Introduction
Schwannoma and neurofibroma are the two most frequent benign tumors that arise from
the peripheral nerves.[1 ]
Schwannomas are benign neoplasms usually encapsulated and composed exclusively of
immature Schwann cells that arise from the peripheral nerve sheath,[2 ] whereas neurofibromas, the most frequent benign tumors in neurofibromatosis type
1 (NF1), contain, besides malignant peripheral nerve sheath tumor (MPNST), proliferating
Schwann cells and other local supporting elements of the nerve fibers, including perineurial
cells, fibroblasts, axons, blood vessels and infiltration of mast cells.[1 ]
[3 ]
[4 ]
Although these tumors are generally easy to distinguish by standard light microscopy,
in some cases, they might closely resemble one another.[1 ]
On the other hand, MPNSTs are malignant neoplasms with nerve sheath differentiation
that originate from the peripheral nerve sheath or the adjacent extraneural soft tissue.
Due to their morphologic heterogeneity and lack of specific immunohistochemical or
molecular criteria, the diagnosis of MPNST is often challenging, including various
sarcomas and benign peripheral nerve sheath tumors, such as cellular schwannoma or
atypical/cellular neurofibroma.[1 ]
[4 ]
[5 ]
[6 ]
[7 ]
Besides the well-sedimented lack of specific markers for peripheral nerve sheath tumors,[4 ]
[6 ]
[7 ]
[8 ]
[9 ] they also share some biomarkers, which makes the diagnosis even harder.[1 ] Thus, it is necessary to outline the antigen patterns for each in order to reduce
the risk for misdiagnosis,[10 ] when the microscopy is not enough. This process must be followed not only for schwannomas,
neurofibromas and MPNSTs, but also between each of these three and their related possible
differential diagnoses.
Immunohistochemistry is an ordinary tool in the diagnosis of soft tissue tumors. It
is a valuable resource when a careful assessment of histopathology and formulation
of differential diagnosis are inconclusive and there still are differential diagnostic
possibilities. Conversely, the use of diagnostic immunohistochemistry regardless of
the histological context frequently results in serious errors and is strongly discouraged.[11 ]
Schwannomas, neurofibromas and MPNSTs are soft tissue tumors and often have histopathological
differential diagnosis with other soft tissue tumors,[6 ]
[11 ] with which they share multispecific markers and antigenic complexities of many tumor
types[11 ] and hence, there is a need of new and specific biomarkers for these cases.
In this review we will briefly expose the common expected histology of the schwannoma,
neurofibroma and MPNST, as well as their variants, followed by the main differential
diagnosis in each situation. Then, a discussion about the well-established and recent
promising markers will be built to set a layout ([Table 1 ]) of alternative and more embracing immunohistochemistry to avoid misdiagnosis and
draft more precisely therapeutic schemes and prognosis.
Table 1
Useful immunophenotypic features in the differential diagnosis of schwannoma, neurofibroma
and malignant peripheral nerve sheath tumor
IHC Marker
Schwannoma
Neurofibroma
MPNST
S100
+++
++
+/+++ (only in the epithelioid MPNST)
CD34
+++ (Antoni A areas)
+++
+/++
Sox10
+++
+++
+/++
EMA
+
0/+
0 (except for MPNST with perineurial differentiation)
Calretinin
+++
+
GFAP
+
+
+
NSE
Leu7
0/+
0/+
GAP43
0/+
0/+
RTK/AXL
TLE1
+++
+/++
+/++
Podoplanin
++ (Antoni A)
0 (except for atypical)
+/++
CD56/PGP9.5
++
++
++
Nestin
0/+
++
++
HMGA2
+/++
EGFR
0
+/++
p16
+/++
0/+
0/+
Neurofibromin
0 (except for a few low-grade cases of MPNST)
Ki -67
+/++
++/+++
P57NTR
+
++
Sox2
++
++
Abbreviations: +, weak expression of the marker; ++, moderate expression of the marker;
+++, strong expression of the marker; 0, no expression of the marker; Blank space,
no information contemplated by this review of the marker; EGFR, epidermal growth factor
receptor; EMA, epithelial membrane antigen; GAP43, growth associated protein 43; GFAP,
glial fibrillary acidic protein; HMGA2 high mobility group AT-hook 2; IHC, NSE, neuron-specific
enolase; MPNST, malignant peripheral nerve sheath tumor; RTK, receptor tyrosine kinases;
TLE1, transducin-like enhancer of split 1.
