Practical Approach to Histological Diagnosis of Peripheral Nerve Sheath Tumors: An Update

Peripheral nerve sheath tumors encompass a wide spectrum of lesions with different biological behavior, including both benign and malignant neoplasms as well as the recent diagnostic category, i.e., “atypical neurofibromatous neoplasm with uncertain biologic potential” to be used only for NF1 patients. Neurofibromas and schwannomas are benign Schwann-cell-derived peripheral nerve sheath tumors arising as isolated lesions or within the context of classical neurofibromatosis or schwannomatoses. Multiple tumors are a hallmark of neurofibromatosis type 1(NF1) and related forms, NF2-related-schwannomatosis (formerly NF2) or SMARCB1/LZTR1-related schwannomatoses. Perineuriomas are benign, mostly sporadic, peripheral nerve sheath tumors that show morphological, immunohistochemical, and ultrastructural features reminiscent of perineurial differentiation. Hybrid tumors exist, with the most common lesions represented by a variable mixture of neurofibromas, schwannomas, and perineuriomas. Conversely, malignant peripheral nerve sheath tumors are soft tissue sarcomas that may arise from a peripheral nerve or a pre-existing neurofibroma, and in about 50% of cases, these tumors are associated with NF1. The present review emphasizes the main clinicopathologic features of each pathological entity, focusing on the diagnostic clues and unusual morphological variants.


Introduction
The interest for peripheral nerve sheath tumors is mainly due to the fact that these tumors can be diagnosed in the context of tumor-predisposing syndromes, such as neurofibromatosis type 1 (NF1) and related forms, NF2-related-schwannomatosis (formerly NF2) or SMARCB1/LZTR1-related schwannomatoses (formerly schwannomatosis) [1]. Based on recent clinical and molecular advances, the diagnostic criteria of the above-mentioned syndromes have been updated [2,3]. The present review focuses on the pathological diagnostic clues of the most common benign and malignant peripheral nerve sheath tumors to aid pathologists in achieving a correct classification. The main clinicopathologic features of each single pathological entity are discussed and summarized in tables. Representative illustrations, including the morphological variants of each single tumor are also provided. Awareness by surgical pathologists of the wide morphological spectrum of these tumors and their development in the context of tumor-predisposing syndromes is crucial for providing correct prognostic information and planning the clinical/radiological follow-up of patients.

Neurofibromas
Neurofibromas are benign peripheral nerve sheath tumors mainly composed of neoplastic cells showing schwannian differentiation admixed to a minor component of cells with fibroblastic and perineurial differentiation. They represent the most common peripheral nerve sheath tumors and are usually diagnosed as apparently sporadic lesions; however, there is increasing evidence that a large number of histologically proven neurofibromas arise within the context of classical neurofibromatosis type 1 (NF1) or its alternative forms ( Table 2), and that, when isolated, they may be caused by mosaic phenomena occurred in the NF1 gene at a somatic level, and it is currently accepted that virtually all individuals affected by NF1 develop neurofibromas (Figures 1 and 2) [8]. Based on the recently revised criteria for NF1 [2] and a newly proposed classification of the different forms of neurofibromatosis and schwannomatoses [9][10][11] (Table 2), a surgical pathologist should be aware of the possibility of this spectrum of disorders when he/she is dealing with an isolated neurofibroma, with multiple lesions or a plexiform neurofibroma from an individual without a personal or familial history of classical NF1 (Table 2); similarly, the chance of being affected by classical NF1 is likely when dealing with the above-mentioned tumors arising in a child with an affected parent by NF1. Mosaic NF1 (Table 2) should be suspected when a surgical biopsy/specimens are referred to a pathologist as an isolated neurofibroma, a single plexiform neurofibroma or when multiple cutaneous and/or nodular neurofibromas were arranged in a clearly segmental/localised distribution. In all these circumstances, a surgical pathologist should always suggest a clinical evaluation and genetic counselling by an NF1 expert.  The majority of neurofibromas occur in the skin and arise from small-sized nerves. These tumors can develop in the deep soft tissues from a major or small-sized nerve [7]; however, there is the possibility that the anatomic association with a nerve cannot be demonstrated, and neurofibroma may present as a soft tissue mass. Notably, although neurofibromas may develop from spinal nerve roots, often as multiple lesions in NF1 patients or as lesions involving, bilaterally, all the spinal roots in individuals with spinal NF1 (Table 2), it is of note that the cranial nerves are spared [7]. Histologically, neurofibroma is a neoplasm with low to moderate cellularity, composed of bland-looking spindled cells with scant cytoplasm and oval, elongated, and regular nuclei without nucleoli. The cells are haphazardly embedded in a variably fibro-myxoid stroma, often containing coarse collagen bundles. Mitoses are absent or very rare (<1 mitosis/50HPF). Mast cells are frequently scattered in tumor stroma. In addition, both isolated and classical NF-associated neurofibromas may contain CD34+/fibroblastic multinucleated giant cells with the cytological features of the so-called "floret-like cells", as commonly seen in spindle cell/pleomorphic lipoma [12]. Immunohistochemically, neurofibroma is typically an S100-protein-and SOX10-positive tu-mor with a variable expression of CD34 and EMA [1,4,7]. Based on the growth pattern, the following types of neurofibromas can be recognized: (i) localized/nodular neurofibroma; (ii) diffuse neurofibroma; and (iii) plexiform neurofibroma [1,4].

