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The vast majority of peripheral nerve sheath tumors derive from the Schwann cell lineage and comprise diverse histological entities ranging from benign schwannomas and neurofibromas to high-grade malignant peripheral nerve sheath tumors (MPNST), each with several variants. There is increasing evidence for methylation profiling being able to delineate biologically relevant tumor groups even within the same cellular lineage. Therefore, we used DNA methylation arrays for methylome- and chromosomal profile-based characterization of 171 peripheral nerve sheath tumors. We analyzed 28 conventional high-grade MPNST, three malignant Triton tumors, six low-grade MPNST, four epithelioid MPNST, 33 neurofibromas (15 dermal, 8 intraneural, 10 plexiform), six atypical neurofibromas, 43 schwannomas (including 5 NF2 and 5 schwannomatosis associated cases), 11 cellular schwannomas, 10 melanotic schwannomas, 7 neurofibroma/schwannoma hybrid tumors, 10 nerve sheath myxomas and 10 ganglioneuromas. Schwannomas formed different epigenomic subgroups including a vestibular schwannoma subgroup. Cellular schwannomas were not distinct from conventional schwannomas. Nerve sheath myxomas and neurofibroma/schwannoma hybrid tumors were most similar to schwannomas. Dermal, intraneural and plexiform neurofibromas as well as ganglioneuromas all showed distinct methylation profiles. Atypical neurofibromas and low-grade MPNST were indistinguishable with a common methylation profile and frequent losses of CDKN2A . Epigenomic analysis finds two groups of conventional high-grade MPNST sharing a frequent loss of neurofibromin. The larger of the two groups shows an additional loss of trimethylation of histone H3 at lysine 27 (H3K27me3). The smaller one retains H3K27me3 and is found in spinal locations. Sporadic MPNST with retained neurofibromin expression did not form an epigenetic group and most cases could be reclassified as cellular schwannomas or soft tissue sarcomas. Widespread immunohistochemical loss of H3K27me3 was exclusively seen in MPNST of the main methylation cluster, which defines it as an additional useful marker for the differentiation of cellular schwannoma and MPNST.

Neurofibromatosis type 1 (NF1) is a common genetic disorder resulting in the development of both benign and malignant tumors of the peripheral nervous system. NF1 is caused by germline pathogenic variants or deletions of the NF1 tumor suppressor gene, which encodes the protein neurofibromin that functions as negative regulator of p21 RAS. Loss of NF1 heterozygosity in Schwann cells (SCs), the cells of origin for these nerve sheath-derived tumors, leads to the formation of plexiform neurofibromas (PNF)—benign yet complex neoplasms involving multiple nerve fascicles and comprised of a myriad of infiltrating stromal and immune cells. PNF development and progression are shaped by dynamic interactions between SCs and immune cells, including mast cells, macrophages, and T cells. In this review, we explore the current state of the field and critical knowledge gaps regarding the role of NF1(Nf1) haploinsufficiency on immune cell function, as well as the putative impact of Schwann cell lineage states on immune cell recruitment and function within the tumor field. Furthermore, we review emerging evidence suggesting a dueling role of Nf1+/- immune cells along the neurofibroma to MPNST continuum, on one hand propitiating PNF initiation, while on the other, potentially impeding the malignant transformation of plexiform and atypical neurofibroma precursor lesions. Finally, we underscore the potential implications of these discoveries and advocate for further research directed at illuminating the contributions of various immune cells subsets in discrete stages of tumor initiation, progression, and malignant transformation to facilitate the discovery and translation of innovative diagnostic and therapeutic approaches to transform risk-adapted care.

