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Meningiomas are extremely common adult brain tumors originating from meningeal coverings of the brain and spinal cord. While most are slowly growing Word Health organization (WHO) grade I tumors, rare variants (clear cell, chordoid, papillary, and rhabdoid), as well as brain invasive (WHO grade II), atypical (WHO grade II), and anaplastic (WHO grade III) meningiomas are considerably more aggressive. This review summarizes the histopathological and genetic features of meningiomas, including differential diagnosis, pitfalls, and grading challenges. Early stages of meningioma tumorigenesis are closely linked to inactivation of one or more members of the 4.1 superfamily, including the neurofibromatosis type 2 ( NF2 ) and 4.1B ( DAL - 1 ) genes, which interact with the 14-3-3 protein family. Other chromosome 22q genes implicated include BAM22 , BCR (breakpoint cluster region), and TIMP - 1 , the last of which is implicated in higher-grade meningiomas. Atypical meningiomas also commonly show chromosomal losses of 1p, 6q, 10, 14q, and 18q, as well as multiple chromosomal gains. While most relevant genes remain unknown, two chromosome 14q candidates ( MEG3 and NDRG2 ) have recently been identified. In addition to alterations of CDKN2A , p14 ARF , and CDKN2B tumor suppressor genes on 9p21, a contribution of the wingless (wnt) pathway with alterations of the E-cadherin and beta-catenin proteins, as well as alterations of the hedgehog signaling pathway have been implicated in anaplastic meningiomas. The integration of histopathological appearance, complex genetic/genomic data, and outcome will likely result in the identification of clinically distinct meningioma subgroups, which in turn can facilitate the development of targeted therapeutic strategies.

This review summarizes the changes in the 5th Edition of the WHO Classification of Endocrine and Neuroendocrine Tumors that relate to the pituitary gland. The new classification clearly distinguishes anterior lobe (adenohypophyseal) from posterior lobe (neurohypophyseal) and hypothalamic tumors. Other tumors arising in the sellar region are also discussed. Anterior lobe tumors include (i) well-differentiated adenohypophyseal tumors that are now classified as pituitary neuroendocrine tumors (PitNETs; formerly known as pituitary adenomas), (ii) pituitary blastoma, and (iii) the two types of craniopharyngioma. The new WHO classification provides detailed histological subtyping of a PitNET based on the tumor cell lineage, cell type, and related characteristics. The routine use of immunohistochemistry for pituitary transcription factors (PIT1, TPIT, SF1, GATA3, and ERα) is endorsed in this classification. The major PIT1, TPIT, and SF1 lineage-defined PitNET types and subtypes feature distinct morphologic, molecular, and clinical differences. The “null cell” tumor, which is a diagnosis of exclusion, is reserved for PitNETs with no evidence of adenohypophyseal lineage differentiation. Unlike the 2017 WHO classification, mammosomatotroph and acidophil stem cell tumors represent distinct PIT1-lineage PitNETs. The diagnostic category of PIT1-positive plurihormonal tumor that was introduced in the 2017 WHO classification is replaced by two clinicopathologically distinct PitNETs: the immature PIT1-lineage tumor (formerly known as silent subtype 3 tumor) and the mature plurihormonal PIT1-lineage tumor. Rare unusual plurihormonal tumors feature multi-lineage differentiation. The importance of recognizing multiple synchronous PitNETs is emphasized to avoid misclassification. The term “metastatic PitNET” is advocated to replace the previous terminology “pituitary carcinoma” in order to avoid confusion with neuroendocrine carcinoma (a poorly differentiated epithelial neuroendocrine neoplasm). Subtypes of PitNETs that are associated with a high risk of adverse biology are emphasized within their cell lineage and cell type as well as based on clinical variables. Posterior lobe tumors, the family of pituicyte tumors, include the traditional pituicytoma, the oncocytic form (spindle cell oncocytoma), the granular cell form (granular cell tumor), and the ependymal type (sellar ependymoma). Although these historical terms are entrenched in the literature, they are nonspecific and confusing, such that oncocytic pituicytoma, granular cell pituicytoma, and ependymal pituicytoma are now proposed as more accurate. Tumors with hypothalamic neuronal differentiation are classified as gangliocytomas or neurocytomas based on large and small cell size, respectively. This classification sets the standard for a high degree of sophistication to allow individualized patient management approaches.

The 2016 World Health Organization Classification of Tumors of the Central Nervous System is both a conceptual and practical advance over its 2007 predecessor. For the first time, the WHO classification of CNS tumors uses molecular parameters in addition to histology to define many tumor entities, thus formulating a concept for how CNS tumor diagnoses should be structured in the molecular era. As such, the 2016 CNS WHO presents major restructuring of the diffuse gliomas, medulloblastomas and other embryonal tumors, and incorporates new entities that are defined by both histology and molecular features, including glioblastoma, IDH-wildtype and glioblastoma, IDH-mutant; diffuse midline glioma, H3 K27M–mutant; RELA fusion–positive ependymoma; medulloblastoma, WNT-activated and medulloblastoma, SHH-activated; and embryonal tumour with multilayered rosettes, C19MC-altered. The 2016 edition has added newly recognized neoplasms, and has deleted some entities, variants and patterns that no longer have diagnostic and/or biological relevance. Other notable changes include the addition of brain invasion as a criterion for atypical meningioma and the introduction of a soft tissue-type grading system for the now combined entity of solitary fibrous tumor / hemangiopericytoma—a departure from the manner by which other CNS tumors are graded. Overall, it is hoped that the 2016 CNS WHO will facilitate clinical, experimental and epidemiological studies that will lead to improvements in the lives of patients with brain tumors.