Localized/Nodular Neurofibroma
This type of neurofibroma is localised in the skin or may be intraneural and, albeit usually isolated, it may be associated to classical NF1 (Tables 2 and 3). It occurs predominantly in young (20-30 years) to middle-aged adults and usually arises in the skin of the trunk, head/neck region, and extremities ( Figure 3). Clinically, it presents as soft, colored papules or a small ovoid/fusiform subcutaneous nodule, usually <2 cm in its greatest diameter, with a glistening tan-white cut section [1]. An anatomic origin from a small-sized nerve of the dermis/subcutaneous tissue cannot be usually documented. It grows slowly as a painless nodule, especially when occurring in the skin, while if it arises from deeply seated nerves, including nerve plexuses and major nerve trunks, it causes sensory or motor deficits related to the affected nerve [1]. Grossly, neurofibroma that arises from major nerves shows a fusiform expansion of the affected nerve and may appear to be encapsulated; conversely, tumors of the small nerves present as well-circumscribed-but not encapsulated-nodules, often lacking an anatomic association with a nerve, and thus they are clinically misdiagnosed as lymph nodes or soft tissue tumors. Histologically, both cutaneous or intraneural neurofibromas present as well-circumscribed and unencapsulated nodules with evidence of a grenz zone if skin-centered, while a peripheral rim of thickened perineurium may be seen in intraneural tumors (Figures 3 and 4). The typical isolated neurofibroma usually shows a low to moderate cellularity and is composed of spindled cells with small wavy or comma-shaped nuclei and poorly defined cellular borders/processes. These cells are haphazardly arranged in a variably fibro-myxoid stroma and may also be arranged, at least focally, in interlacing bundles (Figures 2 and 3). The tumor stroma usually contains collagen fibrils and ropey and/or thicker collagen fibers as well as mast cells and, less frequently, lymphocytes and xanthomatous cells. The incidence of malignant transformation into malignant peripheral nerve sheath tumors is unknown, but it is very low for cutaneous tumors, while intraneural lesions, especially in NF1 patients, may occasionally represent the morphological precursor [1]. Although surgical excision with free margins is usually curative, these lesions can be difficult to be excised in their entirety, and patients should be aware of the possibility of local recurrence.

Immunohistochemical Features
Tipically, S-100 protein and SOX10 positivity can be identified in these tumors. CD34 and EMA stain, respectively, the minor components of admixed spindled fibroblasts and perineurial cells, respectively [1].

Diffuse Neurofibroma
Diffuse neurofibroma occurs more often in the younger age group, including children. Although a number of these lesions may present as isolated/solitary manifestations, they should be regarded as mosaic NF1 lesions or other NF1 stigmata should be carefully searched for [1,9] (Tables 2 and 4). It usually presents as a plaque-like lesion of the dermis/subcutaneous tissue of the head/neck region. Histologically, diffuse neurofibroma presents as an ill-defined dermal/subcutaneous mass with infiltrative margins entrapping (without destroying) cutaneous adnexal and nerve structures and with extension to the subcutaneous fat, especially along connective tissue septa ( Figure 5). The neoplastic cells, closely intermingling with adipocytes, impart the tumor with a honeycomb effect similar to that seen in dermatofibrosarcoma protuberans [1] (Figure 5). In contrast to the other types of neurofibroma, the diffuse type is composed of short spindled to round cells with small, hyperchromatic, wavy or comma-shaped nuclei. The neoplastic cells are haphazardly set in a uniformly fine fibrillary stroma with variably myxoid changes. Some tumors may contain clusters of pseudomeissnerian-body-like structures that seem to be a characteristic feature of diffuse-type neurofibroma [1,7]. Notably, in NF1 patients, it is not uncommon to detect tumor cells admixed with large ectatic vessels and occasionally with branching plexiformtype capillary-like vessels, as seen in myxoid liposarcoma ( Figure 5D). In a minority of tumors, neoplastic cells may focally adopt an epithelioid morphology or contain scattered dendritic cells with intracytoplasmic melanin. Although surgical excision with free margins is usually curative, these lesions can be difficult to radically remove. The risk of malignant transformation into a malignant peripheral nerve sheath tumor is very low [13].

Immunohistochemical Features
Immunohistochemical analyses reveal the S100-protein and SOX10 positivity of tumor cells.

Plexiform Neurofibroma
Plexiform neurofibroma is a type of neurofibroma arising virtually only in NF1 patients (up to 40% of all patients with NF1), and thus its histologically proven diagnosis is considered pathognomonic of this syndrome [1] (Table 5); notably, even an isolated/solitary plexiform neurofibfroma (i.e., a plexiform neurofibroma truly presenting without other NF1 stigmata after careful clinical, laboratory and instrumental work-up, should be regarded as a manifestation of mosaic NF1 ( Table 2); occasionally superficial and deep soft tissues of an entire extremity are involved, giving rise to the rare condition known as "elephantiasis neuromatosa", which is characterized by hyperpigmented, pendulous folds of the overlying skin and is often associated with bone hypertrophy [1]. Grossly, plexiform neurofibroma appears as a multinodular mass composed of irregular/serpentine structures reminiscent of intermingling nerve fascicles (bag of worms appearance) ( Figure 6). Histologically, the tumor consists of multinodular and serpentine nerve-like structures with abundant myxo-edematous stroma containing thick haphazardly arranged collagen fibers (shredded carrots appearance) [1]. Notably, tumors cells are usually associated with thicker collagen bundles, often simulating bundles of smooth muscle cells. Interestingly, the neoplastic proliferation may extend beyond the nerve-like structures into the surrounding tissues with a morphology closely resembling diffuse-type neurofibroma; this latter growth pattern may be occasionally predominant, obscuring the underlying plexiform architecture. The importance of a correct diagnosis of plexiform neurofibroma relies not only on its association with NF1 but also in its highest risk of malignant transformation among all types of neurofibromas [1].

