Type

Microscopically, 3 common elements compose SFTs: bland-appearing spindle cells, a dense collagenous matrix, and prominent blood vessels. How these 3 elements are distributed in various proportions and relate to each other give a variegated histopathological pattern that overlaps with other soft tissue tumors. At first glance, it reminds one to consider a tumor at the intersection of a nerve sheath-(myo)fibroblastic-pericytomatous proliferation. The spindle cell component possess oval to spindly nuclei with a dense chromatin pattern, scant cytoplasms, and indistinct cell borders. These fusiform cells tend to be bundled in barely undulating fascicles that may insinuate themselves along collagen-rich bands, but lack any specific arrangement, thus often resulting in a “patternless pattern.” Focally, one may find a vaguely swirling, storiform or herringbone pattern (Figs. 6 and 7). Regimented neural-like palisading reminiscent of Verocay bodies have only rarely been documented in SFT of the CNS101 or in SFT of other sites72 (Fig. 8). Mitoses are usually sparse, with a mean of 1 mitosis/10 high-power fields (HPF) or less. The vasculature is often prominent, characterized by small and/or large branching vascular spaces, imparting the tumor an illusory (staghorn) HPC-like pattern. The collagenous stroma varies from being scant in areas of high density of spindle cells, to abundant, in the form of thin, elongated, rope-like strands of keloidal collagen. Variation in cell density, alternating hypercellular and hypocellular foci (polymorphic growth pattern) within the same tumor, is a characteristic feature. Nuclear pleomorphism and necrosis are usually absent. Whorls of tumor cells and psammoma bodies are rarely discernable, although whorling was seen in 1 case88 and psammoma bodies were noted in 2, the latter attributed to the incorporation of the arachnoidal membrane.64 Reticulin stain usually outlines the framework around small groups of cells, but a pericellular pattern has also been seen. Analogous to SFT of other sites, these are also the usual findings in the common type of SFT of the CNS. All cases reported showed the classical immunohistochemical profile of SFT as seen in other sites. To reiterate, SFT of the CNS show the immunohistochemical coexpression of vimentin, CD34, CD99, and BCL2. However, although CD34 may be negative in a small proportion of SFTs of the soft tissue (between 5% to 10%,201 and 30%5,63,202), we are not aware of this being the case for SFT of the CNS. Vimentin, CD34, BCL-2, and CD99 were strongly and diffusely positive in practically all CNS cases in which this panel of antibodies was applied (Fig. 9). CD34 was weakly162 or focally positive70,176 in 3 cases, and negative in only 1170; CD99 was either positive focally176(case3) or in a few cells only,171 or completely negative169 in another three cases; and BCL2 was only patchily positive in 1.170 Still, the other 3 markers strongly immunoreacted in all cases.162,171,171 CD34 immunohistochemistry was applied to all reported cases, except for one in which the use of this marker was not mentioned in the report.168 In a few cases, actin and desmin were variably positive.89,101,108 S-100 protein, cytokeratins, and EMA were almost always negative. Cytokeratin was focally positive in 1 case.15 Factor XIIIa and CD57 have been reported as positive in scattered tumor cells.69,101,104,105 Other immunomarkers occasionally investigated were claudin-1, a tight junction-associated protein expressed in approximately 50% of all types of meningiomas, which was negative in all 7 cases of SFTs studied.185 CD117,108,119 which was applied in 10 cases, was found to be negative20,120,125,146,166 in some, and focally 41,108 to diffusely119 positive in others. Immunostaining for steroid hormone receptors was performed in a few cases with estrogen receptor variably positive in 2 cases64 and negative in the remainder of the cases tested,68,75,114,125,166 and progesterone receptor variably negative68,75,114 to focally weakly19,64,166 to diffusely positive.125 In a few cases, p53 was tested and was generally weakly64 to moderately positive in a small proportion (<5%) of tumor cells.16,96,104 In a single case,173 which was initially p53 negative, a more significant fraction (10-40%) of positive tumor cells was found in recurrences following sarcomatous transformation: this observation is consistent with previous studies which correlated p53 expression with higher tumor grade in SFT of the pleura.203 Parenthetically, p53 was also immunohistochemically investigated in (several) extrapleural cases and found to be positive in some, including visceral,18 and orbital51 tumors. Molecular analysis of the p53 gene (PCR-SSCP and sequence analysis) in 1 case disclosed a point mutation at codon 161 in exon 5.16 The proliferation index as assessed with the monoclonal antibody MIB1 to Ki-67 nuclear antigen (in the conventional) type was almost always below 5%, most often under 2%, and occasionally between 2% and 5% (Fig. 10).

