Hybrid Peripheral Nerve Sheath Tumors, Including a Malignant Variant in Type 1 Neurofibromatosis

A Case of Hybrid Cellular Neurothekeoma and Perineurioma With a Folliculin Gene Mutation

Hybrid peripheral nerve sheath tumors (PNSTs) are benign tumors that show features of more than one type of PNST. Benign cutaneous plexiform hybrid tumor of perineurioma and cellular neurothekeoma (BCPHTPCN) is a rare, recently described entity that shows various combinations of histomorphologic and immunophenotypic features of the PNSTs perineurioma and cellular neurothekeoma. Our case describes a middle-aged man initially presenting with an acute papulopustular rosacea flare. Rosacea treatment unmasked a prominent erythematous papule on the forehead. A biopsy was taken, and histopathological examination showed a proliferation of epithelioid, ovoid, and spindled cells arranged in a compact nested and intersecting fascicular growth. Immunohistochemical stains were positive for NKI/C3, CD10, PGP9.5, MITF, and GLUT-1. There was weak reactivity with EMA and minimal reactivity with Claudin-1. Tumor cells were negative for SOX10, CD163, CD68, CD34, ALK, and Pan-TRK. The patient was diagnosed with BCPHTPCN. DNA and RNA sequencing showed a folliculin (FLCN) gene mutation, which is most commonly associated with Birt-Hogg-Dubé syndrome. The patient underwent excision and has remained without recurrence or complications several months post-diagnosis. We hope to expand the clinical and histopathologic characteristics of this peculiar neoplasm, as well as provide additional insight that might improve our understanding of BCPHTPCN tumorigenesis.

Hybrid Benign Peripheral Nerve Sheath Tumors: A Comprehensive Literature Review with Emphasis on Their Clinical, Morphological and Genetic Features

Hybrid Peripheral Nerve Sheath Tumors (HPNSTs) are rare benign neoplasms that exhibit a combination of histological features from multiple types of benign peripheral nerve sheath tumors, including schwannomas, neurofibromas, and perineuriomas. These tumors present a diagnostic challenge due to their morphological and histological variability. In this article, we aim to summarize the key morphological, histological, and molecular characteristics of HPNSTs, providing insights into their diagnostic approaches. We review the different hybrid subtypes, including schwannoma-perineurioma, schwannoma-neurofibroma, and perineurioma-neurofibroma, emphasizing their clinical features, genetic associations, and the role of surgical excision as the gold-standard treatment.

Hybrid neurofibroma-schwannoma of the orbit: a case report and review of the literature

Hybrid peripheral nerve sheath tumor is a rare tumor subtype that infrequently occurs within the orbit. A 50-year-old male presented with a 6-year history of worsening left-sided proptosis and hypoglobus. Magnetic resonance imaging of the orbits revealed a left superior extraconal, muti-lobulated orbital lesion with extension into the left orbital apex originating from the left frontal nerve. The patient underwent surgical resection of the mass via a left orbito-cranial approach. Histopathologic examination was positive for orbital hybrid neurofibroma-schwannoma. Postoperatively, the patient had improvement in proptosis, resolution of symptoms, and improved cosmesis. This case represents the eleventh reported occurrence of orbital hybrid neurofibroma-schwannoma.

Schwannian and Perineuriomatous Differentiation in a Series of Giant Congenital Melanocytic Nevi

ABSTRACT

Close relationship between melanocytes and neural cells is accepted to reflect their common derivation from the neural crest and tumors combining both elements. We present a series of 10 patients with giant congenital melanocytic nevi (CMN) in which a secondary proliferation (11 lesions) with schwannian and/or perineuriomatous differentiation developed in the course of the disease. The age of the patients (4 male and 6 female) at the time of surgery and histological assessment varied from 3 months to 57 years. Histopathologically, the following subgroups were delineated: (1) nodular/tumoriform "neurotization" in CMN, (2) diffuse neurofibroma-like proliferation within CMN, (3) plexiform neurofibroma-like proliferation within CMN, and (4) diffuse perineuriomatous (hybrid schwannomatous-perineuriomatous) differentiation in CMN. We review the pertinent literature, including the role of recently identified Schwann cell precursors which are believed to represent the nerve-associated state of neural crest-like cells that persists into later developmental stages.

