Bone & Soft Tissue Pathology
Case 3 -
Solitary Fibrous Tumor
Matt Van De Rijn
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A 35 y.o. male presented with shortness of breath. An 11 cm pleural based lesion was resected from
the right lung.
Sections show a densely collagenized, hypercellular bland spindle cell lesion with focally
hemangiopericytomatous appearing blood vessels. The tumor is well circumscribed but no clear capsule is
present. Immunohistochemistry shows a strong positive reaction for CD34. The tumor cells are negative
for S100, keratin, EMA, and actins.
Solitary fibrous tumor.
Solitary fibrous tumors are spindle cell tumors that were originally described in the pleura and at
that time were also called localized or fibrous mesotheliomas. Immunohistochemical and ultrastructural
studies subsequently showed that the cells in these tumors do not display features indicative of
mesothelial differentiation . Currently, the histogenesis of these lesions remains controversial,
with many favoring a derivation from mesenchymal fibroblast-like cells
Several growth patterns of SFT have been described
. Perhaps the most helpful to remember are:
Usually, tumors show a combination of different growth patterns in the same lesion , a finding that
can help in differentiating these tumors from other lesions
. The spindled cells in these tumors
have a small amount of indistinct cytoplasm. In most cases little nuclear pleomorphism is seen and the
behavior is benign. However, cases with increased cellularity, increased mitotic activity (>4/10
hpf), nuclear pleomorphism, hemorrhage, or necrosis have also been described and these lesions have been
associated with a malignant behavior in pleural sites , where the degree of resectability (polypoid
and well circumscribed vs. infiltrative) also is a major prognostic factor .
- the "patternless" pattern in which a proliferation of randomly oriented spindle cells is seen, often with alternating hypocellular and hypercellular areas.
- the hemangiopericytoma-like pattern, consisting of a cellular neoplasm with "staghorn" vessels.
- the diffuse sclerosis pattern which shows a paucicellular lesion consisting of dense bundles of collagen permeated by thin strands of spindle cells.
Originally, SFT were negative for most markers tested and the diagnosis of SFT was one of exclusion
and was based on its failure to react for a panel of antibodies. Subsequently, several studies showed a
strong reaction for CD34 in the vast majority of SFT both in pleural
and other sites . As
a result, many previously uncategorized spindle lesions could be classified as SFT and there was a
subsequent marked increase in the number of extrapleural SFT's described in the literature .
The majority of these extrapleural SFT's have behaved in a benign fashion, but it should be mentioned
that even bland appearing extrapleural SFT can recur or even metastasize . With longer follow-up,
more cases of SFT with a malignant behavior will most likely appear in the literature.
Extrapleural SFT have been reported in almost all body sites .
While not all SFT of the pleura are CD34+, most pathologists will require CD34 reactivity in
extrapleural lesions before calling the tumor a SFT. The reactivity for CD34 can be a useful tool to
distinguish the pleural based SFT from mesothelioma, fibrous histiocytoma, fibromatosis, synovial
sarcoma, and spindled cell carcinoma, as these lesions are almost uniformly negative for CD34. Depending
on the site in which SFT's occur, the differential diagnosis will of course vary, as described by
Nascimento . In combination with other antibodies lesions, such as nerve sheath tumors, smooth
muscle tumors and others can also be distinguished.
Hemangiopericytoma has become a highly controversial diagnosis. The diagnosis had always been one of
exclusion as many lesions such as synovial sarcoma can have a similar architectural pattern consisting of
a hypercellular tumor with irregular vessels. With the recognition that the vast majority of HPC
actually are solitary fibrous tumors, this diagnosis has lost most of its reason to exist, as stated in 2
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