Potential Morphological Pitfalls of Malignancy in Neurofibromas: Cytological Atypia and Hypercellularity
Despite the different types, both sporadic and NF1-associated neurofibromas may contain focal or diffuse nuclear atypia in the absence of increased mitotic activity (≥1 mitosis/50 HPF) and/or hypercellularity. Although alarming, cytological atypia alone, in the context of an otherwise specified neurofibroma, is not by itself a criterion of malignancy [1,6]. Nuclear atypia consists of nuclear enlargement (2-3-fold or more), hyperchromasia, and irregular chromatin distribution ( Figure 7); multinucleated/bizarre cells are in the spectrum of cytological atypia. Although similar to the so-called "degenerative atypia" seen in ancient schwannoma, some authors favor the use of the term "neurofibroma with cytological atypia" rather than "ancient neurofibroma", especially in NF1 patients [1]. This is mainly due to the fact that it is impossible to morphologically distinguish "degenerative atypia" from the "malignancy-related atypia" that is one the morphological criteria for malignant transformation along with an increased number of mitoses, hypercellularity, and a loss of neurofibroma architecture. Accordingly, we recommend that all neurofibromas showing cytological atypia alone should be extensively sampled to rule out the presence of areas with hypercellularity, increased mitotic activity, and/or tumor necrosis. Potential diagnostic pitfalls may arise when inflammatory cells, especially lymphocytes and histiocytes, may be extensively found among neoplastic cells, imparting a neurofibroma with a false hypercellular appearance. In addition, it should be emphasized that the diagnosis of neurofibroma with cytological atypia should be rendered with caution when a pathologist is dealing with small biopsies from NF1 patients. Based on these considerations, radiological correlation is crucial, suggesting obtaining multiple core biopsies from areas with the suspicion of malignant transformation to avoid the risk of tumor undersampling. As neurofibromas with cytological atypia are treated conservatively, even in presence of positive margins [1], some authors discourage the term "atypical neurofibroma" to avoid confusion among clinicians who could consider it as a premalignant lesion [1]. Apart from cytological atypia, the term "cellular neurofibroma" should be restricted to those tumors in which hypercellularity is the only worrisome feature and in the absence of increased mitotic activity, cytological atypia, and/or the loss of neurofibroma architecture [6]. Histologically, cellular neurofibroma can be appreciated at low magnification for the presence of areas with greater cellularity and the fascicular arrangement of neoplastic cells [1] (Figure 8). Similar to cytological atypia, hypercellularity alone is not by itself a criterion of malignancy. Cellular neurofibroma should be extensively sampled to rule out the presence of areas with cytological atypia and/or increased mitotic activity. As in neurofibroma with cytological atypia, it should be emphasized that the diagnosis of cellular neurofibroma should be rendered with caution when a pathologist is dealing with small biopsies from NF1 patients. Similarly, radiological correlation is crucial, suggesting obtaining multiple core biopsies from those areas with a high suspicion of malignant overgrowth to avoid the risk of tumor undersampling. Cellular neurofibroma is treated conservatively, even in presence of positive margins.

Immunohistochemical Features
Although it is not routinely recommended, immunohistochemistry may be reassuring, revealing a low percentage of Ki67-positive cells and p53 positivity restricted to a few cells [6].

Atypical Neurofibromatous Neoplasm with Uncertain Biologic Potential (ANNUBP)
The term "atypical neurofibromatous neoplasm with uncertain biologic potential (ANNUBP)" has been recently coined [6] for a subset of NF1-associated neurofibromatous tumors that exhibit at least two of the following features: (i) nuclear atypia; (ii) hypercellularity; (iii) increased mitotic activity (>1 mitosis/50 HPF but <3 mitoses/10 HPF); and (iv) a variable loss of neurofibroma architecture (i.e., the presence of herringbone and/or fascicular storiform growth patterns) ( Figure 9) ( Table 6). These tumors are a provisional diagnostic category created on the evidence of their virtually absent (or very low) risk of metastasis. Similar to neurofibromas with cytological atypia or hypercellularity alone, the diagnosis of ANNUBP should be rendered with caution on small biopsies, suggesting to clinicians that the examined tumor does not fit all the morphological criteria of malignancy. If the radiological suspicion of malignancy is high, a close clinical follow-up and/or additional bioptic sampling is necessary.

Immunohistochemical Features
Although immunohistochemical analyses are not helpful for the diagnosis of ANNUBP, a variable to complete loss of S100 protein/SOX10 expression and a loss of the CD34-positive fibroblastic network, typically found in all types of neurofibromas, may be suggestive [6].

Malignant Transformation in Neurofibromas
Although the risk of transformation into a malignant peripheral nerve sheath tumor (MPNST) is very low (if any) in isolated/solitary cutaneous/nodular or diffuse neurofibromas, it is well-documented in NF1-associated intraneural and/or plexiform neurofibromas [1]. The lifetime risk for the development of MPNST in NF1 patients has been calculated as 8-16% in two population-based studies [14][15][16]. Most NF1 patients develop MPNST during their 3rd to 4th decades, even if malignant transformation can also occur in childhood [notably, children/adults harbouring NF1 gene deletions (i.e., the so-called"NF1 microdeletion syndrome" (Table 2), are at higher risk of malignant transformation]. In the majority of cases, the diagnosis of malignant transformation is straightforward in that it is represented by a highgrade sarcoma, often arising abruptly from areas of classic neurofibroma into hypercellular areas with diffuse cytological atypia, increased mitotic activity (often >10 mitoses/10 HPF), and/or tumor necrosis. Less commonly, the transition is more challenging, being represented by areas of well-differentiated neoplasms with the morphological features of "atypical neurofibromatous neoplasms with uncertain biologic potential (ANNUBP)" in which, however, the mitotic activity ranges from 3 to 9 mitoses/10 HPF in the absence of tumor necrosis (low-grade MPNST). In a minority of cases, all the morphological spectrum ranging from a classic neurofibroma to ANNUBP, to low-grade and high-grade MPNST, can be seen.