Discussion
Schwannoma ([Figs. 1 ] and [2 ])
Fig. 1 Intraoperative photograph of a schwannoma of the left fibular nerve before resection.
The patient is a 42-year-old non-neurofibromatosis male.
Fig. 2 Intraoperative photograph of the left fibular nerve after resection. Same patient
of [Fig. 1 ].
Schwannomas are benign neoplasms that arise from the Schwann cells of the peripheral
nerve sheath. They are well circumscribed masses with a variable admixture of compact
spindle-cell areas with nuclear palisading and Verocay bodies (Antoni A), and reticular
paucicellular areas (Antoni B).[1 ]
[6 ]
[12 ] Neurofibromatosis type 2 (NF2) and schwannomatosis are genetically distinct disorders
that are related to predisposition of developing nerve sheath tumors, mainly schwannomas.[8 ]
In the NF2 syndrome (1:33.000), there are alterations in the NF2 gene coding for the
Merlin tumor suppressor protein that cause all tumors in the syndrome.[2 ]
[8 ]
[13 ] Neurofibromatosis type 2 patients mainly suffer from bilateral vestibular schwannomas,
meningiomas, gliomas and peripheral schwannomas.[8 ]
[14 ]
Schwannomas rarely suffer malignant degeneration and when they do, they turn into
epithelioid MPNSTs, which have a strong diffuse expression of S100.[4 ]
[6 ]
On the other hand, in the schwannomatosis, (recently linked to mutations in the SMARCB1
tumor suppressor gene) schwannomas are the only tumor entity observed.[8 ]
Cellular schwannoma is a variant of schwannoma composed basically of compact, fascicular
growth pattern, proliferating Schwan cells with variable hyperchromasia and pleomorphism
and absence of Verocay bodies.[5 ]
[6 ]
[12 ] Because of its high cellularity, fascicular growth proliferation and increased mitotic
activity, and occasionally more aggressive behavior, the cellular schwannoma can be
a differential diagnosis of spindled (regular) MPNST. Besides, the cellular schwannoma
presents morphologic similarities with fibrosarcoma, leiomyosarcoma, and synovial
sarcoma, particularly in cases with monophasic histology or in needle biopsy specimens[5 ]
[6 ]
[15 ]
There is a rare variant of schwannoma, the melanotic schwannoma, whose histology is
composed of melanin-producing cells with ultrastructural features of Schwann cells.
Its immunohistochemistry is strongly positive for S-100, Leu-7, HMB-45, Melan-A and
vimentin.[16 ]
Neurofibroma ([Figs. 3 ], [4 ], [5 ] and [6 ])
Fig. 3 Intraoperative photograph of a plexiform neurofibroma of the right tibial nerve before
resection. The patient is a 34-year-old NF-1male.
Fig. 4 Surgical specimen of a plexiform neurofibroma. Same patient of [Fig. 3 ].
Fig. 5 Intraoperative photograph of a neurofibroma of the left sciatic nerve before resection.
The patient is a 56-year-old non-neurofibromatosis female.
Fig. 6 Intraoperative photograph of the left sciatic nerve after resection. Same patient
of [Fig. 5 ].
Neurofibromas are a benign peripheral nerve sheath tumor, whose primary neoplastic
cellular components are the Schwann cells, but they also have nonneoplastic peripheral
nerve components, such as fibroblasts CD34 + ,[3 ]
[4 ]
[6 ]
[8 ]
[10 ]
[11 ]
[17 ] perineurial cells, blood vessels, axons and mast cells.[1 ]
[3 ]
[4 ] A total of 10% of neurofibromas are associated with neurofibromatosis 1.[6 ]
Neurofibromatosis 1 (NF1) is the most common genetic disorder of the nervous system,
affecting 1 out of 3,500 newborns worldwide.[3 ]
[18 ]
[19 ] Neurofibromatosis 1 is an autosomal dominant tumor suppressor disorder[8 ]
[15 ] of the Ras signal transduction pathway,[3 ]
[18 ] in which the loss of Schwann cells leads to a cascade of interactions with the neighboring
cells in the microenvironment and additional cell autonomous modifications, resulting
in the formation of multiple neurofibromas.[3 ]
There are three specific clinicopathologic subtypes of neurofibromas based on architectural
growth pattern: localized, diffuse and plexiform.[6 ]
The two localized types of neurofibroma are the localized cutaneous neurofibromas
(LCNs), or dermal neurofibroma, and the localized intraneural neurofibromas (LINs).
