A narrative review of what the neuropathologist needs to tell the clinician in neuro-oncology practice concerning WHO CNS5

Yanghao Hou; Felix Sahm

doi:10.4103/glioma.glioma_31_22

REVIEW

Year : 2022 | Volume : 5 | Issue : 4 | Page : 120-129

A narrative review of what the neuropathologist needs to tell the clinician in neuro-oncology practice concerning WHO CNS5

Yanghao Hou¹, Felix Sahm²
¹ Department of Pathology, Chongqing Medical University, Chongqing, China
² Department of Neuropathology, University Hospital Heidelberg, Germany, and CCU Neuropathology, German Consortium for Translational Cancer Research (DKTK), German Cancer Research Center (DKFZ), Heidelberg, Germany

Date of Submission	31-Dec-2022
Date of Decision	12-Jan-2023
Date of Acceptance	16-Jan-2023
Date of Web Publication	08-Mar-2023

Correspondence Address:
Dr. Yanghao Hou
Department of Pathology, Chongqing Medical University, Yixueyuan Road 1, Chongqing - 400 016
China

Source of Support: None, Conflict of Interest: None

DOI: 10.4103/glioma.glioma_31_22

Abstract

The fifth edition of the World Health Organization Classification of Tumors of the Central Nervous System (WHO CNS5) was released in late 2021. The adoption of new tumor nomenclature, grading criteria, terminology, tumor types, and novel diagnostic approaches, including methylation profiling, will benefit the precise diagnosis of CNS tumors, facilitate optimal patient care and improve diagnostic reproducibility with higher clinical relevance. However, the gap between rapid updates in tumor classification and the clinical management of patients requires frequent and up-to-date communications between neuropathologists and clinicians. This review aims to provide an overview of WHO CNS5, focused on the changes that are most pertinent to the clinical care of patients. Forming concrete ideas for neuropathologists that are necessary to express to clinicians, for a better understanding of how the patient may benefit from the new classification.

Keywords: Central nervous system tumors, glioma, methylation, neuro-oncology, neuropathology, next-generation sequencing, tumor classification, World Health Organization

How to cite this article:
Hou Y, Sahm F. A narrative review of what the neuropathologist needs to tell the clinician in neuro-oncology practice concerning WHO CNS5. Glioma 2022;5:120-9

How to cite this URL:
Hou Y, Sahm F. A narrative review of what the neuropathologist needs to tell the clinician in neuro-oncology practice concerning WHO CNS5. Glioma [serial online] 2022 [cited 2023 Jun 4];5:120-9. Available from: http://www.jglioma.com/text.asp?2022/5/4/120/371294

Introduction

Based on rapid advances in both basic and clinical research for central nervous system (CNS) tumors and the multiple updates of the Consortium to Inform Molecular and Practical Approaches to CNS Tumor Taxonomy^{[1],[2],[3],[4],[5],[6],[7],[8]} (c-IMPACT), the fifth edition of the World Health Organization Classification of Tumors of the CNS^[9] (WHO CNS5) was released in late 2021. In the new edition, general changes including new tumor types and subtypes, terminology, updated grading criteria, and novel diagnostic approaches have thrived. However, the gap between rapid updates in tumor classification and the clinical management of patients requires frequent and up-to-date communications between neuropathologists and clinicians, especially regarding the substantial changes brought by the new edition of CNS tumor classification. This review aims to provide an overview of WHO CNS5, focused on the changes that are most pertinent to the clinical care of patients. Forming concrete ideas for neuropathologists that are necessary to express to clinicians, for a better understanding of how the patient may benefit from the new classification.

Retrieval Strategy

PubMed database was used for literature searching. The following keywords were used for article selection and initial evaluation: CNS tumors; WHO classification; cIMPACT-NOW; clinical implications; DNA methylation; and neuropathology. Most of the studies were published within 5 years. Review and systematic review were selected as searching article types. The authors individually curated the data for a narrative review.

Division of Diffuse Gliomas, Adult-Type, and Pediatric-Type

The WHO CNS5 separates the diffuse gliomas into adult-type and pediatric-type diffuse gliomas,^[9] this categorical division raised heavy discussions and questions, for example, whether certain tumor types that do not fit their age categories should be diagnosed in addition to not elsewhere classified (NEC), or, prompt further diagnostic approaches such as methylation profiling and next-generation sequencing (NGS) for decisive conclusions; whether the age should be taken into an account when choosing treatment. In fact, the weight of age in diagnostic value also differs between clinicians and neuropathologists.

