Reticular and plexiform perineurioma: clinicopathological and immunohistochemical analysis of two cases and review of perineurial neoplasms of skin and soft tissues

What is new in intraneural perineurioma?

Since the initial description of intraneural (IN) perineurioma in 1964, advances in the understanding of the clinical presentation, diagnostic imaging, pathologic features, and genetic underpinnings have changed how this pathology is managed. IN perineuriomas are rare, benign peripheral nerve sheath tumors, most frequently coming to clinical attention when patients present with painless, progressive weakness or sensory loss in adolescence or young adulthood. The gold standard of diagnosis has traditionally been with targeted tissue biopsy demonstrating "pseudo-onion bulb" formation with positive epithelial membrane antigen (EMA) staining. However, modern magnetic resonance imaging is allowing some patients to forgo biopsy. Recent genetic studies of IN perineuriomas have demonstrated common TRAF7 point mutations and rare NF2 mutations, which may present targets for diagnosis or therapy in the future. Current advances have allowed for us to provide improved patient counseling with informed understanding for various clinical scenarios. With the workup and diagnosis now clearly defined, the next frontier is for improving the lives of patients with IN perineuriomas through the interaction between restoration of functional deficits and advances in our understanding of the genetics of this entity.

Hybrid tumors with perineurioma components: a systematic review of the literature and illustrative case

PURPOSE

Hybrid peripheral nerve sheath tumors (HPNST) are a newly recognized class of peripheral nerve sheath tumor, composed of at least two areas characteristic of perineurioma, schwannoma, or neurofibroma. The literature consists only of case reports and small series; therefore, we present an illustrative case and an analysis of all reported cases of HPNST with a perineurioma component in the literature.

METHODS

A systematic search of the literature was performed to identify all reported cases of hybrid perineurioma-schwannoma or perineurioma-neurofibroma in the world's literature. Individual cases were analyzed for demographics, clinical features, imaging, and outcomes.

RESULTS

A total of 159 cases were identified across 41 studies. Hybrid tumors tended to present in mid-adulthood (median 38.5 years), predominantly affected females (57%, 89/156), as a painless (63%, 63/100) mass, or swelling. Ten patients (10/74, 14%) had a history of neurofibromatosis 1, and 2 patients a history of neurofibromatosis 2 (2/74, 3%). The majority (78%, 122/157) of cases occurred superficially, most commonly in the lower extremity (25%, 39/157). Perineurioma-schwannoma was the most reported (86%, 137/159) pathologic diagnosis, with 3 cases presenting with malignant features. Two cases reocurred after resection.

CONCLUSION

HPNST tend to occur in mid-adulthood and present as slowly progressive, painless, superficial masses, with a heterogeneous appearance on imaging. These entities pose a unique diagnostic challenge and likely remain under-recognized in the literature and current clinical practice. They pose low risk of recurrence or malignant transformation, and future work regarding the association with neurofibromatosis and genetic profiles is needed.

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.

[Myxoid soft tissue tumours : An algorithm for differential diagnosis]

Soft tissue neoplasms with myxoid features are collectively not uncommon. Their often complex differential diagnosis makes them significantly over-represented among consultation cases. This applies not only to sarcomas but in particular to benign lesions as well. Generally, myxoid soft tissue lesions are divided into two major groups: (1) myxoid lesions by definition (which can however rarely be non-myxoid) and (2) rare myxoid variants of otherwise non-myxoid entities.Four major diagnostic challenges are responsible for the complexity of myxoid soft tissue neoplasms: (1) Diagnosis of malignancy in many cases is not based on conventional malignancy criteria but is defined by the entity itself, making under-diagnosis of malignancy likely in entities such as low-grade fibromyxoid sarcoma. (2) On the other hand, harmless myxoid lesions with features of high proliferation, e.g. nodular and proliferative fasciitis, tend to be over-diagnosed as malignant by the unworried. (3) The necessity to assess not only cellular morphology/differentiation, but also the stromal, vascular and architectural characteristics adds to the complexity of the differential diagnostic algorithm. (4) Last but not least, recognition of unexpected myxoid variants of non-myxoid entities is basically impossible if focal conventional areas are absent, underlining the need for high suspicion index and sufficient sampling.This review illuminates the various aspects related to the differential diagnostic workup of these challenging entities.