The LCNs are the most common subtype, occurring sporadically in almost all cases,
and virtually in all NF1 individuals, primarily around puberty. They consist of an
intracutaneous relatively well-circumscribed nodule that grows near the budding of
the nerve. The LINs are similar to the LCNs at the structural and citollogic level,
but instead of the skin, they involve the nerve, and can also arise in a major nerve,
typically causing a fusiform expansion of the nerve trunk.[3 ]
[4 ]
[6 ]
[8 ]
[19 ]
[20 ]
The diffuse neurofibroma (DFN) consists of a plaque-like elevation of the skin, usually
in the head and neck region, that spreads through the subcutaneous tissue. Despite
its infiltrative growth, it rarely suffers malignant degeneration.[6 ]
[20 ]
Conversely, the plexiform neurofibroma (PXN) has a potential of 5 to 13% for malignant
transformation and is practically pathognomonic of NF1. Actually, NF1 patients have
a 25 to 50% chance of developing PXN. The PXN is a variant of neurofibroma that involves
numerous adjacent nerve fascicles or multiple components of a nerve plexus.[3 ]
[4 ]
[6 ]
[18 ]
[19 ]
Still, there is the massive soft-tissue neurofibroma (MSTN), a very rare type of PXN
often associated to NF1, which is characterized by large size, infiltration of soft
tissue and skeletal muscle, frequently involving large anatomical regions. When under
the microscopy light, it demonstrates the presence of cellular component. Despite
its aggressive appearance, the MSTN hardly suffers malignant degeneration.[4 ]
[6 ]
There are two variants of neurofibroma with unusual features, the atypical neurofibroma,
with degenerative cytological atypia, and the cellular neurofibroma, with increased
cellularity. They are differential diagnoses of MPNST, mainly the low-grade examples,
but while the cellular neurofibroma lacks the cytological atypia, the atypical neurofibroma
lacks increased cellularity or mitotic activity, and both presents lower mitotic figures
than MPNST. Still, Naber et al write about the hypothesis that the atypical neurofibroma
is actually a transition between a neurofibroma and a MPNST.[8 ]
There are many different possible differential diagnoses for neurofibroma, including
schwannoma, nerve sheath myxoma, neurothekeoma, ganglioneuroma and traumatic neuroma
as well as variety of non-nerve sheath tumors, in particular dermatofibrosarcoma protuberans
(DFSP) and desmoplastic malignant melanoma.[6 ]
Fig. 7 Intraoperative photograph of a MPNST of the right femoral nerve. The patient is a
16-year-old non-neurofibromatosis female.