As previously illustrated in the WHO CNS5 and c-IMPACT-now, the division is primarily aimed at better clinical care of both adults and children with brain tumors,^[4],[9] especially in the case that "pediatric-type" tumors currently have more defined different tumor types and variable prognosis,^[10] a precise diagnosis may lead to an ideal therapy, that potentially improve not only the survival but also the quality of life for the young patient.

By recognition of their clinical and genetic distinctions, and the availability of new diagnostic approaches, most diffuse gliomas can now fit into the category of their age.^[10] Nevertheless, pediatric-type tumors can occur in adults, especially in younger adults, as well as adult-type tumors occur in children (more rarely).^[9] In clinical practice, the majority of diffuse gliomas is correlated with their age and can provide more clinical implications and precise prognosis predictions. The tumors that do not match the age categories are encouraged to investigate their biological feature and clinical behavior, accumulating research for such cases may help expand the knowledge of CNS tumors or recognize new entities. Noteworthy, circumscribed astrocytic gliomas, glioneuronal and neuronal tumors, and ependymal tumors are not yet divided into separate categories by their age.

Tumor Terminology

The WHO CNS5 has now abandoned the long-lasting Roman numerals (Grades I, II, III, IV) for the tumor grading system, due to the visual similarity of the Roman numerals, Arabic numerals (grades 1, 2, 3, and 4) are used instead to prevent mistyping or misreading. The term "CNS WHO grade" still has to be assigned to highlight the difference compared to non-CNS neoplasms.^[11]

The nomenclature of CNS tumor is simplified as well, the descriptive term "anaplastic" is no more included in the diagnosis, because the appearance of anaplasia is not always relevant with higher grade tumor types (previously "anaplastic" usually implies a WHO grade 3 tumor type) and worse prognosis,^[12],[13] especially in some circumscribed and glioneuronal tumors. The use of layered integrated reports suggested by WHO CNS5 will exhibit the histological diagnosis, molecular features, and its CNS WHO grade as concisely as possible, preventing modifier terms that could potentially cause confusion and stereotype false impressions in clinical procedures.

Most of the grades assigned by WHO CNS5 were identical to the previous edition,^[10] but some of them are still under discussion or compromised, as the grade is not always linked to ideal clinical-biological behaviors, for example, almost all WNT-activated medulloblastoma patients that received standard therapy can have long-term survival. Despite the great difference between other embryos untreatable with a desperate outcome, they are all assigned to the WHO grade 4.^[10],[14] Clinicians should be informed by pathologists to prevent false prognosis predictions or inadequate therapeutic options solely based on their WHO grades.

Not Otherwise Specified and Not Elsewhere Classified

As described by cIMPACT-NOW and WHO CNS5,^[6],[9] not otherwise specified (NOS) alerts clinicians that necessary molecular tests to classify the lesion to a certain WHO diagnosis or grades were not performed or failed for technical reasons, the current diagnosis is rather descriptive and not decisive, further molecular approaches are highly encouraged if the integrated diagnosis is treatment relevant.

NEC implies that sufficient molecular tests were well-performed, however, the result from both histology and molecular testing still failed to fit in a recognized WHO tumor entity,^[6] thus, clinicians have to be aware that further diagnostic approaches may not be necessary and the case can be valuable for research. However, in some instances, testing procedures that may not be standard yet could provide more insight, including the search for rare gene fusion events.

For instance, for an H3-wildtype diffuse midline glioma (DMG) NOS, it should be noted in the molecular information of the layered pathology report that EZHIP expression levels, EGFR gene alterations, or methylation profiling are needed to confirm the diagnosis of DMG, H3 K27-altered (H3-wildtype subtypes) according to its new diagnostic criteria. In addition, NEC should be used instead if the molecular testing yielded the negative results above or the tumor was not classifiable by comprehensive analysis including methylation profiling.