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The differential diagnosis is broad, but it can be narrowed to exclude the usual mimics such as fibrous meningioma, schwannoma, and HPC. In problematic cases, the vimentin+/CD34+/EMA-/S100-immunohistochemical profile exhibited by SFT helps to exclude both fibrous meningioma (usually positive for vimentin and EMA and either negative or positive for CD34 and S100 protein) and schwannoma (usually positive for vimentin and S100 protein and negative for EMA and CD34). Regarding HPC, we acknowledge that this is the most important and difficult location-dependent differential diagnosis. The differentiation is still possible in the large majority of cases: most HPC, in contrast to SFT, are uniformly (hyper)cellular and composed of oval to round rather than spindle cells, typically show a pericellular reticulin outline, and immunohistochemically are reactive with CD34 and BCL2 only in a minority of cases and/or in a weak and patchy pattern.69,152 Myofibroblastic lesions, such as meningeal inflammatory myofibroblastic tumor,204 (mammary type) myofibroblastoma,205 and intraparenchymal myofibromatosis of the brain,206 can also occasionally be considered, but the myogenic immunoprofile (alpha smooth muscle actin+/desmin+/h-caldesmon+/calponin+) usually expressed by the latter group of entities is of assistance in the differential. In cases with ambiguous findings, for example, CD34-positive fibrous meningioma with weak EMA expression, ultrastructural examination may assist in the differential diagnosis (see below).

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Histopathological Variants

Comparably with the pleural and soft tissue counterparts, morphological variants occur in SFT of the CNS. Among the variants, some have very scant collagenous matrix and diffuse uniform bland hypercellularity (imparting a monomorphic aspect to the tumor in question), others have abundant collagenized extracellular matrix (ie, the sclerosing or desmoplastic type), and still a few are of the myxoid or the lipomatous subtypes.

Although we recognize that hypercellularity in pleural2,3,207 and soft tissue13 sites is considered an atypical histological feature of SFT, we must emphasize that hypercellularity may be a marker for high risk of recurrence (see below) when associated with cell crowding, cell pleomorphism, nuclear atypia, and increased mitotic activity. A point in need of further analysis and corroboration is the contention that an SFT on any location that features uniformly increased but bland hypercellularity, scarce collagen matrix, and is devoid of pleomorphism should not be considered atypical. Some authors refer to this type of SFT as “cellular SFT.” Others, although noting the hypercellularity, simply call them SFT without any additional qualifier. In the absence of cell crowding, nuclear atypia, increase in mitoses, and low MIB1 proliferation index, we propose the term monomorphic variant, a variant that is unlikely to be more aggressive. We consider the monomorphic hypercellularity to be within the spectrum of conventional SFT, with the caveat that this tumor variant (or “cellular” variant of others) needs to be thoroughly examined to exclude the presence of any other atypical histological feature and/or increased rate of proliferation to distinguish it from a true atypical variant (Figs. 11 and 12).

A sclerosing variant is characterized by diffuse hyalinization of the intercellular matrix or the deposition of keloid-like collagen bundles (Fig. 13).