Malignant peripheral nerve sheath tumors (MPNSTs) are neural crest-malignant neoplasms
with nerve sheath differentiation, which arise from the peripheral nerve sheath or
adjacent extraneural soft tissue.[5 ]
[9 ]
[15 ]
[21 ] Due to their morphologic heterogeneity and lack of specific immunohistochemical
or molecular criteria, the diagnosis of MPNSTs is often challeging.[4 ] They arise most frequently in the proximal segments of the extremities, in the major
nerve trunks (such as the sciatic nerve) followed by the trunk, head and neck.[4 ] And 50 to 66% of the MPNSTs arise from neurofibromas, mostly of PXNs.[7 ]
The MPNST is a tumor derived from the Schwann cells or pluripotent cells of the neural
crest. It displays a heterogenous cellular morphology (including spindle, epithelioid,
pleomorphic or small round cells).[8 ] Generally, the MPNST presents alternating areas of hyper or hypo cellularity or
a diffuse growth pattern of spindle cells.[7 ]
[15 ] The histologic features of the MPNST are frequently, but not specifically, fascicles;
neuroD whorls; palisades or rosette-like arrangements; perineurial/intraneural spread,
when associated with nerve perivascular hypercellularity; subendothelial infiltration
of tumor cells; hemorrhage, and necrosis areas.[4 ]
[5 ]
[6 ]
There is a rare subtype of MPNST, the epithelioid MPNST, that derives more often from
a preexisting schwannoma, which has suffered a malignant degeneration.[4 ]
[6 ] In the epithelioid subtype, heterologous differentiation is observed in 15% of the
MPNSTs, and it includes rhabdomyoblasts, cartilaginous and osseous components, or
less commonly, smooth muscle, glandular (often with a neuroendocrine foci) or liposarcomatous
components,[7 ] particularly in patients with NF1.[6 ] However, there is a certain controversy regarding the frequency of the rhabdomyosarcomatous
differentiation; while Rodriguez et al state that it is the less frequent,[6 ] Guo et al describe it as the most common one.[7 ]
The differential diagnosis is extensive, including benign peripheral nerve sheath
tumors and various sarcomas,[5 ] such as atypical and cellular neurofibroma, cellular schwannoma, synovial sarcoma,
fibrosarcoma, rhabdomyosarcoma, leiomyosarcoma, dedifferentiated liposarcoma, clear
cell sarcoma or ossifying fibromyxoid tumor of soft parts.[6 ]
S100
Although S100 is considered important to separate neoplasms with neural crest origin
from the non-neural ones,[1 ] it is a well-known non-specific immunohistochemical marker, as S100 is also expressed
for non-neural and non-melanocytic sarcomas, such as synovial sarcoma (15%), Ewing
sarcoma (21%), rhabdomyosarcoma (24%), chondromyosarcoma (75%) and extraskeletal myxoid
chondrosarcoma (45%), according to Karamchandani et al.[15 ] The immunohistochemical staining for S-100 protein is more uniform and pronounced
in schwannomas than in neurofibromas,[1 ] whereby MPNST has a scattered expression of S100, seen in only 50 to 60% of the
cases, and the majority of high-grade examples are negative.[4 ]
[7 ]
[9 ]
[10 ] Thus, when there is a diffuse expression of S100 in a suspect MPNST sample, it suggests
melanoma or cellular schwannoma,[4 ] except for the epithelioid MPNST, which expresses diffuse S100.[4 ]
[6 ] In this case, the differential diagnoses would be melanoma, clear cell sarcoma,
epithelioid sarcoma and carcinoma. The absence of melanocytic markers, such as MelanA
or HMB45, helps to discard melanoma and clear cell sarcoma, while the lack of low
weight cytokeratin expression distinguishes epithelioid MPNST from epithelioid sarcoma
and carcinoma.[6 ]
Ultimately, when cellular schwannoma assumes a more aggressive behavior, it can mimic
MPNST. In this situation, the usage of S100 immunohistochemistry is crucial, as cellular
schwannoma usually expresses strong diffuse S100, while MPNST rarely does.[6 ]
CD34
The transmembrane phosphoglycoprotein CD34 is ubiquitously expressed in the fibroblasts
of all subtypes of neurofibromas,[3 ]
[4 ]
[6 ]
[8 ]
[10 ]
[11 ]
[17 ] and in the Antoni A areas (compact areas) of the schwannomas.[8 ]
The MPNST, on the other hand, has a weak to medium intensity of expression of the
protein, and apparently has a kind of inverse expression relation with podoplanin:
when CD34 is highly expressed in MPNST the, the expression of podoplanin is weak and
vice versa.[8 ]
Furthermore, CD34 is also expressed in various types of tumors, such as fibroblastic,
myofibroblastic, fibrohistiocytic, vascular, neural, adipocytic, smooth muscle, melanocytic
and epithelial lesions.[4 ]
Therefore, CD34 may not be a good biomarker for differentiating peripheral nerve benign
lesions, and these from other lesions.
Sox10
Sox10 is useful to distinguish nerve sheath neoplasms from mesenchymal tumors, given
the fact that the marker is rarely seen in non-schwannian or non-melanocytic tumors.[4 ]
[6 ]
[9 ]
[10 ]
[20 ]
[22 ] Sox10 has been considered a specific marker for schwannomas and neurofibromas[9 ] and, it assists in the differentiation, in the intracranial space, of schwannomas
and neurofibromas from meningioma, which expresses 20 to 30% S-100 protein, or gastrointestinal
(GI) schwannomas from GI stromal tumors (that are occasionally S100-protein positive).
