Yonago Acta medica 2008;51:77–82

Solitary Fibrous Tumor of the Thigh: A Patient ReportKensaku Yamaga, Takeshi Minamizaki*, Mari Osaki*, Kohei Shomori, NaokoIshiguro†, Haruhiko Yoshida and Hisao Ito
Division of Organ Pathology, Department of Microbiology and Pathology, School of Medicine,Tottori University Faculty of Medicine, Yonago 683-8503, *Department of Orthopedic Surgery,
Yonago Medical Center, National Hospital Organization, Yonago 683-8518 and †Department ofPathobiological Science and Technology, School of Health Science, Tottori University Faculty ofMedicine, Yonago 683-8503 Japan

We report a rare case of solitary fibrous tumor (SFT) within the thigh muscle. A 37-yearold woman noticed a painless mass in her left thigh, which had gradually enlarged. Computed tomography (CT) showed an intramuscular tumor and enhanced CT showed strong
enhancement within the lesion. Magnetic resonance imaging revealed that the tumor
measured 13.1 × 7.3 × 2.3 cm and was located within the sartorius muscle, with a clearly
defined margin. Malignancy was suspected and the tumor was surgically resected. Thetumor was characterized by a well-circumscribed lesion, and was composed of bland or
short spindle cells, which were patternless, with scattered thick collagen bundles, andvariably gaping thin- or hyalin-walled blood vessels. Immunohistochemically, the tumor
cells were positive for vimentin, CD34, CD99 and focally, bcl-2. The tumor was subsequently diagnosed as an extrapleural SFT, which is classified into an intermediated (rarelymetastasizing) category. Extrapleural SFT should be considered in the evaluation of soft
tissue tumors in the extremities, although it may be rare.
Key words: extremity; soft tissue tumor; solitary fibrous tumor

Solitary fibrous tumor (SFT) was first describedby Klemperer and Robin in 1931 as a pleurabased lesion (Klemperer and Rabin, 1931). While
SFT was known initially as a pleural lesion, it has
been reported in various sites, including soft tissue of the extremities, head, neck, thoracic wall,mediastinum, pericardium and abdominal cavity(Guillou et al., 2002). We describe a 37-year-old
woman who had an SFT of the deep soft tissue inher thigh. Extrapleural development of the tumor
is relatively rare and the diagnosis was difficult
prior to surgery.

Patient Report
A 37-year-old woman noticed a painless mass in
her left thigh a year ago. Six months later, shevisited a nearby hospital because the mass had
begun to gradually enlarge. A soft tissue tumor
was noted in her left thigh. She was referred toour hospital for evaluation and therapy. Physicalexamination showed that the tumor was locatedin the deep soft tissue of the anterior thigh, 10 cm
in diameter, elastic/hard, unmovable and hemispherical. No abnormality was observed in theskin. Computed tomography (CT) showed anintramuscular mass in her left thigh, which waswell margined, and enhanced CT showed strong
enhancement within the lesion (Fig. 1). Magnetic

Abbreviations: CK, cytokeratin; CT, computed tomography;
EMA, epithelial membrane antigen; HPF, high-power field;
MRI, magnetic resonance imaging; MPNST, malignant
peripheral nerve sheath tumor; SFT, solitary fibrous tumor;
SMA, smooth muscle actin

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abFig. 1. a: Plain CT showed an inter-muscle mass which was well-circumscribed.
b: Enhanced CT showed strong enhancement in the tumor.

abFig. 2. The mass was located within the sartorius muscle.
a: T1-weighted imaging exhibited low signal intensity of the mass with scattered lower signal intensity.
b: T2-weighted imaging exhibited high signal intensity of the mass with scattered higher signal intensity.