Peripheral nerve tumors

The chapter is focused on the neoplastic peripheral nerve lesions, which primarily involve "cranial and paraspinal nerves," as outlined in the CNS volume (WHO_Classification_of_Tumours_Editorial_Board, 2021). These include classic peripheral nerve sheath tumors such as schwannoma, neurofibroma, intraneural perineurioma, and malignant peripheral nerve sheath tumors, with their variants as well as new and more precisely defined entities, including hybrid nerve sheath tumors and malignant melanotic nerve sheath tumor (previously melanotic schwannoma).

Trigeminal hybrid nerve sheath tumor - a case report and literature review

BACKGROUND

Hybrid nerve sheath tumors (HNST) contain elements of more than one established sub-type of nerve sheath tumor and have been recently recognized in the 2016 WHO classification of central nervous system tumors. While common in the peripheral nerves and extracranial branches of cranial nerves, only one case has been previously documented of an intracranial HNST arising from a cranial nerve.

CASE DESCRIPTION

We describe a large, multi-compartmental intracranial hybrid nerve sheath tumor arising from the trigeminal nerve in a 22-year-old lady who presented with clinical and radiological features suggestive of a right cerebellopontine angle mass. Histopathological examination following retrosigmoid excision of the tumor revealed histological and immunohistochemical features of a schwannoma and a perineurioma.

CONCLUSIONS

HNSTs are likely to be underreported in the intracranial region. The clinical course of these tumors and the reason for their occurrence in this location are not known.

Hybrid Neurofibroma/Schwannoma of the Oral Cavity: A Rare Case Report and Literature Review

Hybrid peripheral nerve sheath tumors show combined features of more than one type of conventional benign peripheral nerve sheath tumors. There are few cases reported of hybrid peripheral nerve sheath tumors in the head and neck region. A 68-year-old female patient was referred for evaluation of an oral swelling lasting five years. Intraoral examination revealed a small mobile nodule located in the lower vestibule. The patient underwent excisional biopsy and microscopic evaluation showed typical features of neurofibroma enclosing areas with palisading nuclei compatible with Antoni A pattern, which are seen in schwannomas. These regions showed strong and diffuse immunoreactivity for S100 protein and moderate positivity in the neurofibroma area. CD34 was positive in the neurofibroma area and entrapped axons were positive for neurofilament. The final diagnosis was oral hybrid neurofibroma-schwannoma tumor. Hybrid peripheral nerve sheath tumors, although extremely rare, may arise within the oral cavity. To the best of our knowledge, this is the first neurofibroma-schwannoma tumor reported in the oral cavity. Recognizing hybrid peripheral nerve sheath tumors as a distinct clinicopathological entity is important because they may also be associated with syndromic disorders.

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.

Diagnosis and Treatment of Peripheral and Cranial Nerve Tumors with Expert Recommendations: An EUropean Network for RAre CANcers (EURACAN) Initiative

The 2021 WHO classification of the CNS Tumors identifies as "Peripheral nerve sheath tumors" (PNST) some entities with specific clinical and anatomical characteristics, histological and molecular markers, imaging findings, and aggressiveness. The Task Force has reviewed the evidence of diagnostic and therapeutic interventions, which is particularly low due to the rarity, and drawn recommendations accordingly. Tumor diagnosis is primarily based on hematoxylin and eosin-stained sections and immunohistochemistry. Molecular analysis is not essential to establish the histological nature of these tumors, although genetic analyses on DNA extracted from PNST (neurofibromas/schwannomas) is required to diagnose mosaic forms of NF1 and SPS. MRI is the gold-standard to delineate the extension with respect to adjacent structures. Gross-total resection is the first choice, and can be curative in benign lesions; however, the extent of resection must be balanced with preservation of nerve functioning. Radiotherapy can be omitted in benign tumors after complete resection and in NF-related tumors, due to the theoretic risk of secondary malignancies in a tumor-suppressor syndrome. Systemic therapy should be considered in incomplete resected plexiform neurofibromas/MPNSTs. MEK inhibitor selumetinib can be used in NF1 children ≥2 years with inoperable/symptomatic plexiform neurofibromas, while anthracycline-based treatment is the first choice for unresectable/locally advanced/metastatic MPNST. Clinical trials on other MEK1-2 inhibitors alone or in combination with mTOR inhibitors are under investigation in plexiform neurofibromas and MPNST, respectively.