Classic Schwannoma
Classic schwannoma is by far the most common type among all schwannomas and may occur as an isolated/solitary lesion (think always about mosaicism for the schwannomatoses genes) or within the context of any type of syndromic schwannomatoses ( Table 8). The histological hallmark of classic schwannoma is the presence of alternating hypercellular (so-called "Antoni A areas") and hypocellular (so-called "Antoni B areas") areas, variably represented in the same tumor; the former areas may blend imperceptibly or abruptly into the latter ones [1]. Antoni A areas are composed of compact spindled cells with wavy nuclei, dense chromatin, and indistinct cytoplasm, often arranged in short and intersecting fascicles, sometimes forming meningioma-like whorls [1]. Notably, in these areas the neoplastic cells characteristically exhibit nuclear palisading around anucleated central spaces filled with eosinophilic fibrillary cytoplasmic processes (the so-called "Verocay bodies") [1,4] (Figure 13). Conversely, Antoni B areas are less cellular and are composed of spindled to ovoid-shaped cells, haphazardly set in a loose myxoid stroma with microcystic changes, inflammatory cells (lymphocytes and histiocytes), and collagen fibers; in these hypocellular areas, there are numerous large-sized and irregularly spaced blood vessels with thickened hyalinized walls, often filled with fibrin. Mitoses are usually absent or rare. Interestingly, schwannomas arising in the gastrointestinal tract typically show a lymphocytic rim at their periphery.

Immunohistochemical Features
Immunohistochemically, unlike neurofibroma, schwannoma exhibits a diffuse and strong S100 protein expression in that it is almost exclusively composed of spindle cells with schwannian differentiation [1,4,6,17]. Although S100 protein expression is typically weaker and more patchy in Antoni B areas, it may be helpful when dealing with a schwannoma exhibiting extensive degenerative changes, in which the amount of fibro-myxoid stroma and/or cystic stromal degeneration can make diagnosis more challenging. SOX-10 is an additional marker of schwannoma that is expressed in the majority of cases [1]. Interestingly, occasional aberrant expressions of cytokeratins, desmin, and TTF-1 can be found [18,19]. Notably, a mosaic expression pattern of INI1 has been found to be helpful in distinguishing schwannomas, in the context of the different types of schwannomatosis, from apparently sporadically occurring and solitary tumors as [20].

Schwannoma with Degenerative/Ancient Changes ("Ancient Schwannoma")
This term should be reserved to those schwannomas that histologically show degenerativetype atypia as well as marked stromal changes (Table 9). These tumors often are large-sized and long-standing and usually affect the deep soft tissues, especially the retroperitoneum or head and neck region [1,21,22]. Histologically, their characteristic features are nuclear atypia of the "degenerative type" as well as other degenerative changes, including cystic stromal changes, hemorrhages, calcifications, and diffuse stromal hyalinization [1] (Figure 14). Numerous histiocytes/siderophages are often encountered, intermingling with tumor cells. The striking nuclear atypia is the most alarming feature of ancient schwannoma, representing a diagnostic pitfall of malignancy; tumor cells usually contain large, hyperchromatic, and multilobulated nuclei, but they characteristically lack mitotic activity. The disproportion between nuclear atypia and mitotic figures is a diagnostic clue of benignancy [1]. Table 9. Key diagnostic features of schwannoma with degenerative/ancient changes ("ancient schwannoma").

•
Histological definition: schwannoma with degenerative-type nuclear atypia and stromal changes

Clinical Features
• usually large-sized and long-standing mass • frequently deeply located tumors (especially the retroperitoneum and head and neck region)

Cellular Schwannoma
Schwannomas that are exclusively or predominantly composed of Antoni A areas and lacking Verocay bodies (or only focally seen) are labeled as "cellular schwannomas" (Table 10) [1,4]. Unlike the classic type, cellular schwannoma more frequently affects the deep soft tissues, including the retroperitoneal and posterior mediastinal regions, with only a minority of cases occurring at the extremities [23]. The macroscopic appearance is similar to that of classic-type schwannoma, but occasionally a multinodular or plexiform architecture can be appreciated. Histologically, it is composed uniformly of hypercellular Antoni A areas, albeit a minority of tumors can contain Antoni B areas accounting for less than 10% of the entire tumor ( Figure 15). Like in classic-type schwannoma, Antoni A areas are composed of short intersecting fascicles; however, an additional characteristic feature is the presence of long sweeping fascicles arranged in a herringbone and/or storiform growth pattern, closely mimicking fibrosarcoma or leiomyosarcoma [1]. Malignancy can also be seriously considered because mitotic activity is higher (usually <4-5 mitoses/10 HPFs) than that seen in classic-type schwannoma, and foci of necrosis can be found in up to 10% of cases [1,4]. Another alarming feature is the presence of infiltrative margins that can produce bone erosion [1]. Due to the presence of the above-mentioned "alarming" features, cellular schwannoma can be misdiagnosed as a spindle cell sarcoma, especially if the pathologists are unfamiliar with soft tissue tumors. The benign nature of the tumor is supported by the following features: (i) sharply demarcated margins (often encapsulation); (ii) a focal presence of Antoni B areas; (iii) disproportion between hypercellularity, nuclear pleomorphism, and the amount of necrosis and mitoses; and (iv) strong and diffuse S100 protein and SOX10 immunoexpression. The cellular schwannoma has a higher rate (4-40%) of local recurrence, likely due to its deeper anatomic location that makes a complete surgical excision difficult [1]. • higher rate (4 to 40%) of local recurrence than the classic-type schwannoma when surgical excision is incomplete

Immunohistochemical Features
Immunohistochemistry is crucial for achieving a correct diagnosis, especially on small biopsies, as virtually no bland-looking spindle cell sarcoma is diffusely and strongly stained with S100 protein and/or SOX10; typically, the nuclear expression of H3K27me3 is retained, and the lack of expression of epithelial and myogenic markers as well as CD117/DOG1 is useful in the differential diagnosis with other tumor entities, such as synovial sarcoma, leiomyosarcoma, and GIST, respectively.