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A myxoid variant, characterized by abundant myxoid matrix and a grossly gelatinous appearance, is the least recognized and most misleading subtype. The myxoid variant of SFT was first recognized in soft tissue in 1999.208 The authors warned of the possible confusion of this variant with several other benign and malignant myxoid spindle cell neoplasms of soft tissue, including myxoid neurofibroma, myxoid spindle cell lipoma, low-grade myxofibrosarcoma, low-grade fibromyxoid sarcoma, myxoid synovial sarcoma, malignant peripheral nerve sheath tumor, and myxoid liposarcoma. In the CNS, the myxoid SFT needs to be differentiated from the following: chordoid or myxo-chordoid meningioma (usually vimentin+/EMA+/CD34-/S100-), particularly the adult-onset variant209; myxoid peripheral nerve sheath tumor (usually vimentin+/S100+/EMA-/CD34-); the exceedingly rare (CD34 negative) low-grade fibromyxoid sarcoma (either metastatic or primary)210; primary (CD34 negative) intracranial myxoma211,212; metastatic embolus from a cardiac primary; and the recently described angiomyxofibromatous tumor.213 By evaluating the clinical context, microscopical features, and the immunohistochemical results, one cannot confuse it with myxoid chondrosarcoma (vimentin+/S100+/CD34-); chordoma of the skull base (vimentin+/S100+/EMA+/CK+/CD34-), including the intrasellar form214; intraventricular215 or ectopic suprasellar chordoid glioma (GFAP+)216; and meningeal metastatic gelatinous carcinoma.217,218 Notably, myxoid changes in SFT may be focal or diffuse, being the dominant microscopic feature, but focally the typical collagenous matrix is retained (Fig. 14). Combined forms (myxoid and conventional) may also occur (Fig. 15). Genuine myxoid variant and the myxoid component of combined forms exhibit the classic immunoprofile of the usual type (Fig. 16). SFT of soft tissue may contain mature adipocytes,201 and along this line a unique such case has been recently reported in an intraspinal location.170 Lipomatous SFT in this context needs to be distinguished mainly from metaplastic lipomatous meningioma (“lipidized meningioma”)219: again, the two entities can be easily differentiated by means of immunohistochemistry and eventually ultrastructural examination, especially if the fat is abundant obscuring the histological features of the tumor.

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Atypical and Malignant Types

Nuclear atypia and/or cellular pleomorphism, hypercellularity, high mitotic rate (> 4 mitoses/10 HPF), and necrosis are all atypical histological features usually associated with SFT of pleural and extrapleural sites with a tendency to develop local recurrence and/or distant metastases.2,3,6,13,207,218 Each of these atypical morphological features may be discernable in isolation or in various combinations. To date, at least 29 cases (13.18%) of SFT of CNS exhibiting 1 or more atypical features at presentation,14,64,86-89,91,94,101,104,109,110,112,116,129,136,139,156,164,171,174,176 and 5 tumors showing atypical features at recurrence17,77,83,108,173 have been reported (Table 1). The atypical features in the majority of the aforementioned cases were diffuse. In one-third of the cases, the atypical features were focal64,86,87,94,104,109,136,174,176Fig. 17).

Figure 17
Figure 17
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Histologically, atypical or malignant SFT must be differentiated from malignant meningioma, malignant peripheral nerve sheath tumor, HPC, fibrosarcoma, synovial sarcoma, and spindle cell (sarcomatoid) or dedifferentiated chordoma, including the spindle cell type (usually vimentin+/EMA+/Cytokeratins+/S100+). Regarding the differential diagnoses of the 3 first entities, the reader is referred to the previous section on diagnosis of the conventional type, taking into account the fact that malignant variants may show markedly weakened to loss of CD34 expression (Fig. 18), similar to what occurs in their pleural or extrapleural counterparts.203 Immunohistochemistry is critical to differentiate the latter 3 tumors. Spindle cell sarcomatoid chordoma221 retains the standard immunoprofile of the usual type chordoma and dedifferentiated chordoma that by definition exhibits areas of low-grade (classic) chordoma.222 Fibrosarcoma, either primary or metastatic,223,224 does not react with CD34 and is usually negative for BCL2 and CD99. Synovial sarcoma is consistently negative for CD34 and reactive at least focally with EMA or cytokeratins. Parenthetically, we apply the qualifier “atypical” only to the histological features of a SFT, not to other aspects for which this term has been used in the literature, such as unexpected outcomes125,172 or aberrant immunohistochemical expression.162,225 The qualifier “malignant” has been applied by others to indicate SFTs showing more than a single atypical feature, with a sarcomatous appearance (also reported as anaplastic),101 or to tumors showing pure high mitotic activity,149 the latter finding likely having a distinctive role among the atypical morphological features. Thus, malignant histology would simply mean the highest risk category of atypical SFTs in terms of local recurrences and in terms of distant metastases.