New Tumor Types

Advanced molecular approaches prompt the recognition of certain new tumor types, most of them was accepted by WHO CNS5 [Table 1]. They are defined mostly by their distinct methylation signature and key genetical changes, as well as their representative histological features. For unresolved lesions that these tumor types are considered a differential diagnosis, methylation profiling may provide essential molecular evidence for a conclusive diagnosis,^[9] since that most of the new entities can be recognized by their distinct methylation profiles,^[15],[16] and it is the only method for definitively establishing the diagnosis of two new entities: high-grade astrocytoma with piloid features (HGAP) and diffuse glioneuronal tumor with oligodendroglioma-like features and nuclear clusters (DGONC).

Table 1: New tumor types and their grading in WHO CNS5^[9],[10]

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A cribriform neuroepithelial tumor (CRINET), DGONC, and intracranial mesenchymal tumor, FET: CREB fusion-positive are provisional types without an assigned CNS WHO grade, more clinical studies to further confirm these tumor types are required.

HGAP is recognized as a new distinct type of circumscribed astrocytic gliomas.^[16] It is critical to emphasize the difference between pilocytic astrocytoma with histological features of anaplasia ("anaplastic pilocytic astrocytoma") and HGAP. These two distinct entities may share similar morphological changes or even molecular features (KIAA1549:BRAF gene fusions), but the former is regarded as a subtype of pilocytic astrocytoma in WHO CNS5 without an assigned WHO grade, as the prognostic significance of the presence of anaplasia in pilocytic astrocytomas still needs to be determined. However, the 5-year overall survival rate of patients diagnosed with HGAP was nearly 50%, roughly corresponding to CNS WHO grade 3.^[9] The desperate outcomes were also seen in HGAP patients without morphological high-grade features.^[16] The misconception of these two entities may cause failure in prognosis prediction or even inadequate treatments.

New Classification and Grading Criteria

The WHO CNS5 has combined different entities previously divided by different grades, NOS designations, and subtypes into one. As a result, the current category of adult-type diffuse gliomas only includes three types of tumor. Grading and subtypes are illustrated in each chapter^[9] (under headlines of subtypes and grading). Astrocytoma, IDH-mutant are still graded by 2, 3, and 4, in addition to conventional histological grades, the presence of CDKN2A/B homozygous deletion can now directly upgrade the tumor to grade 4 despite their morphological appearance. Noteworthy, these molecular-defined upgrades may not be appropriate in other tumor types like pleomorphic xanthoastrocytomas (PXA).^[17],[18]

Previously, there are a considerable number of cases that were diagnosed as IDH-wildtype diffuse astrocytomas in adults, which were considered as a provisional entity in the 2016 CNS classification, multicenter studies proving most of these tumors showed the similar outcome of glioblastomas,^[19] with the molecular signature of either TERT promoter mutation, EGFR gene amplification, or combined gain of entire chromosome 7 and loss of entire chromosome 10 (7⁺/10⁻). In WHO CNS5, the two morphological changes: necrosis, microvascular proliferation, and the three genetic alterations above are independently regarded as five parameters sufficient for the diagnosis of glioblastoma, IDH-wildtype. Since the type of "IDH-wildtype diffuse astrocytoma" is no longer included in WHO CNS5,^[9],[10] IDH-wildtype diffuse gliomas should now be considered as NOS, further molecular testing may be required for a decisive diagnosis, especially for younger age patients, to differentiate the molecular-defined glioblastoma and pediatric-type diffuse low-grade gliomas.^[20]

New Diagnostic Approach: Methylation Profiling

Methylation profiling using bisulfite converted DNA and illumina microarray technology to determine the methylation status of comprehensive cancer-relevant CpG sites (over 850000 preselected CpG sites for EPIC bead chip, "850K") has repeatedly been validated as a reliable and powerful diagnostic tool for the classification of CNS tumors.^[21]

The milestone study by Capper et al. and multicenter large-scale research^[22],[23] has proven the independent ability of this technology in CNS tumor diagnostics. Beyond the utility of precise classification and subtyping recognized CNS tumors, new entities that share distinct epigenetic features are constantly discovered by this technology;^{[15],[16],[20],[24]} most of these rare and unresolved lesions are hard to be solved by histology and NGS.