Plexiform Schwannoma
Plexiform schwannomas represent an uncommon variant of schwannoma (about 5% of all cases) that characteristically exhibit a multinodular/plexiform architecture, often appreciated at macroscopic examination (Table 11). Unlike plexiform neurofibromas, which most often arise within the context of classical NF1 and are virtually pathognomonic to NF1, most plexiform schwannomas are (at least initially) isolated/solitary, with only a few cases arising within the context of NF2-related schwannomatosis (NF2/MERLIN schwannoma predisposing syndrome) [1,4,9,[24][25][26][27][28][29]; one must think, however, in the cases of truly isolated/solitary lesions (after extensive work-up), about mosaicism for the schwannomatoses genes (Table 7). They commonly affect the skin (dermis/subcutis) of the head and neck region and the distal extremities; more rarely, deep soft tissues are involved. Although usually encapsulated, they may lack the capsule. Histologically, the hallmark is the multinodular/plexiform growth pattern. Apart from this peculiar growth pattern, plexiform schwannoma is usually a cellular schwannoma, being mainly composed of Antoni A areas ( Figure 16) [24][25][26][27][28][29]. Pathologists should be aware of this hypercellularity to avoid a misdiagnosis of sarcomatous transformation/overgrowth in a plexiform schwannoma. The rare occurrence of plexiform schwannomas in deeper soft tissues or in large peripheral nerves may represent a further diagnostic challenge, as they may exhibit increased cellularity and mitotic count and thus pose differential diagnostic problems with malignant peripheral nerve sheath tumors [24][25][26][27][28][29]. The absence of tumor necrosis, significant nuclear atypia, and the relatively low number of mitoses are in contrast with malignancy [1]. Although a higher rate of local recurrence has been reported, additional studies on larger series are needed.

Immunohistochemical Features
The strong and diffuse expression of S100 protein and SOX10, along with the retained expression of H3K27me3 in tumor cells, are helpful to exclude a malignant peripheral nerve sheath tumor [1].

Epithelioid Cell Schwannoma
Epithelioid cell schwannoma is a rare variant of schwannoma composed exclusively/ predominantly of epithelioid tumor cells showing schwannian differentiation (Table 12) [1]. This tumor arises as an isolated/solitary lesion, usually sporadically, without any evident association with any of the different forms of schwannomatoses [1,4]. Epithelioid schwannoma is usually a well-circumscribed and small-sized tumor arising in the skin or in the superficial soft tissues, especially of the extremities. Histologically, it is composed of smallto intermediate-sized rounded to epithelioid cells with abundant eosinophilic cytoplasm and well-defined cellular borders; nuclei are rounded and contain prominent nucleoli and nuclear pseudoinclusions; and a variable number of large-sized cells with the morphology of deciduoid-like cells can be seen (Figure 17) [30][31][32]. The neoplastic cells, often set in a fibro-myxoid stroma, are usually arranged singly or in small nests or cords. A characteristic feature is the presence of collagen rosettes consisting of the condensation of neoplastic cells around central cores of dense collagen. These rosettes can be occasionally seen in classic-type schwannoma (the so-called "neuroblastoma-like schwannoma) [33][34][35]. The diagnosis of epithelioid cell schwannoma can be suggested by the identification, at least focally, of tumor areas with the morphology of classic schwannoma. Nuclear atypia and mitoses can be documented, but they do not represent signs of malignancy. Actually, epithelioid cell schwannoma is considered a benign tumor with a very low risk of malignant transformation, similar to that documented in the other types of schwannomas.

Immunohistochemical Features
As for the other types of schwannoma, even epithelioid cell schwannoma is strongly and diffusely stained with S100 protein and SOX10 [1]. Notably, in the last years there is convincing evidence that approximately 40% of cases lack nuclear expression of SMARCB1/INI1 [36].

Unusual Features in Schwannomas
Schwannomas may rarely contain glands and epithelial structures that are believed to represent "true" epithelial differentiation rather than entrapped normal elements [37]. In addition, large cysts lined by Schwann cells with rounded/epithelioid morphology mimicking epithelial differentiation may be occasionally encountered (pseudoglandular schwannomas) [38]. Occasionally, schwannomas may exhibit, at least focally, a small cell component with scant cytoplasm arranged around collagen-filled spaces or around vessels, forming collagen rosettes and/or perivascular collagen pseudorosettes (neuroblastoma-like schwannoma) [39,40]. Like in neurofibromas [41], rare cases of schwannomas exhibiting intralesional mature adipocytes, along with lipoblast-like cells with signet-ring cell morphology, have been labeled as "schwannomas with lipoblastic differentiation" [42].

Perineuriomas
Perineurioma is a benign peripheral nerve sheath tumor composed almost entirely of neoplastic cells that exhibit morphological, immunohistochemical, and ultrastructural features consistent with perineurial differentiation [1,4]. Different from other peripheral nerve sheath tumors, perineurioma is a sporadic tumor with only a few cases reported to be associated with NF1 or other types of schwannomatosis [1]. Based on its location, perineurioma can be distinguished as: (i) intraneural perineurioma or (ii) soft tissue perineurioma [49,50].

Intraneural Perineurioma
Intraneural perineurioma, also known as "localized hypertrophic neuropathy", is a rare benign tumor growing within and expanding the nerve fascicles (Table 13) [49]. This tumor primarily affects the extremities of young adults and children, and it usually presents as a solitary, slowly growing, painless mass, often causing neurologic motor and/or sensory defects. The most commonly affected nerves are the ulnar, median, peroneal, sciatic, and radial nerves [1,49]. Grossly, the involved nerve shows segmental and fusiform expansion with variable extension; if the affected nerve is small in size, a plexiform-type architecture can be appreciated. The histological hallmark is a proliferation of bland-looking spindleshaped cells with wavy to round nuclei arranged in concentric layers (onion-bulb-like arrangement) surrounding the more centrally located Schwann cells and axons [1,4]. This growth pattern is better appreciated when evaluating cross, rather than longitudinal, sections of the involved nerve. The clinical behavior is benign.  Immunohistochemically, perineurial cells are stained with EMA; immunoreactivity has also been reported with claudin-1 and GLUT-1 [1].