resonance imaging (MRI) revealed that the tumor
measured 13.1 7.3 2.3 cm and was locatedwithin the sartorius muscle, with a clearly defined hemispheric margin. T1-weighted imaging
exhibited low signal intensity of the tumor with
scattered lower signal intensity (Fig. 2a). In contrast, T2-weighted imaging exhibited high signal
intensity of the tumor with scattered higher signal
intensity (Fig. 2b). Tl scintigraphy demonstrated
marked uptake in both early and delayed images.
Blood glucose level was normal.
Based on the clinical findings and imaging
examination described above, we diagnosed themass as a malignant tumor of the soft tissue, suchas synovial sarcoma, alveolar soft tissue sarcoma
and malignant peripheral nerve sheath tumor(MPNST). Core needle biopsy was performed forhistopathological diagnosis. Although the tumor
was diagnosed as a spindle cell tumor and suspected of low-grade malignancy, histological type
could not be diagnosed. Thereafter, modified
wide resection was performed.
Pathological findings (operative specimen)
Gross findings
The tumor was capsulated and well circumscribed. On sectioning, it had a whitish and firm
appearance with partial hemorrhage and no necrosis (Fig. 3).
Histopathological examination
The tumor was well circumscribed. It was composed of alternating cellular and less cellular78

Solitary fibrous tumor of the thighareas, and of cytologically bland spindle or shortspindle cells arranged without an obvious pattern,with scattered thick collagen bundles, and variably gaping thin- or hyalin-walled blood vessels,
which had a hemangiopericytoma-like appearance, and a fibromyxoid stroma (Fig. 4). Mitoticfigures were less than 1/HPF. Based on the morphologic features, there was no convincing evidence of malignancy.
Immunohistochemical examination
The tumor cells were positive for vimentin, CD34,
CD99, and focally for bcl-2, whereas alphasmooth muscle actin (SMA), desmin, S-100 pro-Fig. 3. The mass presented as well-circumscribed andhad a whitish and firm appearance with focal hemorrhage.

abcdFig. 4. Histopathological findings. Hematoxylin and eosin stain. Bar = 100 μm.
a: The tumor was composed of an alternating cellular area.
b: Thick collagen bundles and hyalin-walled blood vessels were scattered in fibromyxoid stroma.
c: Bland spindle or short spindle cells were arranged without an obvious pattern.
d: Mitotic figures (arrow) were rare.

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abcdFig. 5. The tumor cells were positive for a) vimentin, b) CD34, c) CD99 and focally d) bcl-2.

tein, cytokeratin (CK) (AE1/AE3) and epithelial
membrane antigen (EMA) were negative (Fig. 5and Table 1). MIB-1 labeling index was about 2%.

vessels, hemangiopericytoma-like appearance andimmunohistochemical positivity for CD34 andCD99.

Genetic diagnosis
The fusion gene SYT-SSX1 or SYT-SSX2, whichis specific in synovial sarcoma (Fisher et al.,2002), was not detected by reverse transcriptasepolymerase chain reaction.

Discussion
SFT is a ubiquitous mesenchymal tumor of possible fibroblastic type, showing a prominent haemangiopericytoma-like branching vascular pattern(Guillou et al., 2002). SFT was first described byKlemperer and Robin in 1931 as a pleura-based lesion (Klemperer and Rabin, 1931). After that, SFT
began to be reported as extrapleural SFT in various locations. In the latest World Health Organization classification, extrapleural SFT is classified
into the intermediate (rarely metastasizing) cate-Pathological diagnosis
This case was finally diagnosed as SFT of the left
thigh, based on clinical findings and histologically bland spindle or short spindle cells arranged
without an obvious pattern, with scattered thickcollagen bundles, thin- or hyalin-walled blood
80Solitary fibrous tumor of the thigh