Figure 18
Figure 18
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Other Morphological Studies

No study of preoperative transcranial cutting needle biopsy, fine-needle aspiration cytology or fine-needle aspiration biopsy have been reported on primary SFT of the CNS, quite likely reflecting the technical unfeasibility of reaching such tumors. In our review, we found a single case of lung metastasis from an intracranial meningeal SFT in which the authors were able to confirm the diagnosis by fine-needle aspirate, but they did not provide illustrations.83 The only report in the literature documenting the cytological findings of a histologically confirmed primary SFT of the CNS is the one by Kanahara et al,74 illustrating a scrape smear from a single spinal SFT showing naked spindle-shaped nuclei associated with abundant thin and thick collagen fibers. Nevertheless, we suspect that other cases of SFT have been examined cytologically during intraoperative consultation because smear analysis from squash preparations is a common technique in neuropathology practice. One of us (M.B.) had the opportunity to examine squash preparations from 3 cases submitted for intraoperative consultation. One was an intraspinal tumor interpreted as neurinoma, a second was an intracranial lesion felt to be a spindle cell tumor, probably malignant (Fig. 19), and a third was a recurrent lesion, previously diagnosed as fibrous meningioma, and interpreted as consistent with myxoid (fibrous) meningioma. These 3 cases were definitively diagnosed as SFTs (the latter of the myxoid type) on permanent sections after immunohistochemical stains were applied. Not surprisingly, the cytological findings by Kanahara et al,74 similar to those of extracranial SFT, illustrated elsewhere,226–228 and ours are all similar. The cytological preparations show oval cells lying singly, or more often in loose or tight aggregates enmeshed in a collagenous matrix with irregular ropy strands of collagen.

Figure 19
Figure 19
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Ultrastructural examination has been applied to the study of SFT of the CNS,64,66,68,70,71,73,78,81,83,85,86,91,105,116,127,130,140,149 with the purpose to establish a diagnosis and/or to investigate the lineage of the constituent tumor cell. The SFT cells show prototypical features of mesenchymal cells with oval to spindle-shaped nuclei, well-developed rough endoplasmic reticulum, cytoplasmic organelles such as the Golgi apparatus and mitochondria, occasional collections of actin-like microfilaments at the cell periphery, absent intercellular interdigitations, external basal lamina, and specialized intercellular junctions, all concordant with fibroblastic differentiation (Fig. 20). Another consistent characteristic of this tumor, paralleling the light microscopic findings, is an abundant collagenous matrix. Amianthoid fibers, also called giant collagen fibrils, that is, collagen fibers with a diameter of 140 nm or larger, previously described in SFT of soft tissue,23 were observed in 1 case.149 (In another case of CNS SFT, amianthoid fibers were diagnosed by light microscopy150; however, according to some authors, this diagnosis must be based on unltrastructural examination only).220 In 1 case,127 the peculiar intracytoplasmic structure designated ribosome lamella-complex was documented in approximately 50% of the tumor cells. The ribosome lamella-complex is mostly observed in certain hematologic malignancies229 (in decreasing order of frequency: hairy cell leukemia, monoblastic leukemia, lymphocytic leukemia, and Sezary's cell leukemia) and has occasionally been seen in solid tumors, but it is particularly rare in soft tissue tumors, a notable example being an ossifying fibromyxoid tumor.230

Figure 20
Figure 20
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Special Studies

In only 1 case of SFT of the CNS, DNA analysis was performed by static cytometry130 and in another by flow cytometry.71 Both were found to have a diploid DNA content similar to the other few cases of SFT of other sites that were so investigated.14,231–233 No case of SFT of the CNS was analyzed cytogenetically. Cytogenetic analysis of SFT from various sites has shown a wide range of karyotypic changes, such as translocations, gains, deletions, and inversions, involving a number of chromosomes2,3,4,5,9,12,22 and others, and correlating with tumor size (present in tumors >10 cm and absent in tumors <10 cm), but no consistent or specific aberration of diagnostic value was found.234 Of note is a pleural SFT harboring a rearrangement of the chromosomal segment 12q13, an abnormality that has also been found in soft tissue and meningeal HPC, implying a pathogenetic link.235–239