Molecular Features
Although intraneural perineurioma had been considered as a reactive process (localized hypertrophic neuropathy), the identification of clonal cytogenetic abnormalities involving chromosome 22 as well as the recent discovery of TRAF7 mutations in 60% of cases seems to confirm its neoplastic nature [51][52][53][54][55].

Soft Tissue (Extraneural) Perineurioma
Soft tissue perineurioma presents as a well-circumscribed mass, usually occurring in the subcutaneous tissue of the extremities and trunk; less commonly, it is located in the dermis, deep soft tissue, or in visceral locations (Table 14) [1,50]. This tumor usually occurs in middle-aged adults and is slightly more frequent in females; children are rarely affected. The clinical presentation is that of a painless and slowly growing mass. Grossly, soft tissue perineurioma is a well-circumscribed unencapsulated mass ranging in size from 1 cm to 20 cm. The cut surface shows a firm or rubbery consistency and a yellow-tan to whitish color. Histologically, it is a well-circumscribed (rarely infiltrative) and unencapsulated tumor composed of a proliferation of slender fibroblast-like cells with long bipolar cytoplasmic processes variably arranged into a fascicular, storiform, whorled, or lamellar (Pacinian) growth pattern ( Figure 18) [1,50]. Cellularity is variable, ranging from paucicellular to densely cellular areas. The tumor stroma is usually collagenized, with focal edematous or myxoid changes. Nuclear atypia, mitoses, and tumor cell necrosis are virtually absent; however, larger tumors may exhibit ischemic-type foci of necrosis. Rare cases may exhibit a minor component of plumper and even epithelioid tumor cells [56]. An intratumoral lipomatous component, including an unusual "pseudolipoblastic" morphology mimicking liposarcoma, has also been reported [57]. Occasionally, some cases exhibiting ossification foci, granular cell change, and Pacinian-like bodies have been described [58,59]. Apart from the classic type, soft tissue perineurioma may exhibit in at least two different histological variants, reticular and sclerosing perineurioma [60][61][62][63]. The former is characteristically composed of slender spindled cells, usually set in a myxo-edematous stroma showing cytoplasmic anastomosing processes, resulting in a reticular or lace-like appearance [60,61]. Sclerosing perineurioma is an unusual variant, mainly occurring in young male adults and involving almost exclusively the superficial soft tissues of the hand. Histologically, it is composed of a proliferation of spindled to rounded/epithelioid cells with pale cytoplasm, indistinct cell borders, and slightly hyperchromatic nuclei arranged in corded, trabecular, or whorled growth patterns and set in an abundant fibro-sclerotic stroma ( Figure 19) [62,63]. Despite the morphological variant, perineurioma may exhibit potential alarming/atypical features in approximately 10-20% of cases, including degenerative nuclear atypia (pleomorphic and multinucleated cells with nuclear pseudoinclusions), mitotic activity, hypercellularity, and the infiltration of skeletal muscle. Notably, all patients with perineuriomas showing these atypical features have experienced a benign clinical course [63].

Molecular Features
Although not associated with NF1 or any type of schwannomatosis, soft tissue perineuriomas share molecular alterations with other nerve sheath tumors, including NF2 point mutations and deletions of chromosome 13 and 22q12 (NF2) and the deletion of 17q11 (including NF1) [1,51]. Interestingly, TRAF7 mutations, seen in intraneural perineuriomas, are not observed in the soft tissue counterpart [52].

Hybrid Tumors
Although most peripheral nerve sheath tumors can be easily diagnosed by surgical pathologists and are correctly classified as neurofibromas, schwannomas, and perineuriomas, a minority of neoplasms are difficult to categorize into one specific diagnostic category. Most of these tumors exhibit a plexiform architecture and are represented by a variable combination of neurofibromatous and schwannomatous areas (hybrid tumors) ( Figure 20). Most tumors are associated with the different types of schwannomatosis and less frequently with NF1 [64]. It has been suggested that the hybrid tumors arising in the context of NF1 patients may be better defined as "plexiform neurofibromas with schwannomalike nodular proliferations" [1]. Notably, about 45% of these tumors have been shown to be associated with chromosome 22 monosomy [65,66]. Apart from hybrid tumors with neurofibroma/schwannoma features, there is increasing evidence of a distinct category composed of a mixture of perineurioma and schwannoma areas [1,67,68]. Although histologically similar to soft tissue perineurioma, such tumors are composed of two different cytotypes, namely, perineurial cells (EMA/claudin/GLUT1-positive) variably arranged into fascicular, storiform, or reticular arrangements and spindled cells (S100/SOX10-positive) with wavy nuclei, closely reminiscent of schwannian differentiation. Only rarely, hybrid tumors with both neurofibroma and perineurioma features have been reported in the literature [69][70][71][72]. Their classification is still to be defined, as it is not clear if they represent a "true" distinct category or simply neurofibromas rich in perineurial cells.