Although classifying tumor types by the difference in methylation features has been reported for decades,^[25] it is the first time that has been officially suggested as a clinical diagnostic approach in WHO CNS5.^[9] In the new edition, most CNS tumors are assumed with a distinct methylation profile, mentioned mostly in the section on tumor definition, diagnostic molecular pathology, and essential and desirable diagnostic criteria. The detailed description may be critical in the selection of different diagnostic approaches and how the result of methylation profiling can be integrated into diagnosis. The DNA methylation-based CNS tumor classifier on the webpage (www.molecularneuropathology.org) allows the upload of methylation raw data derived directly from 450K or 850K Illumina arrays (.idat file) and provides methylation profiling reports based on the reference tumors and standardized algorithms.^[21],[22] However, this is so far only available as research-use-only on the website. Moreover, other bioinformatical pipelines and local approaches for methylation profiling still await to reach a consensus,^[21],[26] regulatory issues between countries made this technology still limited and not widely available.

In addition to analyzing the methylation class, the tumor refers to, and the MGMT promoter methylation status from the 850K methylation data (STP27 Model),^[27] whole-genome copy number profile (CNP) can also be obtained by the sum of methylated and unmethylated signal intensities.^[21] Due to an extraordinary number of probes from 850K, CNP inferred by 850K contains more information and showed advantages compared to other methodologies (FISH, NGS). For example, 1p/19q whole-chromosome codeletion is the essential diagnostic evidence for oligodendroglioma; many of the pathology departments are using FISH for a cost-effective approach to detecting chromosomal changes; however, the limited number designed from FISH probes usually cannot objectively represent the whole chromosome changes, and partial deletion in chromosome 1p plus 19q deletion can be reported as a false-positive result of 1p/19q codeletion,^[28] and has constantly been regarded as diagnostic evidence for oligodendroglioma in IDH-wildtype diffuse gliomas, which arouses diagnostic confusion. This false-positive result happens also in the detection of whole-chromosome 7 gain in combination with 10 loss (7⁺/10^-), and most of the FISH probes designed for the prediction of 7⁺/10⁻ are based on aiming at single locus: Gene EGFR on chromosome 7 and PTEN on chromosome 10. The result from FISH cannot be truly representative for the whole chromosome change and may cause an upgrade-misdiagnosis directly as molecular-defined IDH-wildtype glioblastoma WHO4. However, CNP derived from DNA methylation provides a clear and global observation for the whole chromosome, as well as amplifications and homozygous deletions for diagnostic or grading relevant genes^[21] (e.g., EGFR, MYCN, CDKN2A/B, and MYB/MYBL1), also suggestive evidence for gene-fusion events^{[29],[30],[31]} (KIAA1549/BRAF, FGFR1/TACC1, and PRKCA/SLC44A1). From a combination analysis of CNP and methylation profiling, tumor types can be correctly classified even if the tumor co-occurred with rare chromosome changes that are typical for other tumor types, for example, PXA with 7⁺/10⁻,^[13] DLGNT with 1p/19q whole chromosome deletion.^[32] Tumors with typical diagnostic chromosome changes and their rare variants are depicted in [Figure 1] and [Figure 2]. The comparison of 1p/19q and 7⁺/10⁻ results derived from FISH and methylation profiling is documented in [Table 2].

Figure 1: Methylation-based T-SNE analysis including six selected cases with reference cases. Case 1 and Case 3 exhibit typical diagnostic-relevant chromosome changes in accordance with their methylation group refers. Case 2 detected a 1p/19q co-del by FISH. Case 4, 5, and 6 are tumors that exhibit glioblastoma-like 7+/10-chromosome changes but belong to ganglioglioma and PXA. AIDH: Astrocytoma, IDH-mutant, GBM_MYCN: Diffuse pediatric-type high-grade glioma MYCN subtype, GBM_RTKI: Glioblastoma, IDH-wildtype, RTKI methylation subtype, GBM_RTKII: Glioblastoma, IDH-wildtype, RTKII methylation subtype, HGAP: High-grade astrocytoma with piloid features, LGG_GG: Ganglioglioma, OIDH: Oligodendroglioma, IDH-mutant and 1p/19q-codeleted, PXA: Pleomorphic xanthoastrocytoma. Original source of Figure 1 was selected from GEO reference dataset (GSE109381).^[23] The raw data were selected and reanalyzed via R package "Rtsne" using 20000 most variable CpG sites

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Figure 2: Methylation-based copy number variations of the six selected cases in Figure 1. Copy-number profile analysis was proceeded by R package "conumee" using the raw data mentioned above. Original source of Figure 2 was selected from GEO reference dataset (GSE109381).^[23]