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.

Hybrid Schwannoma/Perineurioma: Morphologic Variations and Genetic Profiles

To clarify the morphologic spectrum and molecular profiles of hybrid schwannoma/perineurioma (HSP), we investigated 15 tumors clinicopathologically and cytogenetically. HSP was classified into 2 morphologic types: mixed cellular and combined tumor types. The former comprising of 14 tumors mostly arose in the subcutaneous tissue of the extremities and the trunk of middle-aged adults. They were well-circumscribed and composed of elongated spindle-shaped tumor cells arranged in storiform and whorl patterns. Immunostaining revealed a mixed cellular proliferation of S-100 protein-positive and SOX10-positive Schwann cells and epithelial membrane antigen-positive, claudin 1-positive, and GLUT1-positive perineurial cells. During follow-up, no tumors were found to have recurred in any cases. In contrast, in the combined tumor type arising in the mediastinum of a young male with neurofibromatosis type 2, the intraneural perineurioma-like areas, characterized by small whorl-like structures, were present in plexiform schwannoma-like areas. No recurrence was noted in the case. Molecular analyses (array comparative genomic hybridization and fluorescence in situ hybridization) revealed LOH 22q in 2 tumors of 5 studied: one each of the mixed cellular and combined tumor types. Although the same diagnostic term, HSP, has been applied to both mixed and combined types, they should be separated from each other.

Benign peripheral nerve tumors

Benign peripheral nerve tumors encompass a wide range of neoplasms and non-neoplastic tumor like lesions. Some of these lesions if not encountered in the setting of genetic syndromes, are occurring sporadically. The principles of oncology should be respectfully followed in every step of diagnostic approach and surgical management. Albeit, classified as benign, some of them do have different level of malignant potential, thus the treating physicians should be aware of that to avoid possible pitfalls with devastating outcomes. This article reviews the most common benign peripheral nerve tumors discussing the clinicopathological findings, imaging appearance and the current trend in their approach.

What's new in nerve sheath tumors

Peripheral nerve sheath tumors are commonly encountered and frequently pose challenges to the pathologist and the clinician. This review discusses the wide range of entities with an emphasis on new discoveries in the past decade. Clinical, histologic, immunohistochemical, and pathogenetic findings are discussed with an emphasis on clinical implications and differential diagnosis.

Soft Tissue Perineurioma in a Child With Neurofibromatosis Type 1: A Case Report and Review of the Literature

Soft tissue perineurioma is a rare benign peripheral nerve sheath tumor, especially in children. This manuscript presents an unusual case of soft tissue perineurioma in a 10-year-old boy with neurofibromatosis type 1. The patient presented with a lump in the region of the right breast. A subcutaneous, well circumscribed mass was removed. The cut surface was cream with a vaguely nodular appearance. Histology showed a spindle cell lesion with variable architecture and biphasic morphology. There was no nuclear atypia or pleomorphism. The differential diagnosis and other soft tissue lesions that can be seen in neurofibromatosis type 1 are discussed.

Benign Cutaneous Peripheral Nerve Sheath Tumor with Hybrid Features: Report of Two Cases with Schwannoma/Perineurioma and Schwannoma/Neurofibroma Components

Hybrid benign peripheral nerve sheath tumors are rare tumors exhibiting areas of more than one neural neoplasm such as schwannoma/neurofibroma, neurofibroma/perineurioma, or schwannoma/perineurioma. These tumors usually arise in the skin of the extremities and trunk, and less commonly affect extracutaneous sites. Here we report two cases of these tumors exhibiting hybrid features of schwannoma/perineurioma and schwannoma/neurofibroma located in the scalp and the ankle, respectively.

Hybrid peripheral nerve sheath tumors: report of five cases and detailed review of literature

BACKGROUND

Hybrid peripheral nerve sheath tumors (PNSTs) have been recognized recently and were first included in the 4th edition of World Health Organization (WHO) Classification of Tumors of Soft tissue and Bone, published in 2013. These tumors show combined features of more than one type of conventional benign peripheral nerve sheath tumors. The most common combinations are those of schwannoma/perineurioma followed by combinations of neurofibroma/schwannoma and neurofibroma/perineurioma. A detailed literature review of published cases is presented. We have discussed the types and etiology, epidemiology and sites of localization, gross and microscopic appearances and immunohistochemical features of hybrid PNSTs and association of these tumors with tumor syndromes.