Reticular Perineurioma of the Hand: Diagnosis and Treatment of a Rare Case of Hand Mass

Reticular perineurioma is a rare and recently delineated morphologic variant of benign perineurioma of skin and soft tissues. Because of its nonspecific gross appearance, varying histologic patterns, and potential range of cellularity, perineurioma of the hand is likely to be confused with more commonly encountered tumor or tumor-like conditions such as schwannoma, neurofibroma, fibromyxoid tumors, and giant tumor of tendon sheath. We report the case of a 20-year-old woman who presented with a slowly growing mass of the hand, which was eventually identified as a reticular perineurioma.

Extra-acral cutaneous sclerosing perineurioma with CD34 fingerprint pattern

Sclerosing perineuroma is a variant of extraneural perineurioma that, as a rule, occurs in acral sites. However, it has also been occasionally reported in non-acral regions. Recently, CD34 expression in a pattern reminiscent of the human fingerprint has been observed in a subset of perineuriomas, but this immunohistochemical finding has not been documented in non-acral sclerosing perineuriomas. We report a case of sclerosing perineurioma presenting CD34 expression in a fingerprint-like pattern on the skin of the neck (a previously unreported site for this neoplasm) of a 56-year-old man. In addition, alpha smooth-muscle actin showed a similar pattern of expression, suggesting that the cell population implicated in the remarkable immunolabeling is most probably fibroblastic/myofibroblastic. Other immunohistochemical findings included epithelial membrane antigen and claudin1-positive lesional cells, and the absence of S100, glucose transporter protein 1, MUC4 and desmin.

Pseudolipoblastic perineurioma: an unusual morphological variant of perineurioma that may simulate liposarcoma

Perineuriomas are rare peripheral nerve sheath tumors arising from or differentiating along the lines of normal perineurial cells. They can be divided into intraneural and soft tissue types, with the latter category including a significant number of morphological variants. Herein, we further expand their morphological spectrum to include "pseudolipoblastic" perineuriomas. These lesions occurred in the tongue of a 30-year-old man and in the triceps of a 67-year-old woman and were characterized by bland, epithelioid cells with striking intracytoplasmic vacuolization. The architecture varied, with some areas showing a striking "net-like" or "microreticular" pattern and smaller areas having a more typical spindled and whorled appearance. Clinical follow-up (5months and 52months, respectively) showed no evidence of local recurrence or metastasis. Multiple perineurial markers, including epithelial membrane antigen, claudin-1, GLUT-1, and collagen IV, were diffusely positive. Both cases were submitted in consultation out of concern that they represented high-grade liposarcomas. To the best of our knowledge, this unusual morphological variant of perineurioma has not been reported. These tumors appear to be entirely benign and should be cured with simple excision. Pseudolipoblastic perineuriomas should be distinguished from round cell and epithelioid pleomorphic liposarcomas, as well as from other tumors that may show prominent intracytoplasmic vacuolization.

[Reticular perineurioma of the tibial nerve]

Reticular (retiform) perineurioma is a rare variant of soft tissue perineurioma developed from the perineurium. This benign tumor is characterized by strands of spindle cells in a fibro-myxoid matrix surrounding pseudocystic mucoid spaces. We report a tibial nerve reticular perineurioma in a 35-year-old patient.

Peripheral Nerve Sheath Tumors

This article presents an overview of the diagnostic categories of benign and malignant nerve sheath tumors, including neuroma, neurofibroma, nerve sheath myxoma, perineurioma, schwannoma, and malignant peripheral nerve sheath tumor. The discussion emphasizes histologic patterns; ancillary studies, such as immunohistochemistry; and differential diagnoses. The information is of value to practicing pathologists in both community and academic settings.

Cutaneous microcystic/reticular schwannoma: a poorly recognized entity

BACKGROUND

Microcystic/ reticular schwannoma is exceptionally rare yet distinctive morphological variant of schwannoma with predilection for visceral sites lacking association with neurofibromatosis.

AIMS

To further delineate clinicopathological features of cutaneous microcystic/reticular schwannoma and to discuss its differential diagnosis.