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Table 2: 1p/19q and 7+/10- results derived from fluorescence in situ hybridization and methylation profiling for tumors with typical chromosome changes and rare variants

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After the publication of WHO CNS5, new entities and variants of CNS tumors emerged continually with distinct methylation features, such as oligosarcoma,^[33] PLAGL1-fused neuroepithelial tumor,^[34] high-grade glioma with pleomorphic and pseudopapillary features,^[35] and primary mismatch repair deficient IDH-mutant astrocytoma.^[36] These new entities are not yet accepted by the WHO, yet methylation profiling may help recognize these tumors that are mostly diagnosed as NEC, the clinical behavior and indication of prognosis in the studies may be informative, which can be noted in the comment of the pathological report. The accumulative clinical investigation of such cases by both clinicians and pathologists will help to refine or expand the classification of CNS tumors.

Diagnostic Approach: Next-Generation Sequencing

DNA mutational analysis by NGS has been used as the primary assay to detect molecular pathological alterations for decades due to its advantages of efficient, feasible, and well-developed analytical pipelines for the evaluation of gene alterations. Both fresh frozen and formalin-fixed paraffin-embedded samples are suitable for NGS even with low tumor purity.^[37] Panels designed especially for CNS tumors can be easier to use and cost-effective in clinical practice. Candidate genes available for targeted therapy (BRAF mutation and NTRK and ROS1 fusions) should be verified by NGS but cannot regard as a surrogate from methylation profiling. NGS can detect mutations (SNVs and small indels), small CNVs, and some fusions based on adequate bioinformatical analysis and enough depth of coverage. Still, it is not optimal for large CNV evaluations (mentioned above) compared to methylation profiling, especially the customized NGS panels.^[37]

WHO CNS5 has pushed the weight of molecular evidence one step further in CNS tumor diagnosis, and key molecular changes are of greater importance when considering the integration of diagnosis. By combining histological findings with NGS results, most CNS tumors can be identified. However, accumulating studies and WHO CNS5 updates revealed an increasing amount of different types of tumors that share a similar gene mutation that used to be considered more specific (H3 K27M, FGFR1, and BRAFv600E). Moreover, these genetic changes may not occur and still cannot exclude the relevant tumor entities. In clinical practice, differential diagnosis often still exists or NGS may not have yielded informative results, especially for those low-grade pediatric gliomas. These cases may be more suitable for methylation profiling.^[22]

Although DNA sequencing can detect a series of genetic fusions, full mRNA sequencing covers even more structural alterations and can identify those uncovered, rare, or novel fusions that can be neglected by DNA sequencing.^[38] Compared to mutational changes, gene fusions are more specific and have a higher probability of occurrence to a certain tumor entity^[31],[39] (YAP1-MAMLD1, ZFTA-RELA, SLC44A1-PRKCA), which usually provide critical and decisive evidence for the diagnosis, especially in the classifications of low-grade pediatric gliomas, glioneuronal tumors, or ependymomas that may not behave specific mutational changes. A false-negative result of a specific gene fusion by DNA sequencing usually leads to an inconclusive diagnosis.

Noteworthy, as the evidence for targeted therapy, druggable fusions can be reliably detected (NTKR-, ROS1-,) by RNA sequencing,^[37],[38] which will certainly benefit patients.

Clinical Implications

Besides the adaption to the new classification scheme of WHO CNS5, the correlation between refined tumor types and clinical implications are important as well. A detailed description of suggested treatment for the new classification of CNS tumors is reviewed by Horbinski et al.^[11] Here, we highlight some most concerning key points:

1. DMG, also known as diffuse intrinsic pontine glioma, has changed its terminology from H3 K27M-mutant to DMG, H3 K27-altered.^[9] Studies revealed that the overexpression of EZHIP in H3-wildtype cases mediates PRC2 inhibition,^[40] which is considered as an endogenous mimic of H3 p. K28M (K27M); such entity is accepted as a subtype of DMG; although they are H3 gene wildtype, this subtype showed a slightly better prognosis (os: 16M) compared to H3-mutant DMGs.^[40],[41] Another subtype of DMG H3-wildtype harbored a genetic alteration of EGFR (mutation or amplification). A similar methylation profile was identified for all these DMG subtypes.^[40],[41]