Table 1. Results of immunohistochemistry

gory (Fletcher et al., 2002). SFTs usually develop
in the pleura, but are rarely of extrapleural origin.
Extrapleural SFTs may be found at any locationin middle-aged adults between 20 and 70 years(median 50 years), with no sex predominance.
Approximately 40% of extrapleural SFTs are located in the subcutaneous tissue, while others arelocated in the deep soft tissue of the extremities orextracompartmentally in the head and neck region(especially the orbit), thoracic wall, mediastinum,pericardium, retroperitoneum and abdominalcavity. Other described locations include the meninges, spinal cord, periosteum, as well as organs
such as the salivary glands, lungs, thyroid, liver,gastrointestinal tract, adrenal glands, urinary
bladder, prostate and spermatic cord and testes(Guillou et al., 2002). Fifteen cases of SFTs in
the thigh have been reported (Suster et al., 1995;
Hasegawa et al., 1996, 1999; Brunnemann et al.,
1999; de Saint Aubain Somerhausen et al., 1999;
Morimitsu et al., 2000; Nakamura et al., 2005;
Anders et al., 2006; Hoshino et al., 2007; Inoue etal., 2007; Martorell et al., 2007). Interestingly, it
has been reported that SFTs are associated withhypoglycemia due to secretion of insulin-likegrowth factors by the tumor in about 5% of cases(Fukasawa et al., 1998). MRI is not sufficient for
final diagnosis. Therefore, pathological findings
are very important (Anders et al., 2006).
Histopathologically, most SFTs present aswell circumscribed, often partially encapsulatedmasses. Typical SFTs show a patternless architecture characterized by a combination of alternating hypo- and hypercellular areas, separated
from each other by thick bands of hyalinized,
somewhat keloidal collagen and branching haemangiopericytoma-like vessels. Myxoid changes,
areas of fibrosis and interstitial mast cells arecommonly observed. Mitotic figures are generally scarce. Immunohistochemically, tumor cells inSFTs are known to be immunoreactive for CD34
(90–95% of cases) and CD99 (70%) as confirmedin the present tumor. Approximately 20% to 35%of the tumors are also variably positive for EMA,
bcl-2 and SMA (Guillou et al., 2002).

Antibodies

SourceVimentin
CD34
CD99
Bcl-2
EMA
CK (AE1/AE3)
S-100 protein
DesminAlpha-SMA
HHF-35

Nichirei
Nichirei
Dako
Dako
Nichirei
Dako
Nichirei
Dako
Dako
Dako

Dilution /Results
antigen retrieval
PD / mw
PD / mw
1:50 / mw
PD / mw
PD / mw
1:50 / mw
PD / mw
1:50 / mw
1:50 / mw
PD / mw+++

Focally +––––––+, positive; –, negative; mw, microwave; PD, prediluted.

In the present case, clinical and histopathological findings at operation were typical, asdescribed above. Differential histopathologicaldiagnoses were monophasic synovial sarcoma
and MPNST. CD34 is usually negative, CK andEMA are usually positive, and stromal collagenis usually wiry and scanty in monophasic synovial sarcomas (Fisher et al., 2002). The fusion
gene SYT-SSX1 or SYT-SSX2 was not detectedin the present tumor. We could also distinguish
this case from MPNST by the following criteria:
S-100 protein is usually positive, CD34 is usually negative, the cells are arranged in sweeping
fascicles and palisading, and mitotic figures arereadily encountered in MPNST (Woodruff et al.,2000). On the other hand, histopathological findings were atypical upon needle biopsy before surgery. The needle biopsy specimen was composedof a cellular area, but thick collagen bundles andhyalin-walled blood vessels were not conspicuous.
However, if we had noted that CD34 was strongly
positive and that there were a few collagen bundles in the needle biopsy specimen, we might
have been able to diagnose SFT before surgery in
the present case.
Morphologic distinction between benign andmalignancy SFT is often difficult. Malignant SFT
are usually hypercellular lesions, showing at least
focally moderate to marked cytological atypia,
tumor necrosis, numerous mitoses (> 4 mitoses/1081

K. Yamaga et al.

HPF) and/or infiltrative margins. Although there
was no convincing evidence of malignancy inthe present case, the biological behavior of thelesion is difficult to predict because rare cases
show abrupt transition from conventional benignappearing SFT to high grade sarcoma.
We have presented a case of SFT located in
the thigh, which is relatively rare. A rare locationof an uncommon lesion led to a confusing diagnosis. Although it may be rare, extrapleural SFT
should be considered in the evaluation of soft tissue tumors in the extremities.

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Corresponding author: Kensaku Yamaga, MD82