RESULTS

We analyzed three cutaneous microcystic/reticular schwannomas, occurring in two males and one female (mean age: 37.6 years). The tumors presented as a non-painful slightly raised papule (mean: 0.7 cm) on upper arm (n = 2) and back (n = 1). No recurrences were observed despite marginal excision (mean follow up: 42 months). Histopathologically, a multilobular proliferation was present in the dermis composed of bland tumor cells forming distinctive microcystic, reticular, lace-like or pseudoglandular structures, containing abundant myxoid/mucinous material. By immunohistochemistry, tumor cells lining microcystic structures corresponded to Schwann cells (diffuse S100 positive, variable GFAP positivity). A discontinuous EMA-positive perineurium was present at the periphery of some of the lobules.

CONCLUSION

Cutaneous microcystic/reticular schwannoma expands the spectrum of benign peripheral nerve sheath tumors with reticular morphology encountered in the skin. Other tumors in this group include reticular perineurioma and hybrid tumors with reticular morphology, e.g. reticular perineurioma/schwannoma and reticular perineurioma/neurofibroma.

Malignant epithelioid peripheral nerve sheath tumor with prominent reticular/microcystic pattern in a child: a low-grade neoplasm with 18-years follow-up

Malignant peripheral nerve sheath tumors (MPNSTs) constitute <2% of soft tissue neoplasms in children and display a wide histologic spectrum including low-grade and epithelioid variants, although most are high-grade spindle cell sarcomas. Here, we describe an unusual case of a large retroperitoneal epithelioid MPNST diagnosed in a 7-year-old girl without family history or clinical features of neurofibromatosis type 1. The patient was treated by repeated surgical interventions, polychemotherapy, autologous stem cell transplantation, and irradiation therapy. Over the years, she developed multiple disseminated abdominal recurrences but is currently alive with very slowly progressing disseminated intra-abdominal disease 18 years from initial diagnosis. Histologically, the tumor was composed of medium-sized polygonal and ovoid-to-spindled cells set within a copious myxoid matrix with a prominent reticular and microcystic pattern reminiscent of the recently described reticular/microcystic schwannoma. Immunohistochemistry revealed strong and diffuse expression of S100, CD56, CD57, collagen IV, and neuron-specific enolase, with negativity for perineurial cell markers (claudin-1, epithelial membrane antigen, and glucose transporter-1) and other lineage-specific mesenchymal and epithelial antigens. This unusual variant of low-grade MPNST must be differentiated from a variety of other entities, in particular benign perineurioma, myxoid neurofibroma, and benign reticular/microcystic schwannoma. Confinement of the recurrent disease to the abdominal cavity emphasizes the necessity of primary curative wide excision of this highly recurring but nonmetastasizing low-grade neoplasm.

On pseudo-onion bulb intraneural proliferations of the non-major nerves of the oral mucosa

Perineurial cells (PCs) participate in reactive and neoplastic processes, of the latter pure perineurial being intraneural (IP) and soft tissue perineuriomas with oral examples being reported in both. In our review of over 500 peripheral nerve sheath tumors including granular cell tumor, we identified a single ostensible case of IP occurring on the tongue of a 45-year-old African-American male that was characterized by classic perineurial pseudo-onion bulbs (PsOb), proliferating PCs among these PsOb, sclerosis apparently due to long term duration and a plexiform pattern. We have also encountered 37 examples of apparently reactive, hyperplastic or traumatic, PsOb intraneural pseudoperineuriomatous proliferation (IPP) simulating microscopically some of the properties of IP. The majority of the lesions occurred in women and close to 80 % affected the tongue. Three microscopic patterns were appreciated. Type I lesions were those where IPP was seen only focally, type II where it was seen in roughly half of the lesion, and type III where the majority of the lesional tissue or the lesion itself was characterized by IPP. Immunohistochemically, IPP featured PsOb with generally a single layer of PCs decorated by epithelial membrane antigen, glut-1 or claudin-1, and decreased numbers of S-100 positive Schwann cells. The number of axons was not apparently altered. A prominent collagenous intraneural component was occasionally evident among PsOb and the affected nerve featured discontinuous or absent perineurial envelop. While type I and II IPP can be distinguished from IP, the distinction from type III lesions can be problematic. However, the discontinuity of the perineurium of the affected nerve, the spacing and collagenization among PsOb, the limited perineurial cell layer defining the pseudo-onion bulbs, the absence of proliferating PCs between PsObs and the decreasing number of Schwann cells may be of help in the distinction from IP.