For the diagnosis of DMG H3 K27-altered, as a diffuse glioma located in the midline location that is H3 gene wildtype, the rest of the two subtypes should be confirmed by molecular testing aiming at the expression of EZHIP and alterations of EGFR. However, H3 gene mutation alone cannot definitively confirm the diagnosis of DMG H3 K27M, while rare lower-grade gliomas (pilocytic astrocytomas, gangliogliomas, subependymomas, and ependymomas) located in the midline with an H3 gene mutation has been constantly reported, but these tumors do not carry the rapidly lethal prognosis like DMG.^{[42],[43],[44]} For unresolved lesions, methylation profiling for such cases is of great help, for confirming the diagnosis of DMG or even differentiating its subtypes, meanwhile, to exclude the lower grade gliomas that harbor H3 gene alterations.

2. Infant-type hemispheric glioma (IHG) is accepted as a distinct tumor that refers to pediatric-type diffuse high-grade gliomas, but without an assigned WHO grade, the occurrence in infants is found to have greater outcomes than those in older children,^[9] and IHG subtypes are characterized by RTK fusions in the NTRK family (ALK, ROS, and MET); these fusions offer the opportunity of targeted therapy. Studies showed that specific inhibitors could change the prognosis of IHGs,^{[45],[46],[47]} and the efficacy of inhibitors and the outcomes for different IHG subtypes still await further investigation. Methylation profiling could help confirm the diagnosis of IHG, and distinguish it from other high-grade gliomas, but druggable fusions should be confirmed by molecular testing before patient-targeted therapy. In general, molecular testing is highly suggested for high-grade pediatric gliomas, especially for the diagnosis of IHG, which is considered the most important therapeutic change by WHO CNS5.

3. Low-grade pediatric gliomas, diffuse astrocytoma, MYB- or MYBL1-altered are mostly well defined but may be focally present a diffuse growth pattern from its imaging; like some of the pilocytic astrocytomas, it is hard to define a diffuse or circumscribed growth pattern under the observation through microscopy.^[11] Histologically, monomorphic round tumor cells and low-grade appearance are usually seen, occasionally glioneuronal areas may be observed, but some cases can be challenging to differentiate between tumor and normal or reactive brain tissues.^[20] In the study, 11 out of 22 (50%) of this tumor type identified a structural variation in the fusion of MYB or MYBL1, which helps confirm the diagnosis. Methylation profiling is ideal and helpful for the identification of tumor, especially for cases without MYB or MYBL1 alterations.^[20]

4. Polymorphous low-grade neuroepithelial tumor of the young and diffuse low-grade glioma, MAPK pathway-altered harbored both MAPK pathway activating genetic alterations relevant to BRAF or FGFR,^[48] the latter is still uncertain as a distinct type or a group of low-grade gliomas. Polymorphous low-grade neuroepithelial tumor of the young shows a relatively distinct methylation profile that closes to ganglioglioma.^[49] The revised category of low-grade diffuse pediatric gliomas generally suggests a benign behavior, but long-time progression and recurrence are rarely reported.^[9] Molecular-targeted therapy, BRAF, FGFR, or MEK inhibitors may be suitable for cases with draggable molecular evidence and should be encouraged in clinical trials.^[11]

Before the publication of WHO CNS5, a considerable amount of these entities were diagnosed as IDH-wildtype astrocytic or oligodendrogliomas grade 2, based on their histological similarities, since the chance of recurrence, tumor progression, treatment options, and prognosis are of great difference,^[9] a definitive diagnosis of an IDH-wildtype diffuse glioma is necessary, especially for younger patients.^[19]

5. For circumscribed astrocytic gliomas, pilocytic astrocytomas are the most frequent type encountered; it is reported that PAs with BRAF fusions responds well to MEK inhibitors.^[11],[50] The frequency of BRAF fusions occurring in PA is 60%, but particularly PA located in the cerebellum, the fusion less occurred in supratentorial PA.^[9] Radiation therapy is not recommended for pilocytic astrocytomas, concerning its radiation-induced mutagenesis that may prompt tumor recurrence, unless some sporadic pilocytic astrocytomas or brainstem locations that unable to be surgically resected.^[51]

Pilomyxoid astrocytoma was a variant type of pilocytic astrocytoma without a recommended grade in the 2016 WHO classification, now it is been regarded as a morphological subtype of pilocytic astrocytoma, likely as CNS WHO grade 1.^[9] However, it is warranted that this subtype is more likely to act aggressively;^[52] this information has to be noticed in the comment of the pathological report and suggests for a closer follow-up.