Molecular analyses were performed in 2 SFT of the CNS. One case of cranial meningeal SFT was analyzed for the p53 gene along with 12 other cases from various extrapleural sites: polymerase chain reaction-single strand conformation polymorphism of exons 5 to 8 did not show any aberrant bands, except for 1 case (from the peritoneum), and the subsequent sequential determination showed point mutation G to A transition at codon 161 (GCC to ACC) in exon 5 encoding corresponding aminoacid Ala to Thr, implying that the p53 pathway may be involved in the genesis of such tumors.16 In the other case, which was a spinal SFT, comparative genomic hybridization evidenced deletion of 9p21, losses on 2q, 3p, 16q, and 19q and gains on 7q, whereas the study of the methylation status of the 5′-CpG island of MGMT, p16, and DAPK1 that was made did not show hypermethylation of these genes.165

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BIOLOGICAL BEHAVIOR AND PROGNOSTIC FACTORS

Although the biological behavior of SFT of CNS is generally considered to be benign or indolent, provided that gross total resection is achieved, the survival data need to interpreted with caution115 as many cases had short-term follow-up or were published soon after the diagnosis without any follow-up (Table 1). Although rare, unfavorable outcome is on record. In our review, we found 26 cases with local recurrences (even multiple recurrences over many years),15,64,101,112,116,125,126,137,139,152,156,172–174,176 2 cases of meningeal spread through cerebrospinal fluid dissemination,117,127 and 5 cases with extracranial metastases, usually after recurrence.83,108,110,139,147 Extracranial metastatic deposits, which were specified in 4 cases, occurred in the soft tissue of the neck and chest wall,83 viscera (lung,83,108,110,147 and liver108,147), and bone.108 These figures would suggest a global recurrence and metastatic rate less than 15%, possibly an underestimation because many cases had very short-term follow-up (eg, 2 histologically malignant cases had an unremarkable but very short follow-up of 10 91 and 12,175mo only). The majority of the recurrent or aggressive tumors were intracranial (relative frequency: 27 of 170; 15.88%), whereas a minority, including 1 with distant metastases, were spinal tumors (relative frequency: 6 of 50, ie, 12.00%).64,16,101,139,150 The vast majority of recurrences occurred when the SFTs exhibited atypical histological features or when tumor resection was incomplete, either partial, subtotal or near total (Table 2). Parenthetically, the tumor which was diagnosed in intrauterine life had no follow up at all, due to the provoked termination of pregnancy.171 A few local recurrences have been observed even when the primary tumors were of the usual variety (10 cases)15,126,128,134,150,152,173,176; and some atypical SFTs recurred despite reported total gross resection of the primary (17 cases).101,116,126,127,137,139,150,152,172,174,176 (Tables 1 and 2). Furthermore, 1 totally removed case of usual histology in the primary, metastasized without local recurrence 4 years later, and retained the usual histology in visceral metastases,147 in a way, this behavior being reminiscent of the well-known phenomenon of (grade 1) “benign” metastasizing meningiomas.240,241 Multiple recurrences were frequent, and in the unfortunate patients, recurrences and/or tumor-related deaths supervened many years later, in some with intervals of 17,127 21,176 26,110 and 30 years 116 after surgery.

Table 2
Table 2
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Regarding the biological behavior of SFTs, we previously addressed the significance of diffuse monomorphic hypercellularity in isolation. Whether focal nuclear atypia and/or (focal) cell pleomorphism alone or associated with hypercellularity influence the biological behavior remains to be evaluated. There is 1 case report in which an SFT with focal nuclear pleomorphism176 did not recur 42 months after resection. Another case in which the SFT showed that focal (<10% of tumor volume) hypercellularity and nuclear pleomorphism174 recurred 6 months later when it showed full-blown histologic features of malignant SFT, including cerebral invasion, marked nuclear pleomorphism, and brisk mitotic activity (15/10 HPF). Consequently, the presence of focal histological atypia, such as cellular pleomorphism and hypercellularity in an otherwise typical SFT, needs to be mentioned in the diagnosis, and may require follow-up at closer intervals and/or more aggressive therapeutical management. Tumor necrosis is always an eye-catching gross and/or histological finding and is an important criterion to designate an atypical SFT. However, necrosis may be radiation induced. The pathologist must have all the clinical information to exclude radiation-induced necrosis (Fig. 21). Spontaneous tumor necrosis versus secondary necrosis is an important distinction to make to establish whether it is an SFT recurrence with the usual histology versus a recurrent atypical SFT.