Peripheral nerve sheath tumors are common neoplasms, with classic identifiable features, but on occasion, they are diagnostically challenging. Although well-defined subtypes of peripheral nerve sheath tumors were described early in the history of surgical pathology, controversies regarding the classification and grading of these tumors persist. Advances in molecular biology have provided new insights into the nature of the various peripheral nerve sheath tumors, and have begun to suggest novel targeted therapeutic approaches. In this review, we discuss current concepts and problematic areas in the pathology of peripheral nerve sheath tumors. Diagnostic criteria and differential diagnosis for the major categories of nerve sheath tumors are proposed, including neurofibroma, schwannoma, and perineurioma. Diagnostically challenging variants, including plexiform, cellular and melanotic schwannomas are highlighted. A subset of these affects the childhood population, and has historically been interpreted as malignant, although current evidence and outcome data suggest they represent benign entities. The growing current literature and the author's experience with difficult to classify borderline or “hybrid tumors” are discussed and illustrated. Some of these classification gray zones occur with frequency in the gastrointestinal tract, an anatomical compartment that must always be entertained when examining these neoplasms. Other growing recent areas of interest include the heterogeneous group of pseudoneoplastic lesions involving peripheral nerve composed of mature adipose tissue and/or skeletal muscle, such as the enigmatic neuromuscular choristoma. Malignant peripheral nerve sheath tumors (MPNST) represent a diagnostically controversial group; difficulties in grading and guidelines to separate “atypical neurofibroma” from MPNST are provided. There is an increasing literature of MPNST mimics which neuropathologists must be aware of, including synovial sarcoma and ossifying fibromyxoid tumor. Finally, we discuss entities that are lacking from the section on cranial and paraspinal nerves in the current WHO classification, and that may warrant inclusion in future classifications. In summary, although the diagnosis and classification of most conventional peripheral nerve sheath tumors are relatively straightforward for the experienced observer, yet borderline and difficult-to-classify neoplasms continue to be problematic. In the current review, we attempt to provide some useful guidelines for the surgical neuropathologist to help navigate these persistent, challenging problems.

Meningiomas account for up to 30% of all primary intracranial tumours. They are histologically classified according to the World Health Organization (WHO) classification of tumours of the nervous system. Most meningiomas are benign lesions of WHO grade I, whereas some meningioma variants correspond with WHO grades II and III and are associated with a higher risk of recurrence and shorter survival times. Mutations in the NF2 gene and loss of chromosome 22q are the most common genetic alterations associated with the initiation of meningiomas. With increase in tumour grade, additional progression-associated molecular aberrations can be found; however, most of the relevant genes are yet to be identified. High-throughput techniques of global genome and transcriptome analyses and new meningioma models provide increasing insight into meningioma biology and will help to identify common pathogenic pathways that may be targeted by new therapeutic approaches.

The classification of adenohypophysial neoplasms as “pituitary neuroendocrine tumors” (PitNETs) was proposed in 2017 to reflect their characteristics as epithelial neuroendocrine neoplasms with a spectrum of clinical behaviors ranging from small indolent lesions to large, locally invasive, unresectable tumors. Tumor growth and hormone hypersecretion cause significant morbidity and mortality in a subset of patients. The proposal was endorsed by a WHO working group that sought to provide a unified approach to neuroendocrine neoplasia in all body sites. We review the features that are characteristic of neuroendocrine cells, the epidemiology and prognosis of these tumors, as well as further refinements in terms used for other pituitary tumors to ensure consistency with the WHO framework. The intense study of PitNETs has provided information about the importance of cellular differentiation in tumor prognosis as a model for neuroendocrine tumors in different locations.

The diagnosis of malignant peripheral nerve sheath tumors can be challenging and other spindle cell sarcomas commonly enter in the differential diagnosis. Loss of trimethylation at lysine 27 of histone-H3 (H3K27me3) by immunohistochemistry was recently described in malignant peripheral nerve sheath tumors. However, its specificity remains controversial. We therefore studied 82 synovial sarcomas, 39 malignant peripheral nerve sheath tumors, and 10 fibrosarcomatous dermatofibrosarcoma protuberans for H3K27me3 loss by immunohistochemistry. The diagnoses were based on morphology, immunophenotype, and genetics based on WHO classification. H3K27me3 immunohistochemistry was scored by two pathologists based on fraction of cells with nuclear staining (score 0 to 3+). Loss of H3K27me3 (score 0) was seen in 44% of malignant peripheral nerve sheath tumors and 9% of synovial sarcomas yielding positive and negative predictive values of 71% and 77%, respectively. Loss of H3K27me3 was seen in 10% of fibrosarcomatous dermatofibrosarcoma protuberans, yielding positive and negative predictive values of 94 and 29% in the differential diagnosis of malignant peripheral nerve sheath tumor versus fibrosarcomatous dermatofibrosarcoma protuberans. Partial loss (score 1–2) was common in all three tumor types. Among malignant peripheral nerve sheath tumors, there was no significant association between H3K27me3 loss and gender, tumor site, or size, and progression-free or overall survival. Patients with tumors with H3K27me3 loss were younger than those with tumors with retained H3K27me3 expression ( P =0.011). H3K27me3 expression was lost in 50 and 31% of sporadic and Neurofibromatosis-associated malignant peripheral nerve sheath tumors, respectively ( P =0.25). Complete H3K27me3 loss is a moderately sensitive and relatively specific marker for the diagnosis of malignant peripheral nerve sheath tumor when the differential diagnosis includes synovial sarcoma and fibrosarcomatous dermatofibrosarcoma protuberans. Partial loss has limited diagnostic utility. H3K27me3 status does not show significant association with clinical outcome in malignant peripheral nerve sheath tumors.