Malignant Peripheral Nerve Sheath Tumors (MPNSTs)
MPNSTs represent about 5% of all sarcomas and may arise from a peripheral nerve or a pre-existing neurofibroma; in about 50% of cases, these tumors arise in the context of NF1, while in the remaining cases they appear to be sporadic (40% of cases) or associated with a previous history of radiation [1,4,6,[73][74][75]. A significant number of the sporadic tumors arise in the deep soft tissues without any anatomic evidence of association with a peripheral nerve or pre-existing neurofibroma (soft tissue MPNST) [73][74][75]. The sporadic tumors usually occur in patients with an age ranging from 30 to 50 years, while the NF1asociated tumors can manifest in younger patients, including in childhood [73][74][75]. The most common sites are the extremities, trunk, and head/neck region; among the major nerves involved, the sciatic nerve is the most common one, followed by the brachial and sacral plexuses and the paraspinal nerves. Clinically MPNSTs present as growing painless or painful masses; in NF1 patients, any rapid enlargement of a pre-existing neurofibroma is suggestive of malignant transformation. Grossly, tumor masses are large-sized (usually >5 cm) with a white-gray firm to fleshy cut surface; necrotic areas and hemorrhage are common. Histologically, the following subtypes of MPNSTs are recognized (

Classic MPNST
The most common subtype of MPNSTs is the classic type, basically a spindle cell sarcoma that can be divided into low-and high-grades based on the following morphological features: (i) the loss of neurofibroma architecture; (ii) hypercellularity; (iii) cytological atypia; (iv) mitotic activity; and (v) tumor necrosis [1,[73][74][75]. Low-grade MPNSTs are uncommon (10% of all MPNSTs) and usually arise from a pre-existing NF1-associated neurofibroma ( Figure 21) [1,76]. Although morphologically similar to atypical neurofibromatous neoplasms with uncertain biologic potential (ANNUBP), they differ in that mitotic activity ranges from >3 to 9 mitoses/10 HPF; tumor necrosis is absent by definition [6]. Accordingly, before making a diagnosis of atypical neurofibromatous neoplasm with uncertain biologic potential (AN-NUBP) (>1 mitosis/50 HPF but <3 mitoses/10 HPF), an extensive sampling of the tumor and a meticulous search of mitoses are necessary to rule out the possibility of a low-grade MPNST [6]. Most MPNSTs (85-90%) are high-grade spindle cell sarcomas with diffuse moderate-to-severe cytological atypia, high mitotic activity, and tumor necrosis. A high grade is equally assigned to tumors in the presence of both tumor necrosis and mitotic activity ranging from 3 to 9 mitoses/10 HPF or if mitotic activity is >10 HPF, even in the absence of tumor necrosis [6]. MPNSTs are usually composed of relatively uniform spindled cells arranged in densely cellular fascicles and/or whorls (fibrosarcoma-like appearance), often alternating and interdigitating with more hypocellular and myxoid areas (marble-like appearance) ( Figure 22). Some tumors may exhibit, at least focally, other growth patterns, including herringbone, nodular, curlicue, and palisading patterns. The neoplastic cells usually have pale eosinophilic cytoplasm with ill-defined cellular borders and hyperchromatic elongated nuclei. The tumor stroma is variably fibro-myxoid in nature. Although not pathognomonic, in most cases there is a perivascular condensation/accentuation of neoplastic cells that appear to push or herniate into the vascular lumens. Extensive geographic tumor necrosis with a peritheliomatous pattern (viability of neoplastic cells limited to perivascular regions) is a common feature [1]. Unusual features include the presence of collagen bands and/or nodules (collagen rosettes), focal epithelioid or, more rarely, small cell morphology (likely neuroepithelial differentiation) [1]. In a minority of cases, MPNSTs may contain areas of marked nuclear anaplasia closely reminiscent of undifferentiated pleomorphic sarcoma [1]. Notably, a small subset of MPNSTs (10% of cases), especially those arising in NF1 patients, may contain cellular lines of divergent differentiation (heterologous elements), including a variable number of relatively mature rhabdomyoblasts (so-called "Triton tumor") ( Figure 23) and osteosarcomatous, chondrosarcomatous, and more rarely, angiosarcomatous or liposarcomatous components; well-differentiated glands, occasionally with malignant cytology, composed of cuboidal/columnar clear cells can be exceptionally appreciated [1,[77][78][79]. The 5-year survival rate is 51%, and the prognosis seems to be closely related to radical surgery (10-year survival: 80% versus 14%, respectively, for patients who had radical or incomplete surgery) [1,6,14,76].

Immunohistochemical Features
Immunohistochemically, a diffuse staining for S100 protein and/or SOX10 is a rare feature, with only 40% of cases showing only focal expression [1]. The loss of trimethylated histone 3 at lysine residue 27 (H3K27me3) expression due to the inactivation of polycomb repressor complex (PRC2) for mutations of the SUZ12 gene is a diagnostic tool [1,5,80]. As a mosaic expression pattern of H3K27me3 (alternating of positive and negative cells) can be obtained in other sarcomas, especially synovial sarcoma, with which MPNST shares some morphological features, only a complete loss of H3K27me3 expression should be considered useful for the diagnosis of MPNSTs in the appropriate clinicopathologic context [1].

Epithelioid Cell MPNST
Epithelioid cell malignant peripheral nerve sheath tumor (MPNST) is a rare subtype, accounting for approximately 5% of all MPNSTs [1,4,[81][82][83][84][85]. By definition, these tumors are composed predominantly or exclusively of Schwann cells with a polygonal/epithelioid morphology. Their incidence and age distribution mimic those of its more common spindle cell counterpart, with most cases occurring mainly in adults (40-50 years) with a slight male predominance. Epithelioid cell MPNSTs arise sporadically in the deep soft tissues from major nerves of the extremities or trunk, including the sciatic, tibial, peroneal, facial, antebrachial cutaneous, and digital nerves; occasionally some cases have been documented to occur in superficial soft tissues or in the skin. Only a few cases have been documented in the context of NF1 patients [81]. Clinically, they present as slowly growing painful or painless masses. Notably, a significant number of epithelioid cell MPNSTs develop from pre-existing sporadic schwannomas, often exhibiting an epithelioid cell morphology, and they develop only exceptionally in patients with schwannomatosis [1]. Grossly, they present as well-demarcated multinodular masses that are firm in consistency and have a fleshy greyish cut surface. Histological examination shows a tumor with a multinodular growth pattern composed of nests and/or cords of large-sized epithelioid cells with abundant eosinophilic to amphophilic cyto-plasm and large rounded nuclei with vesicular chromatin and prominent nucleoli ( Figure 24). Cellularity is variable, ranging from densely cellular tumors to hypocellular myxoid lesions. The tumor stroma is variably fibro-myxoid. Mitoses and necrosis are relatively common. Although most tumors are composed exclusively/predominantly of epithelioid cells, a minor spindle cell component closely resembling a classic MPNST can be occasionally seen. Unusual histologic features, including clear cell changes and a rhabdoid cell morphology, have been reported. Due to their morphological appearance, differential diagnosis with metastatic epithelioid cell melanoma, undifferentiated carcinoma, or primary or metastatic proximal-type epithelioid cell sarcoma is extremely difficult on morphology alone unless an origin from a nerve or schwannoma can be demonstrated. Epithelioid MPNSTs are malignant lesions with significant metastatic potential, usually to the lungs. It is still to be established if superficially located tumors have a better prognosis than deep-seated ones [1].