Figure 21
Figure 21
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In borderline cases, the evaluation of the MIB1 labelling index (LI) may be prognostically helpful (see below). In fact, assessment of the proliferative index in the 2 aforementioned cases revealed that the former had a diffusely low MIB1-LI (<2%), including the pleomorphic focus, whereas in the latter—despite having a predominantly low MIB-LI (average 1%)—it exhibited a high MIB1-LI of 10% in the focus of hypercellularity and atypia, and subsequently rose to 25% (diffuse nuclear staining) in the recurrent tumor.

Similar to meningioma, invasion of the cerebral parenchyma, which has been reported in a few cases at presentation14,64,94,139,156,171 or at recurrence,110,125,174 should be regarded as an adverse prognostic factor. Frank bone infiltration,125,139,143,156,173 venous sinus invasion,89,104,112,118,125,128,134,139,176 encasement of internal carotid arteries,112,158 and large size may also play a role, but we did not find sufficient follow-up data to reach a definitive conclusion. Bone erosion or adherence to bone136,158,163,167 alone apparently has no prognostic significance (provided frank invasion is ruled out). In conclusion, incomplete surgical resection, atypical or malignant histological features, and brain infiltration are the best predictors of an unfavorable outcome (Table 2). However, similar to pleural and extrapleural SFT of other sites, reliable prediction of behavior in SFT after complete resection is difficult, or probably unpredictable in this context as rare occasional tumors with usual histology at presentation (or on recurrence) did recur (or recurred again) or gave rise to systemic metastases, even when grossly totally removed.127,139,176 In one of these cases, multifocal intracranial and intraspinal metastatic spread was documented at autopsy.127 In comparison with its soft tissue counterpart, most cases of which have been found to behave in a benign manner,4,17 SFT of the CNS is definitely more aggressive and this is probably related to incomplete surgical excision in the latter, whereas the well-recognized better prognosis of the former should be attributed to the ease of complete radical resection.

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Ki-67/MIB1 Labeling Index as a Prognostic Parameter

A high MIB-1 proliferation index should be considered as a function defining atypical SFT of the CNS. Ki-67/MIB1-LI was higher than 5% in 9 of 17 atypical cases in which it was determined, and there are cases on record that recurred despite having no atypical histological features, except for MIB1-LI>5% (Table 2). In fact, a morphologically “usual” SFT in the series of Vassal et al172 with an MIB1-LI of 10% as the only atypical finding recurred at 27 months. In a case of SFT reported by Suzuki et al84 with an MIB1-LI of 6.2%, the tumor recurred 15 years later. Furthermore, there were 3 additional cases117,125,176 with a MIB1-LI of greater than 5% but in absence of any cytological atypia or other adverse histological features, which nevertheless showed unfavorable behavior, with brain invasion,125 multiple local recurrences,125,176 and multiple intracranial metastases with cerebrospinal fluid dissemination,117 which eventually resulted in the deaths of at least 2 patients.

There is only 1 case on record 156 of a usual SFT of the CNS with MIB1-LI of 10% to 15% with no evidence of recurrence at 5 years. The significance of this must be considered in light of the fact that SFTs may recur after relatively long periods of time. There are in fact at least 5 cases84,116 of recurrences that occurred at 8,110 8.5,139 11,139 and 2 at 15 years; hence, an interval of 5 years may be considered to be relatively short. Therefore, we advocate inclusion of high MIB1-LI (ie, >5%) as an adverse prognostic parameter in assessing the prognosis of SFT of the CNS (Fig. 22), a concept that is reinforced by the fact that Ki-67/MIB1-LI is a useful measure of tumor grade and risk of recurrence in the prognostication of meningiomas. Incidentally, the figure proposed for SFT is near the cutoff index (>4%) that has been recently proposed242 for predicting an increased rate of relapses in meningiomas, which was arrived at on the basis of a vast literature search of articles correlating Ki-67/MIB1-LI with histological grades of meningiomas (ie, tumors with which SFT was previously confused).