Intraoral perineurioma, soft tissue type: report of five cases, including 3 intraosseous examples, and review of the literature

Soft tissue type perineuriomas (STP) are uncommon tumors, oral examples being very rare. They have been described in the mandible, gingiva, lips, retrotonsillar mucosa and maxillary vestibule. Herein, we report the clinicopathologic features of five STP, two affecting the buccal mucosa and three the mandible. Three patients were women and two men. All tumors were characterized by a proliferation of cytologically bland, mitotically inactive spindled cells with round, ovoid or spindled nuclei, embedded in a variably collagenous and myxoid matrix. Interestingly, two mandibular tumors featured psammoma bodies and one, in addition, contained irregular calcifications. Tumor cells showed the immunohistochemical profile of perineurial cells including epithelial membrane antigen, Glut-1, claudin-1 and collagen type IV. S100 and neurofilament proteins were not expressed by the tumor cells. A few minute, peripherally situated, entrapped nerves were identified. All tumors were reported gross-totally excised and no recurrences have taken place. The clinical characteristics of STP are summarized and its differential diagnosis relative to other spindle cells tumors and meningioma is discussed.

Extraneural sclerosing perineurioma of the buccal mucosa: a case report and clinicopathologic review

The perineurioma is an infrequently encountered benign peripheral nerve sheath tumor composed of a clonal proliferation of perineurial cells. Rare cases of perineurioma have been reported in the oral cavity. An extraneural sclerosing perineurioma arising in the buccal mucosa of a 17-year-old male is presented. Histopathologically, the tumor is composed of a well circumscribed nodular proliferation of spindle cells arranged in a storiform growth pattern, in some areas subtly arranged around vascular channels. The tumor cells reveal positive immunostaining for epithelial membrane antigen (EMA), collagen type IV and vimentin, and negative immunostaining for S-100 protein, consistent with a perineurial origin. To the best of our knowledge, this case represents the first report of an extraneural sclerosing perineurioma involving the oral cavity.

Endobronchial perineurioma: an unusual soft tissue lesion in an unreported location

We report the first case of an endobronchial perineurioma, a rare benign neoplasm typically occurring in soft tissue. A 53-year-old nonsmoking female presented with a three-month history of persistent bronchitis. A CT scan followed by bronchoscopy demonstrated an endobronchial lesion involving the left mainstem bronchus. Removal of the lesion by bronchoscopy was accomplished. The tumor was composed of bland spindle cells in a variably collagenized stroma. These cells had long cytoplasmic processes. No mitotic activity or necrosis was observed. Neoplastic cells were immunoreactive for epithelial membrane antigen (EMA), CD34, and claudin-1. Smooth muscle actin (SMA), desmin, and S-100 immunostains were all negative. Based on the morphologic appearance and immunophenotype, a diagnosis of perineurioma was rendered.

[Sclerosing perineurioma: case report and literature review]

Sclerosing perineurioma is a rare benign neoplasm composed exclusively of perineural differentiation cells spread in a dense fibrous stroma. It affects primarily the skin of fingers and that of the palm of the hands. It appears as a hard papule or nodule, normochromic and asymptomatic. A case of sclerosing perineurioma in the left palm of a 16-year-old female is described, with detailed demonstration of clinical aspects, histopathology and literature review published in the English language.

Early Colonic Perineurioma: A Report of 11 Cases

Colonic perineurioma has been depicted as characterized by a mucosal proliferation of monomorphic spindle perineurial cells leading to an evident separation, distortion, and entrapment of colonic crypts. The authors, however, believe that a sizable subset of the cases differ in that they display only a limited perineurial proliferation leading to only mild crypt separation without crypt entrapment. This morphological variant (early perineurioma) has not yet been documented. The authors herein present the clinicopathological and immunohistochemical features of 11 cases. Polyp size ranged from 2 to 4 mm, and 8 (73%) were located in the sigmoid. Histologically, they revealed small, frequently noncontiguous nests or bundles of uniform round to oval cells, causing slight separation of parallel or mildly distorted crypts, which displayed a serrated/hyperplastic architecture in 8 (73%) cases. Immunostaining for perineurial markers showed strong expression for claudin-1, GLUT-1, and collagen type IV and weak reactivity for epithelial membrane antigen. In conclusion, early perineurioma is a morphological variant of colonic perineurioma in which the perineurial proliferation is limited and consequently more difficult to recognize. Using perineurial markers is helpful in reaching an accurate diagnosis.