CASE PRESENTATION

We have included five cases which were diagnosed in our department as we believe that publication of these new cases is relevant for the improved understanding of these specific tumors. Four of our five patients were males, mean age was 24 years. There was wide variation in the location of these tumors. Mean size of excised tumors was 5.5 cms in the greatest dimensions. Three out of five cases represented hybrid schwannoma/perineurioma histologically. No significant nuclear atypia, mitotic activity or necrosis seen. All five cases were completely excised. All five patients are alive and well at the time of writing with no recurrence.

CONCLUSION

Hybrid PNSTs are distinct tumors and are usually benign. However, rare case reports have described local recurrence and at least two recent case reports have described malignant transformation in these tumors. Further studies on large number of cases are required to determine the exact pathogenetic basis of these tumors.

Whorling cellular perineurioma: A previously undescribed variant closely mimicking monophasic fibrous synovial sarcoma

The authors present a distinctive perineurioma (PN) variant which morphologically strongly resembles monophasic fibrous synovial sarcoma (MSS). The patients were 3 males and 1 female. The age ranged from 15 to 61years (mean: 44years). Locations included the sole, lower jaw, palm and foot. The tumor size ranged from 1.3cm to 2.5cm in the largest dimension (mean 1.8cm). Morphologically, all tumors had an identical, monotonous appearance. The perineurial cells were closely packed and created a confluent cellular whorls and/or sheets in a scarce stroma, with only focally discernible long, slender cytoplasmic processes typical for perineurial differentiation. The nuclei were rounded or slightly elongated to tapered, without nuclear atypia. Mitoses were rare to completely absent. Atypical mitoses, hemorrhage, necrosis or calcifications were not present. The proliferative index (Ki-67) was 1-3%. All analyzed tumors were positive for EMA, Claudin-1, GLUT-1 and negative with S100 protein, CD34, OSCAR, CK7 and TLE-1. Two cases were tested by fluorescence in situ hybridization and neither showed alterations of the SYT gene. One case studied by electron microscopy showed characteristic features of perineurial differentiation. Follow-up was available for two patients both of which showed no evidence of disease at 8years and 6months, respectively. Based on their bland morphology, perineurial features and presumably benign clinical outcome we propose the term "whorling cellular perineurioma" for these tumors, which may represent an extremely cellular variant of sclerosing PN. Awareness of this PN subtype and its distinction from MSS is of utmost clinical significance.

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.

Microscopic intraneural perineurial cell proliferations in patients with neurofibromatosis type 1

Benign peripheral nerve sheath tumors (PNSTs) showing more than one line of differentiation (hybrid PNSTs) have been increasingly recognized, mainly due to awareness of their existence and as a consequence of increased use of immunohistochemisty during the last decade. Two recent studies suggested overrepresentation of hybrid tumors among benign PNSTs in patients with neurofibromatosis type 1 (NF-1). This study was performed to assess the presence of perineurial cells in microscopic (early) neurofibromatous lesions and normal-looking peripheral nerves in specimens from 5 patients with NF-1 using markers of perineurial cell differentiation (epithelial membrane antigen, claudin-1, and glucose transporter 1). In 2 patients, multiple normal looking nerve fibers as well as hypertrophied nerves and microscopic tumor nodules showed variable intraneural perineurial cell proliferations that frequently occupied the whole nerve fascicle resulting in multiple microscopic reticular perineurioma-like nodules (microscopic hybrid neurofibromatosis/perineuriomatosis). None of the cases showed the onion skin pattern of intraneural perineurioma. However, other nerve fibers within the same specimens showed normal compact rim of perineurium without any detectable intraneural perineurial cells. Both patients had concurrent multiple larger PNSTs (plexiform neurofibromas, hybrid neurofibroma/perineurioma and lesions with features intermediate between the 2 types). One specimen harboring high-grade malignant PNST and 2 specimens with large solitary neurofibromas displayed no intraneural perineurial cells. These observations suggest that intraneural perineurial proliferations are part of the early lesions in the setting of constitutional NF-1 inactivation and support the concept of pure and hybrid perineuriomatous lesions as novel member of the spectrum of PNSTs in NF-1.