Figure 22
Figure 22
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SFT Versus HPC

Analogous to HPC of soft tissue,201 some authors have proposed that HPC of the CNS should be included at one end of the spectrum of SFT (as “cellular” SFT),243 a semantic argument that is less than universally accepted.69,101,152,244–246 HPC of the CNS is a well-defined clinico-pathological entity, regardless of the controversy surrounding its histogenesis. HPC of the CNS is prone to local recurrence, carries a higher risk for extracranial metastases, with consequently more tumor-related deaths compared, not just with meningeal SFT, but with HPC and SFT of the somatic soft tissue. HPC of the CNS (meninges) has a local recurrence rate of approximately 80% to 85% and of extracranial metastases of 20% according to the collective experience of several large series from various sources, reported over the last 2 decades.101,247–248 In 2 recent studies, the biological behavior of HPC of the CNS contrasted even more sharply with that of (hemangiopericytoma/)SFT of soft tissues. HPC of the CNS had a local recurrence rate reaching 92%249 and 87.5%,152 a 50% rate of extracranial metastases,152 and greater than 60% tumor-related deaths.152 Therefore, although HPC of the soft tissues is a vanishing entity, the aggressive clinical behavior in the CNS justifies keeping the HPC nomenclature as separate from SFT.69,101,152,244–246 Notwithstanding, we acknowledge the existence of individual cases that can be very difficult to classify, with overlapping or transitional forms between the 2.69,176

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THERAPEUTICAL OPTIONS AND FOLLOW-UP MANAGEMENT

Surgery is the only current treatment for SFT of intracranial and intraspinal locations. The tumors are usually well circumscribed and often amenable to gross total resection. Very large and hypervascular lesions may require preoperative embolization. Craniotomy for intracranial and laminectomies and stabilization for intraspinal lesions are standard approaches. Transsphenoidal resection of intrasellar tumors, usually misinterpreted as pituitary adenoma, would result in incomplete resection, thus requiring subsequent craniotomy or postoperative gamma-knife radiosurgery. Adjunctive radiotherapy is not necessary when complete resection is achieved. Radiotherapy, either as gamma-knife radiosurgery or external beam radiation therapy is reserved for those cases with incomplete (subtotal or partial) resection or in some cases of recurrence or those with malignant histology. A few cases were so treated101,117,122,125,134,138,152,162,176 and some had partial or satisfactory responses,122,134,138,162,176 but recurrences or progression are also on record.117,125,139,152,176

Close follow-up is mandatory with biannual MRI scans for cases in which tumor removal was incomplete and for those with atypical histological features or a high proliferation index. Extended follow-up is suggested in all cases, including for those completely removed and/or with usual histology. Follow-up for the usual SFT of the CNS completely resected may include annual imaging for 5 years and every 5 years thereafter.123 No case of SFT of the CNS can be considered definitively cured.

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CONCLUSIONS

SFT of CNS is a rare tumor, distinct from fibrous meningioma and HPC, of which 215 cases, both intracranial and intraspinal, have been reported so far. Immunohistochemistry is critical to establish a correct diagnosis. The biological behavior is generally benign, and recurrence after gross total resection is uncommon. Recurrence may occur with subtotal resection or in cases with atypical morphology. Malignant variants and tumor-related deaths are relatively rare. Surgery is the only current treatment available. The usefulness of radiotherapy is not well documented. Stereotactic and external beam radiotherapy may be indicated for residual tumor or for inoperable recurrences. Active long-term surveillance of patients who had SFT of the CNS is mandatory.

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ACKNOWLEDGMENTS

The authors thank Dr M. Carandente and Mrs L. Giuliani, who are affiliated with IRCCS “Casa Sollievo della Sofferenza” Hospital, San Giovanni Rotondo, Italy, for their valuable assistance in the bibliography search during the preparation of the manuscript.

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Keywords:

solitary fibrous tumor; meninges; brain tumor; spinal cord tumor; central nervous system; CD34

© 2011 Lippincott Williams & Wilkins, Inc.

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