CD34-reactive tumors of the skin. An updated review of an ever-growing list of lesions

Over the past few years, a growing number of cutaneous tumors expressing CD34 is being reported. The list contains benign and malignant neoplasms as well as reactive and hamartomatous lesions of diverse lineages of differentiation, including fibroblastic, myofibroblastic, fibrohistiocytic, vascular, neural, adipocytic, smooth muscle, hematopoietic, melanocytic and epithelial. The more frequent diagnostic difficulties are found in spindle cell proliferations, mainly in those of the fibrocytic lineage. In part, this is because of the fact that in this area are, aside to well-defined entities, histologically and clinically diverse, recently reported cutaneous CD34-reactive lesions, whose definitions, limits and relationships are not completely established. The CD34 expression plays a key role in the differential diagnosis of some tumors, such as dermatofibrosarcoma protuberans, epithelioid sarcoma or pleomorphic hyalinizing angiectatic tumor of soft parts, with important therapeutic consequences. In others, as in desmoplastic trichilemmoma, it can help to resolve diagnostic problems in concrete cases. Finally, in many of the CD34-positive lesions, the diagnosis with the hematoxylin and eosin stain is straightforward. However, in all of them, the knowledge of the immunohistochemical profile contributes to our understanding of the cutaneous pathology.

Multiple hybrid granular cell tumor-perineuriomas

Hybrid peripheral nerve sheath tumors (PNSTs) are recently recognized, rarely reported entities. Very few hybrid PNSTs with elements of perineurioma have been reported. We report a case of a 47-year-old man who presented with multiple (>20) cutaneous nodules on all 4 extremities, present since childhood. The patient and his family had no history of other PNSTs, other neural-origin tumors, or genetic disorders, including neurofibromatosis. Five excised lesions are well circumscribed but unencapsulated nodular tumors composed of cytologically bland spindled cells with tapering nuclei and indistinct cytoplasm arranged in a storiform pattern in a collagenous matrix. Scattered among these cells are large granular eosinophilic cells. The spindled cells are epithelial membrane antigen-positive and S-100 protein negative. The cytoplasm of the granular cells contains abundant S-100 protein, NK1/C3, and CD68-positive granules. Sparse nerve fascicles contain neurofilament-positive axons. Each tumor also contains poorly marginated areas that histologically resemble pure granular cell tumor, and superficial biopsies of these tumors can histologically resemble pure granular cell tumors. Considering a Schwannian origin for both perineurial and granular cells can provide a logical explanation for the pathogenesis of this case. To the best of our knowledge, our case is the first report of multiple hybrid granular cell tumor-perineurioma.

Epithelioid perineurioma: an unusual variant

An unusual epithelioid variant of perineurioma of the groin occurring in a 53-year-old man is described. The lesion appeared to be associated with a femoral nerve branch. The tumour was characterised by the presence of a syncytial proliferation of epithelioid cells, mimicking a meningioma of syncytial type. In addition there was a minor component of a conventional perineurioma. The tumour cells were EMA+, claudin-1+ and collagen type IV+. Bcl 2 was focally expressed. This case highlights the possibility of a common histogenetic pathway for meningiomas and perineuriomas. Although ultrastructural evidence of possible meningiomatous differentiation within an otherwise histologically typical perineurioma has been described, this is the first reported case of an unconventional epithelioid variant of perineurioma, histologically resembling meningioma.

Fibroblastic polyp of the colon and colonic perineurioma: 2 names for a single entity?