Background and Clinical Significance: Neurofibromatosis type 1 (NF1) predisposes individuals to various peripheral nerve sheath tumors (PNSTs), including benign neurofibromas, malignant peripheral nerve sheath tumors (MPNSTs), and intermediate lesions known as atypical neurofibromatous neoplasms of uncertain biologic potential (ANNUBP), previously often termed atypical neurofibroma. These atypical lesions are considered premalignant precursors to MPNST. Case Presentation: We present the case of a 33-year-old male with NF1 who developed a rapidly growing, painful mass in his right calf. Clinical examination revealed signs consistent with NF1. Magnetic resonance imaging showed a large, heterogeneous mass in the lateral compartment. Biopsy revealed a neurofibroma with hypercellularity, moderate atypia, scarce S100 positivity, focal CD34 positivity, and an elevated Ki-67 proliferation index of 10–12%, consistent with ANNUBP. The patient underwent wide surgical resection, including the fibula and peroneal muscles. At the 30-month follow-up, there was no local recurrence, though the patient had a mild residual limp. Discussion: This case highlights the clinical presentation, diagnostic features, and management considerations for ANNUBP in NF1, emphasizing the importance of recognizing warning signs and the role of pathology in guiding treatment for these high-risk precursor lesions.

Background To intraindividually compare the diagnostic performance of positron emission computed tomography (F-18-FDG-PET/CT) and diffusion-weighted magnetic resonance imaging (DW-MRI) in a non-inferiority design for the discrimination of peripheral nerve sheath tumours as benign (BPNST), atypical (ANF), or malignant (MPNST) in patients with neurofibromatosis type 1 (NF1). Results In this prospective single-centre study, thirty-four NF1 patients (18 male; 30 ± 11 years) underwent F-18-FDG-PET/CT and multi-b-value DW-MRI (11 b-values 0 – 800 s/mm²) at 3T. Sixty-six lesions corresponding to 39 BPNST, 11 ANF, and 16 MPNST were evaluated. Two radiologists independently assessed the maximum standardized uptake value (SUV max ) and mean and minimum apparent diffusion coefficient (ADC mean/min ) as well as the ADC in areas of lowest signal intensity in each lesion (ADC dark ). The AUCs of DW-MRI and F-18-FDG-PET/CT were compared to determine whether the ADC is non-inferior to SUV max (non-inferiority margin equal to -10%). Follow-up of ≥ 24 months (BPNST) or histopathological evaluation (MPNST + ANF) served as diagnostic reference standard. Both SUV max and ADC parameters demonstrated good diagnostic accuracy (AUC SUVmax 94.0%; AUC ADCmean/min/dark 91.6% / 90.1% / 92.5%). However, non-inferiority could not be demonstrated for any of the three ADC parameters (lower limits of the confidence intervals of the difference between the AUC of ADC mean/min/dark and SUV max -12.9% / -14.5% / -11.6%). Inter-rater reliability was excellent for both imaging techniques (Krippendorff’s alpha all > 0.94). Conclusions Both PET/CT-derived SUV max and MRI-derived ADC allow sensitive and non-invasive differentiation of benign and (pre)-malignant peripheral nerve sheath tumours. Nevertheless, DW-MRI cannot be considered as non-inferior to F-18-FDG-PET/CT in this prospective single-centre study.

Opinion statement Malignant peripheral nerve sheath tumors (MPNST) are aggressive soft tissue sarcomas characterized by high risk of local recurrence and distant metastasis. The only known curative therapy is complete resection. Adjuvant radiation is recommended for larger lesions or those with more aggressive histology. Given the dismal prognosis in tumors that cannot be cured by surgery alone and the lack of systemic therapy with proven benefit, targeted therapies based on knowledge of activation of the Ras pathway and downstream effectors have been trialed in MPNST, thus far without proven benefit. However, novel or combination therapies based on recent preclinical advances are highly desirable and are the subject of ongoing clinical trials.