When it comes to the situation to differentiate pilocytic astrocytoma with histological anaplasia and HGAP, methylation profiling is highly suggested since it is regarded as the only method for definitively confirming this new tumor type, and also to exclude other potential mimics of pilocytic astrocytomas.^[16] The clinical behavior and the sensitivity to therapies of HGAP remained to be further investigated.

6. New changes in ependymoma including the rename of RELA-fused ependymoma to ZFTA (formerly C11orf95) fusion-positive, supratentorial ependymoma, ZFTA fusions (mainly with RELA) are regarded as the key oncogenic driver of this entity. Myxopapillary ependymoma has upgraded from WHO CNS grade 1 to grade 2, due to its tendency of recurrence and spreading features, particularly in children.^[53],[54]

7. Medulloblastomas, the most common malignant brain tumors of childhood,^[55] classification is refined by their molecular features, especially methylation patterns. Group 3 and group 4 medulloblastomas have been combined into medulloblastoma, non-WNT/non-SHH. However, eight heterogeneity subgroups were demonstrated by DNA methylation profiling. These eight subtypes shared disparate outcomes: group 2 and group 3 exhibited poor outcomes with a 5-year overall survival rate of 55% and 45%, whereas group 4, group 6, and group 7 showed better prognosis with a 5-year overall survival rate of over 80%.^[56],[57] A clear classification of medulloblastomas is of great importance to pediatric patients, not only for its precise prognosis prediction but also an indication for optimal therapy, which would greatly influence the outcome. The overall survival rate of WNT-activated medulloblastomas is close to 100% as if the patient went through appropriate surgical approaches and adjuvant therapeutic regimens.^[58] Nevertheless, without a precise classification and optimized therapy, the tumor is still highly aggressive and behaves as WHO grade 4.^[11] As the frequent and critical technic used in the research reveals these subtypes of medulloblastoma, methylation profiling should be suggested as the ideal approach for confirming and subtyping the diagnosis of medulloblastoma, meanwhile, excluding the histological mimics including newly recognized other CNS embryonal tumors and offers precise prediction of prognosis relevant with heterogeneity medulloblastoma methylation subtypes.

8. Other CNS embryonal tumors, based on the new discovery of their molecular characteristics, the overarching term "CNS primitive neuroectodermal tumor," are been abandoned by WHO CNS5.^[9] Instead, CRINET, CNS neuroblastoma, FOXR2-activated, and CNS tumor with BCOR internal tandem duplication are newly added to this category.^[10] Previously, most of the CRINET was been diagnosed as choroid plexus carcinomas, as they share similar morphological appearance, but studies revealed that CRINET exhibits similar molecular features with ATRT-TYR subgroup, but with different histological features and favorable overall survival of 10 years.^[59] Methylation profiling for CRINET can be helpful to differentiate with choroid plexus carcinoma. The methylation distinction between CRINET and ATRT-TYR subgroup can be now identified by the methylation-based brain classifier (v12.5).

9. Meningiomas are combined into a single type in WHO CNS5 with 15 morphological subtypes. In the new edition, meningiomas with focal papillary and rhabdoid morphological appearance alone without other high-grade indications (PBRM1, BAP1 gene mutations, or deletions) are not sufficient for WHO grade 2 or 3. TERT promoter mutation or homozygous CDKN2A/B deletion is regarded as new independent molecular parameters sufficient for WHO grade 3 meningioma, despite its morphological grading or subtypes.^[9]

The new classification of WHO CNS5 will no doubt group more homogeneous patients both biologically and in their prognosis; the enrolment of the patient in clinical trials based on this new refined classification enables a more objective observation of the impact of an established or novel therapy to the patients. WHO CNS5 thus regarded molecular testing at least as important as histological findings, a precise diagnosis usually cannot be achieved solely based on histological features, and even the decision of high grade or low grade may still be pending on molecular results. In this situation, patients and clinicians need to wait longer for the integrated diagnosis that finally points toward the optimal therapy, as well as the necessity of clinical trial enrolment.^[11]