Fibroblastic polyps of the colon and intestinal perineuriomas are unusual mucosal lesions with identical clinical and histologic features, and apparent different immunohistochemical and ultrastructural characteristics. However, immunohistochemical distinction was solely based on the results obtained with epithelial membrane antigen (EMA), an antibody whose reactivity on perineuriomas is difficult to demonstrate. Likewise, accurate ultrastructural diagnosis may be flawed by sampling error, preservation artifacts, or paucity of specific diagnostic features. In a recent short communication, it was suggested that both lesions may represent the same entity. To further evaluate this hypothesis, 28 colorectal polyps with clinical and histologic features of colonic fibroblastic polyps/perineuriomas (including 10 cases previously reported as fibroblastic polyps) were stained immunohistochemically for 4 markers of perineurial differentiation, that is, claudin-1, GLUT-1, collagen type IV, and EMA (the latter performed using an extended protocol for antigen retrieval and a kit for signal amplification). In addition, electron microscopy was performed in 4 cases. EMA and claudin-1 stained 26 of 28 (93%) polyps whereas GLUT-1 and collagen type IV were expressed in all of them. EMA reactivity was mostly focal and weak whereas the other markers displayed a diffuse and strong signal. Ultrastructural examination revealed elongated cells with features of perineurial differentiation including long, slender cytoplasmic processes with pinocytotic vesicles and an external lamina. Our findings support the hypothesis that fibroblastic polyps and perineuriomas of the colon represent the same entity. We suggest reclassifying fibroblastic polyps reactive to perineurial markers as perineuriomas. To reach an accurate diagnosis, we recommend employing at least 2 markers of perineurial differentiation, and performing EMA immunostaining with high antibody concentration, prolonged incubation time, and/or extended protocol for antigen retrieval.

GLUT-1 expression in mesenchymal tumors: an immunohistochemical study of 247 soft tissue and bone neoplasms

GLUT-1, an erythrocyte-type glucose transporter protein expressed in juvenile hemangiomas, has recently been shown to be a sensitive marker of perineurial cells and their tumors in a small number of cases. However, GLUT-1 expression has not been systematically examined in other mesenchymal neoplasms. Prompted by a recent report of GLUT-1 expression in epithelioid sarcoma, a tumor not generally felt to show perineurial differentiation, we examined GLUT-1 expression in a wide variety of mesenchymal tumors. Sections from 247 mesenchymal tumors of a variety of histologic subtypes were retrieved from our archives and immunostained for GLUT-1 using heat-induced epitope retrieval and the DAKO ADVANCE detection system (DAKO, Carpinteria, CA). Scoring was as follows: negative (<5% of cells), 1+ (5%-25% of cells), 2+ (25%-50% of cells), and 3+ (>50% of cells). All benign nerve sheath tumors showed a peripheral rim of positive normal perineurial cells, with 2 neurofibromas and 3 schwannomas showing more extensive staining. Three of 4 perineuriomas showed strong GLUT-1 expression. All juvenile hemangiomas were GLUT-1 positive. GLUT-1 expression was also seen in a wide variety of benign and malignant mesenchymal tumors. However, GLUT-1 expression was absent in nonjuvenile hemangioma endothelial tumors and in almost all low-grade lesions that enter the histologic differential diagnosis of perineurial tumors, including low-grade fibromyxoid sarcoma, dermatofibrosarcoma protuberans, and myxofibrosarcoma. We conclude that GLUT-1 expression in mesenchymal tumors is by no means specific for perineurial differentiation, but may instead represent upregulation of this protein within hypoxic zones, secondary to upstream activation of proteins such as hypoxia-inducible factor 1-alpha.

The normal and neoplastic perineurium: a review

Peripheral nerves consist of 3 layers with differing characteristics: the endoneurium, perineurium, and epineurium. The perineurium represents a continuum with the pia-arachnoid from the central nervous system and extends distally with the sheath of capsular cells of peripheral sensorial organs and propioceptive receptors. It is made of layers of flattened cells surrounded by a basement membrane and collagen fibers, forming concentrically laminated structures around single nerve fascicles. Functionally, the perineurium modulates external stretching forces (that could be potentially harmful for nerve fibers), and along with endoneurial vessels, forms the blood-nerve barrier. Multiple pathologic conditions associated with the perineurium have been described. Perineurial invasion is considered an important prognostic factor in several malignant neoplasms. Perineuriomas are true benign infrequent perineurial cell neoplasms that have been divided in 2 categories: those with intraneural localization and a more common extraneural (soft tissue) group, including sclerosing and reticular variants. Sporadic cases of malignant perineuromas have been reported. Interestingly, neurofibromas and malignant peripheral nerve sheath tumors may also display perineurial cell differentiation. The histologic appearance of perineuriomas may overlap with other soft tissue spindle cell neoplasms. Immunohistochemistry is imperative for the diagnosis, although in certain cases ultrastructural studies may be needed. Typical perineuriomas are positive for epithelial membrane antigen, glucose transporter-1-1, and claudin-1, and negative for S-100 protein and neurofilaments. Perineuriomas have mostly simple karyotypes, with one or few chromosomal rearrangements or numerical changes and it seems that specific cytogenetic aberrations may correlate with perineurioma subtype.