Still, Karamchandani et al suggest that Sox10 is a useful marker for differentiating
cellular schwannoma from fibrosarcoma, leiomyosarcoma, and synovial sarcoma.[15 ] However, Sox10 is not a reasonable biomarker for MPNST, as less than half of MPNSTs
express Sox10, which corresponds to entrapped or residual Schwann cells , and the
high-grade MPNST examples are nearly all S100 negative.[10 ]
[20 ] Although Sox10 expression is also not exclusive of neural-crest tumors,[9 ]
[10 ] it is pointed by several authors to be a more specific marker than S100.[9 ]
[10 ] Moreover, it is suggested that Sox10 is used in combination with S100 when diagnosing
MPNST, though even using both, there is a non-staining chance.[9 ]
[10 ]
[20 ] Thus, the study and, hence, a set of a immunohistochemical identities are crucial
to avoid misdiagnosis of neural crest tumors, mainly the MPNSTs.
EMA
Epithelial membrane antigen (EMA) is normally expressed in perineurial cells; therefore,
it is positive stained in perineuriomas and MPNST with perineurial differentiation,
but negative in schwannomas, neurofibromas or other types of MPNST.[1 ]
[6 ]
[11 ]
[12 ]
Calretinin
Although neurofibromas and schwannomas are generally easy to distinguish by standard
light microscopy, when the schwannoma does not display the characteristic palisading
nucleus, and it consists only (or mostly) of Antoni B areas, it can be hard to differentiate
it from a neurofibroma.
Furthermore, as S-100 might not reliably distinguish these two neoplasms, calretinin
is a good alternative. Samson et al have shown that whereas the schwannoma generally
presents a positive staining of calretinin, with a strong staining, neurofibromas
rarely stain, and when they do, it is a week staining.[1 ] Therefore, calretinin is suggested to be a promising immunohistochemical tool for
the differentiation between schwannomas and neurofibromas.
GFAP, Neuron-specific Enolase, Leu-7 and GAP43
Neuron-specific enolase (NSE) is a nonspecific biomarker supportive of nerve sheath
differentiation.[4 ] Schwannoma and neurofibroma normally do not, but occasionally can, present Leu-7
and the glial fibrillary acidic protein (GFAP).[4 ]
[20 ]
[22 ] Le LQ et al stained Schwann cells of mouse plexiform neurofibroma with anti-growth
associated protein 43 (GAP43), suggesting that in can be used in the immunohistochemical
diagnosis of human neurofibromas.[3 ]
RTK and AXL
Receptor tyrosine kinase (RTKs) is a receptor expressed on the cellular membrane of
neurofibromas and MPNSTs, and its exterior portion is called AXL, whose ligand is
the GAS6. Johansson et al have shown that MPNST and neurofibromas, the plexiform and
nodular subtypes as well as the dermal subtype, related to NF1 or not, may express
increased AXL protein on their surface. Furthermore, they suggest that elevated levels
of soluble AXL in the plasma (sAXL) can be detected in the presence of PXN and MPNST,
as well as higher numbers of dermal neurofibromas are followed by higher levels of
sAXL. Therefore, according to the latter data, sAXL could be used as a burden marker
for neurofibroma.[19 ] However, AXL is not exclusively expressed by neurofibromas and MPNST, thus, it is
more a burden marker, than a diagnostic tool.[19 ]
TLE1
Transducin-like enhancer of split 1 (TLE1) is commonly expressed by peripheral nerve
sheath tumors, including 100% of schwannomas, 33% of neurofibromas, and (in a more
variable and weaker way) 30% of MPNST.[4 ]
[6 ] Therewithal, TLE1 typically has a strong diffuse expression in synovial sarcoma.[3 ]
[6 ] Thus, the difference between the conformation and intensity of the TLE1 expression
in the MPNST and synovial sarcoma could be a discriminatory marker.[3 ]
[4 ]
[6 ]
Podoplanin
Podoplanin is a mucin-type transmembrane glycoprotein, expressed in various human
cell types as well as in various tumors, such as hemangioblastomas, tissue of testis,
germ cell tumors, ovarian serous carcinomas and squamous cell carcinomas. It has been
associated with invasion potential of adjacent tissue, and hence, podoplanin is highly