Objective Neurofibromatosis type 1 (NF1) patients have a 13% risk of developing a malignant peripheral nerve sheath tumor (MPNST). Many MPNSTs are histopathologically complex, with regions exhibiting features of the original benign plexiform neurofibroma (PNF), of an atypical PNF, or of MPNST showing varying degrees of de-differentiation. This study analyzed the genetic alterations associated with this pathological heterogeneity in order to identify the genetic processes involved in transformation from a benign to an aggressive malignant tumor. Methods A histological and molecular analysis of a single MPNST tumor that was subdivided into three histopathologically distinct regions, a benign PNF (region 1), an atypical PNF (region 2), and a high-grade MPNST (region 3), was carried out. Tumor DNA from each region was analyzed in conjunction with the patient's lymphocyte DNA. Somatic mutation analyses included loss-of heterozygosity (LOH), MLPA analysis, NF1 gene sequencing, and a microarray comparative genomic hybridisation (array CGH) analysis. Results The patient had a germline NF1 splice-site mutation. The NF1-associated LOH analysis found that LOH increased in the three tumor areas, with 9, 42, and 97% LOH evident in regions 1, 2, and 3, respectively. Additional genetic changes, including losses of TP53, RB1, CDKN2A, and of several oncogenes and cell-cycle genes, were found only in the malignant MPNST (region 3). Array CGH also identified genomic gains and losses in DNA from region 3. Discussion This is the first study that correlates the histological and molecular changes associated with MPNST development, confirming the significant cellular and genetic heterogeneity that poses both diagnostic and therapeutic challenges.

Neurofibromas (ICD-O 9540/0) are infiltrative, benign tumors of peripheral nerves that can occur as solitary or diffuse masses in the dermis, subcutis, soft tissues, or peripheral nerves. Neurofibromas are considered World Health Organization (WHO) grade I neoplasms [1].

There is much ambiguity surrounding the diagnosis of nerve sheath tumors, including atypical neurofibroma and low-grade MPNST, and yet, the distinction between these entities designates either benign or malignant behavior and thus carries presumed profound prognostic importance that often guides treatment. This study reviews the diagnostic criteria used to designate atypical neurofibroma from low-grade MPNSTs and reviews existing literature the natural history of each of these tumors to see if the distinction is, in fact, of importance.

Neurofibromatosis type I (NF1), neurofibromatosis type 2 (NF2), and schwannomatosis represent a diverse group of genetic tumor predisposition syndromes with a shared feature of tumors affecting the peripheral nerve sheaths.Purpose of ReviewMany advancements have been made in understanding the biologic underpinnings of these conditions, and in 2016 the first drug was approved by the FDA to treat pediatric symptomatic unresectable plexiform neurofibromas.Recent FindingsMek inhibitors have provided a much-needed therapeutic avenue for NF1 patients with unresectable plexiform neurofibromas (PN), both for reduction of tumor bulk and for improvement in symptoms. Selumetinib is the first FDA approved drug for PN, but is only approved for children. Some research suggests that alternative Mek inhibitors and other mixed tyrosine kinase inhibitors may have better efficacy in adults. Vascular endothelial growth factor (VEGF) inhibitor bevacizumab can prolong hearing and delay the need for surgery in NF2 patients with bilateral vestibular schwannomas.SummaryThis article provides an update regarding considerations and approaches when treating the tumors associated with the neurofibromatoses (NF), including risk and prognosis metrics, clinical trial results, surgical techniques, and radiation therapy recommendations.

Malignant peripheral nerve sheath tumor (MPNST) is an aggressive soft tissue sarcoma with limited therapeutic options and a poor prognosis. Although neurofibromatosis type 1 (NF1) and radiation exposure have been identified as risk factors for MPNST, the genetic and molecular mechanisms underlying MPNST pathogenesis have only lately been roughly elucidated. Plexiform neurofibroma (PN) and atypical neurofibromatous neoplasm of unknown biological potential (ANNUBP) are novel concepts of MPNST precancerous lesions, which revealed sequential mutations in MPNST development. This review summarized the current understanding of MPNST and the latest consensus from its diagnosis to treatment, with highlights on molecular biomarkers and targeted therapies. Additionally, we discussed the current challenges and prospects for MPNST management.