Intraneural perineurioma: a systematic review with illustrative cases

CONTEXT

Intraneural perineurioma may be confused with other "onion bulb" Schwann cell entities (localized hypertrophic neuropathy, reactive/demyelinating processes, or inherited polyneuropathies of Charcot-Marie-Tooth/Dejerine Sottas) due to similar clinical, radiologic, and histologic features. Perineurial and Schwann cells can only be differentiated by ultrastructure and immunohistochemsitry.

OBJECTIVE

To identify and summarize the clinicopathologic features of true cases of intraneural perineurioma from the English language literature.

DATA SOURCES

A systematic review was performed on definitive intraneural perineuriomas identified through Medline. Baylor College of Medicine-affiliated hospitals' anatomic pathology databases yielded 2 illustrative intraneural perineurioma cases.

STUDY SELECTION

Intraneural perineurioma inclusion criteria consisted of characteristic histology and confirmation of perineurial cell lineage by either immunohistochemistry (epithelial membrane antigen positive, S100 protein negative) and/or ultrastructural analysis (thin cytoplasmic processes with an incomplete basal lamina, poorly formed tight junctions, and pinocytotic vesicles).

DATA EXTRACTION

Clinicopathologic data were extracted from all identified articles, with subsequent statistical analysis of the following parameters: age, sex, race, tumor location, tumor size, duration of symptoms prior to diagnosis, treatment modalities and outcomes measures, follow-up assessment for tumor recurrence and metastasis, clinical features (history of trauma, motor/sensory abnormalities, clinical/family history), and diagnostic workup (routine histology, immunohistochemistry, ultrastructural analysis, and molecular/cytogenetic characteristics).

CONCLUSIONS

Intraneural perineurioma is a neoplastic proliferation of perineurial cells with unique immunohistochemistry and ultrastructural features, and it is distinct from other onion bulb Schwann cell-derived entities. Despite harboring molecular abnormalities of the long arm of chromosome 22, intraneural perineurioma has not been associated with neurofibromatosis. Intraneural perineurioma is a benign peripheral nerve sheath tumor that does not recur or metastasize.

Divergent differentiation in malignant melanomas: a review

The aim of this review was to document and discuss diagnostic problems associated with divergent differentiation ('metaplastic change') in malignant melanomas, defined as the development in these tumours of morphologically, immunohistochemically and/or ultrastructurally recognizable non-melanocytic cell or tissue components. Types of divergent differentiation reported in malignant melanoma include: fibroblastic/myofibroblastic, Schwannian and perineurial, smooth muscle, rhabdomyosarcomatous, osteocartilaginous, ganglionic and ganglioneuroblastic, neuroendocrine and probable epithelial. Divergent differentiation is certainly a rare phenomenon and, when it occurs, can be missed by unwary pathologists and lead to diagnostic uncertainty. A carefully chosen immunohistochemical panel and the input of electron microscopy can help to clarify the nature of the cellular differentiation of these tumours and lead to a correct final diagnosis. The clinical significance of such aberrations is uncertain, nor are the underlying mechanisms as yet well defined.

Perineurioma: a distinctive and underrecognized peripheral nerve sheath neoplasm

CONTEXT

Perineuriomas are benign peripheral nerve sheath neoplasms composed of perineurial cells with characteristic immunohistochemical and ultrastructural features. They have been traditionally classified into two main types according to their location--intraneural and extraneural--and overlap histologically with many other tumors, which may be diagnostically challenging to general surgical pathologists.

OBJECTIVE

To review the clinical, morphologic, immunohistochemical, ultrastructural, cytogenetic, and molecular genetic aspects of perineurioma, as well as to discuss its clinicopathologic variants and differential diagnosis.

DATA SOURCES

English-language literature published between 1966 and 2005 was reviewed.

CONCLUSIONS

The correct identification of perineuriomas is important to avoid unnecessary overtreatment. The histologic diagnosis should be confirmed through immunohistochemical studies (including epithelial membrane antigen, S100 protein, and more recently described antibodies such as claudin-1 and GLUT1) or electron microscopy. Cytogenetic and molecular genetic studies are still of limited value for the diagnosis of perineuriomas but may play a fundamental role in excluding important differential diagnoses and also in helping elucidate the biology of these poorly known neoplasms.