expressed in the invasive front of the MPNST. The glycoprotein is negative for dermal
and plexiform neurofibromas, except for the perineurial cells, and positive in the
compact area of atypical neurofibroma. Podoplanin is also positive in Antoni A (cellular)
areas, but not in the Antoni B (reticular) areas of the schwannomas. Moreover, the
difference between the staining pattern of schwannomatosis and NF2-associated tumors,
and vestibular schwannoma tumors was not statically relevant.[6 ]
[8 ]
CD56 and PGP 9.5
CD56 and PGP 9.5 are sensitive biomarkers for peripheral nerve sheath tumors, like
schwannomas and neurofibromas; thereby, they could be used as trial markers. However,
their expression is not specific for MPNST.[7 ]
[23 ]
Nestin
Nestin, an intermediate filament protein expressed in neuroectodermal stem cells,[4 ]
[7 ] has shown higher sensitivity for MPNST,[7 ] with more extensive pattern with cytoplasmatic staining than other neural markers[4 ] (such as CD56, PGP 9.5 or S100), whereas with weak or no expression in benign nerve
sheath tumors and other sarcomas.[4 ]
[7 ] On the other hand, Shimada et al found similar expression of nestin in rhabdomyosarcoma,
leiomyosarcoma and desmoplastic melanoma.[24 ] And hence, nestin, if in combination with other markers, may be an useful marker
in the diagnosis of MPNST.
HMGA2
The high mobility group AT-hook 2 (HMGA2) has been considered as a specific biomarker
for MPNST when compared with its close morphologic mimic, such as synovial sarcoma;
therefore, immunohistochemical staining of HMGA2 could be a useful marker to separate
MPNST from synovial sarcoma.[4 ]
[7 ]
[23 ]
EGFR, p16, Neurofibromin and Ki -67
Pekmezci et al have evaluated several possible useful biomarkers for the differential
diagnosis of MPNST and cellular schwannoma and concluded that these 4 biomarkers had
promising use: epidermal growth factor receptor (EGFR), Ki -67, p16 and neurofibromin.[5 ]
Epidermal growth factor receptor is considered an important component in the pathogenesis
of MPNST.[5 ] Pekmezci et al have shown that the expression of EGFR is exclusive of the MPNST
in comparison with cellular schwannoma (31% and 0%, respectively). Furthermore, other
authors had previously showed a lack of EGFR expression in schwannomas, in a general
way.[5 ]
They show that there is a substantial loss of neurofibromin (the neurofibromatosis
type 1 protein product) in almost all MPNST, except for few low-grade cases, while
it is retained in other spindle-cell tumors.[5 ]
The same authors also observed a significant loss of expression of p16 in MPNST (73%)
against a total preservation in cellular schwannoma.[5 ] The biomarker p16 is typically expressed in neurofibromas, and its loss could actually
point to a malignization.[6 ] The amplified expression of EGFR as well as the loss of expression of p16 and neurofibromin
can help with the differential diagnosis of MPNST.[5 ]
Lastly, Pekmezci et al observed that in cellular schwannoma slices demonstrating variable
labeling with focal stained areas, MPNST showed a diffuse expression. Setting a sensitivity
of 87% and a specificity of 96% for Ki -67.[5 ] The Ki -67 usage was also cited by Rodriguez et al.[5 ]
[6 ] Actually, some authors are already using the Ki -67 expression in addition to the World Health Organization (WHO) classification of
tumors of soft tissue and bone as a criterium to denote malignancy in perineuriomas.[25 ]
[26 ]
p57NTR and SOX2: still an Obscure Area
Pekmezci et al highlight SOX2 and p57NTR, two neural stem-cell proteins, as promising
biomarkers: p57NTR has been found more often in MPNSTs than in schwannomas and healthy
tissue, but when comparing their conclusions with those of other authors, we found
that there is still controversy in the literature. On the other hand, SOX2 has been
shown in their study as a marker for both MPNSTs and schwannomas, while lacking expression
in normal peripheral nerve tissue. However, further studies are needed for a better
understanding of its role in peripheral nerve tissue tumors.[5 ]