Interaction between Clinician and Neuropathologist

The update of WHO CNS5 requires fast adaption of neuropathologists to the new classification, for cases that may have been solved solely by histology, it may now be necessary to integrate molecular testing for a definitive diagnosis.^[9] The integration of the diagnosis may be challenging when molecular features conflict with variable morphological changes or clinical behaviors. Thus, it is even harder and more time-consuming for clinicians to well understand the impact of a refined diagnosis on the patients suggested by WHO CNS5. Before this new edition of classification, the experience of treatment previously based on an uncertain or misclassified diagnosis may establish a false impression upon treatment selection or prognosis prediction. Likewise, adult-type diffuse gliomas account for more than 70% and may be the most common tumor type that occurs in neurosurgeons.^[60] An impression exists that the molecular feature, IDH-mutation, for adult glioma is a good prognostic marker, and IDH-wildtype adult glioma tends to progress and indicate bad prognosis, no matter what descriptive diagnosis has been made. On the one hand, most of the provisional tumor diagnoses "IDH-wildtype astrocytoma" or descriptive diagnoses like "IDH-wildtype diffuse astrocytic glioma" are actually molecular-defined glioblastomas.^[19] Even though the original pathological grading may be 2 or 3, the patient still yielded rapid progression, recurrence or bad prognosis. In this situation, when inconclusive diagnosis or astrocytic IDH-wildtype diffuse gliomas occurred, clinicians will tend to consider radical treatment identical to glioblastomas, followed by a tendency for bad prognosis prediction due to consideration of IDH-wildtype status. However, the entity of "IDH-wildtype astrocytoma" now has been dislodged by WHO CNS5.^[10] An inconclusive diagnosis of IDH-wildtype tumors occurring in adults should now be suggested by neuropathologists for molecular testing, aiming at a definitive diagnosis before the treatment that is solely based on the experience of previously already relevant findings (IDH mutation and MGMT promoter methylation). Especially in younger patients, diagnosis may be those of pediatric-type low-grade gliomas^[20] that may not be necessary for radiotherapy or chemotherapy after total resection, or circumscribed astrocytic gliomas that behave anaplastic morphological changes, which potentially may be suitable for MEK inhibitors.^[11]

For unresolved lesions or NOS/NEC diagnosis, a tumor board or a multidisciplinary treatment is suggested. Neuropathologists may usually receive limited materials that are insufficient to represent its true growth patterns, or some of the circumscribed tumors can also locally show an inconclusive diffusely growing pattern to adjacent parenchyma in the evaluated tissue, but generally grow circumscribed radiographically (PXA and pilocytic astrocytomas). In this situation, radiologists may provide valuable or even critical information for differential diagnosis, for example, the differential diagnosis between glioblastoma and PXA, when other methods (including methylation) were not conclusive.

New diagnostic approaches, NGS, and methylation profiling will inevitably increase the cost of the diagnosis. Based on the facts that only limited treatment options are provided for most CNS tumors, or the subclassification and grading may not influence the actual therapy, the utility and necessity of new technologies were often discussed and questioned by clinicians,^[11] neurosurgeons, or patients. In this situation, accurate information on the value of a refined and certain diagnosis should be delivered by neuropathologists; usually, on a tumor board or a multidisciplinary treatment, certain parameters should be evaluated, likewise, the chance of tumor type or even grading that could be corrected by molecular results, especially for those IDH-wildtype younger or pediatric patients. The payors for such testing differ from county to country. If it is not included in the health insurance and has to be paid by the patient themselves, advice for these testing may be given to the patients after the discussion between pathologists and clinicians.

Limitations

The amount of literature does not allow a comprehensive review of all underlying studies resulting in the update of the classification within this piece. Thus, some aspects have been more focused and do not represent a completely unbiased reproduction of their relevance in the literature.

Conclusions

WHO CNS5 emphasized the advanced progress and the role of molecular diagnostics in CNS tumor classification. The adoption of new tumor nomenclature, grading system, terminology, tumor types and novel diagnostic approach, methylation profiling, will benefit the precise diagnosis of CNS tumors, facilitate optimal patient care and improve diagnostic reproducibility with closer clinical relevance. Neuropathologists are encouraged to interact more with clinicians, concerning certain changes and challenges brought by WHO CNS5, aiming at better clinical management for the patients.

Acknowledgments

Nil.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

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