Immunohistochemical Features
Unlike classic MPNSTs, epithelioid cell MPNSTs typically show a diffuse and strong immunohistochemical expression of S100 protein and SOX-10 [1]; in addition, the loss of nuclear expression of SMARCB1 (INI-1) has been documented in approximately 50% of cases [84], while the nuclear expression of H3K27me3 is typically retained [1]. Unlike in melanoma, Melan-A, MITF, and HMB-45 are not expressed. Although cytokeratin expression can be occasionally documented, posing differential diagnostic problems with metastatic carcinoma, the latter lacks S100 protein and SOX10 expression [1].

Perineurial Malignant Peripheral Nerve Sheath Tumors (Malignant Perineurioma)
Malignant perineurioma, also called "Malignant Peripheral Nerve Sheath Tumor with perineurial differentiation" represents an extremely rare variant of MPNST that is not related to NF1 or schwannomatoses (Table 16) [1,[85][86][87]. The most commonly reported sites of occurrence include the extremities, trunk, and face as well as visceral sites, the mediastinum, and the retroperitoneum of adult patients. These tumors are distinguished from their benign counterpart by the presence of severe nuclear atypia, hypercellularity, and high mitotic activity. Malignant perineurioma has been further classified, on the basis of malignant histological features, into low-grade and high-grade perineurial MPNST. Low-grade tumors are histologically reminiscent of soft tissue perineurioma (so-called "perineural sarcoma") from which they differ for worrisome features, such as infiltrative growth, areas of increased cellularity, cytologic atypia, and mitotic figures [1]; tumor necrosis is lacking. Immunohistochemically, they express markers consistent with perineurial differentiation (EMA, claudin-1, and GLUT1). On the other hand, high-grade perineurial MPNST is a pleomorphic spindle cell sarcoma showing prominent cytologic atypia and numerous mitotic figures. These tumors lack obvious morphological features of perineurioma. Therefore, an extensive sampling of these neoplasms and a careful morphologic evaluation are required to detect, at least focally, features suggestive of perineurial differentiation, such as whorls and/or cells with delicate overlapping cell processes. Moreover, positivity for EMA, claudin-1, and/or GLUT1 is detected, at least focally. Given their rarity, the biologic behavior of these tumors is still unclear. To date, malignant perineuriomas, despite their potential for distant metastases and local recurrences, are believed to be less aggressive when compared to conventional malignant peripheral nerve sheath tumors. The most challenging differential diagnosis for low-grade malignant tumors is represented by low-grade fibromyxoid sarcoma, as this tumor may show similar morphology as well as immunohistochemical expression of EMA and claudin-1. However, the immunohistochemistry for MUC4 and the FISH analysis for FUS rearrangement are distinctive features of low-grade fibromyxoid sarcoma that were never observed in perineurial tumors.

Malignant Melanotic Schwannian Tumor (So-Called "Melanotic Schwannoma")
Formerly known as "melanotic schwannoma", malignant melanotic schwannian tumor is a neoplasm with an uncertain risk for aggressive behavior, usually arising from the spinal or autonomic nerves near the midline [1,[88][89][90][91]. Although sporadic and in most cases solitary, this tumor can occur in <5% of patients with Carney complex, and about 20% of these patients may present with multiple lesions [91]. Clinical symptoms, such pain and neurologic manifestations, are related to the tumor location. Grossly, the tumors are circumscribed and variably encapsulated and characteristically show a black-brown cut surface. Histologically, tumors are composed of relatively bland-looking spindled to polygonal cells arranged in fascicles; less commonly, nuclear palisading and/or whorls may be detected, suggesting schwannian differentiation. They characteristically contain heavy deposits of intracytoplasmic melanin pigment, while nuclei often show clear pseudoinclusions and marked nuclear pleomorphisms (hyperchromatism and/or nucleomegaly) as well as prominent nucleoli ( Figure 25). Notably, numerous psammoma bodies are present in most cases. Although tumors with >2 mitoses/10 HPF may have a metastatic risk, the clinical behavior of malignant melanotic schwannian tumor is uncertain in that metastases may occur even in the absence of adverse morphological features. As metastases have been variably reported in 26-53% of cases [90,92,93], the definition of "tumor with uncertain biological behavior" seems to be appropriate [1].

Molecular Features
Molecular studies have identified PRKAR1A mutations in most cases, despite their occurrence in the context or without Carney complex [91,92].

Conclusions
Peripheral nerve sheath tumors are relatively common lesions that exhibit a wide morphological and biological spectrum. In the presence of conventional morphological and immunohistochemical features, the histological diagnosis is usually straightforward, but they may represent diagnostic challenges. As these neoplasms may arise in the context of tumor-predisposing syndromes, such as NF1 and the different types of schwannomatosis, it is crucial for surgical pathologists to provide a correct diagnosis in order to plan the appropriate clinical management of the patients.  Data Availability Statement: All data presented in this manuscript are available from the corresponding author upon reasonable request.

Conflicts of Interest:
The authors declare no conflict of interest.