Clinical, pathological, and genomic features of EWSR1-PATZ1 fusion sarcoma

A case of pelvic EWSR1-PATZ1 fusion sarcoma treated with carbon ion radiotherapy

EWSR1-PATZ1 fusion sarcoma is a type of round-cell sarcoma with EWSR1-non-EST fusion that was newly categorized in the 2020 World Health Organization classification of soft tissue and bone tumors. In general, local disease is managed via surgical resection; however, at present, there is no standard therapy for locally advanced or metastatic disease. Here, we report our experience with a middle-aged male patient with pelvic EWSR1-PATZ1 fusion sarcoma who was treated with carbon ion radiotherapy and maintained stable disease for 13 months. The patient's clinical course suggests that carbon ion radiotherapy may be effective in patients with locally advanced EWSR1-PATZ1 fusion sarcoma.

Updates on WHO classification for small round cell tumors: Ewing sarcoma vs. everything else

The WHO Classification of Soft Tissue and Bone Tumours currently recognizes four categories of undifferentiated small round cell sarcoma: Ewing sarcoma, round cell sarcoma with EWSR1-non-ETS fusions including NFATc2 and PATZ1, CIC-rearranged sarcoma, and sarcoma with BCOR genetic alterations. These neoplasms frequently pose significant diagnostic challenges due to rarity and overlapping morphologic and immunohistochemical findings. Further, molecular testing, with accompanying pitfalls, may be needed to establish a definitive diagnosis. This review summarizes the clinical, histologic, immunohistochemical, and molecular features of these neoplasms. In addition, differential diagnosis and areas of uncertainty and ongoing investigation are discussed.

Sarcomas with EWSR1::Non-ETS Fusion (EWSR1::NFATC2 and EWSR1::PATZ1)

The wide application of increasingly advanced molecular studies in routine clinical practice has allowed a detailed, albeit still incomplete, genetic subclassification of undifferentiated round cell sarcomas. The WHO classification continues to include provisional molecular entities, whose clinicopathologic features are in the early stages of evolution. This review focuses on the clinicopathologic, molecular, and prognostic features of undifferentiated round cell sarcomas with EWSR1/FUS::NFATC2 or EWSR1::PATZ1 fusions. Classic histopathologic findings, uncommon variations, and diagnostic pitfalls are addressed, along with the utility of recently developed immunohistochemical and molecular markers.

Prognostic and predictive biomarkers in paediatric solid tumours

Characterisation of histological, immunohistochemical and molecular prognostic and predictive biomarkers has contributed significantly to precision medicine and better outcomes in the management of paediatric solid tumours. Prognostic biomarkers allow predictions to be made regarding a tumour's aggressiveness and clinical course, whereas predictive biomarkers help determine responses to a specific treatment. This review summarises prognostic biomarkers currently used in the more common paediatric solid tumours, with a brief commentary on the most relevant less common predictive biomarkers. MYCN amplification is the most important genetic alteration in neuroblastoma prognosis, and the histological classification devised by Shimada in 1999 is still used in routine diagnosis. Moreover, a new subgrouping of unfavourable histology neuroblastoma enables immunohistochemical characterisation of tumours with markedly different genetic features and prognosis. The predominant histology and commonly observed cytogenetic abnormalities are recognised outcome predictors in Wilms tumour. Evaluation for anaplasia, which is tightly associated with TP53 gene mutations and poor outcomes, is central in both the International Society of Paediatric Oncology and the Children's Oncology Group approaches to disease classification. Characterisation of distinct genotype-phenotype subclasses and critical mutations has expanded overall understanding of hepatoblastoma outcomes. The C1 subclass hepatoblastoma and CTNNB1 mutations are associated with good prognosis. In contrast, the C2 subclass, NFE2L2 mutations, TERT promoter mutations and high expression of oncofetal proteins and stem cell markers are associated with poor outcomes. Risk stratification in sarcomas is highly variable depending on the entity. The prognosis of rhabdomyosarcoma, for example, primarily depends on histological and molecular characteristics. Advances in our understanding of clinically significant biomarkers will translate into more precise diagnoses, improved risk stratification and more effective and less toxic treatment in this challenging group of patients.

[Neuroepithelial tumor with PATZ1 fusion - case report and focus on an ill-defined entity]

The neuroepithelial tumor with PATZ1 fusion is a recently described tumor type, at the border between central nervous system and mesenchymal tumors. The histopathological diagnosis of this neoplasm, not recognized by the 2021 WHO classification, is challenging due to its varied and non-specific morphologic features. Most cases are densely cellular with monomorphous nuclei. Perivascular pseudo-rosettes of the ependymal type and astroblastic features are frequent. Blood vessels may be hyalinized. The tumor may display low- or high-grade features. OLIG2 and GFAP are variably expressed. Guided by DNA methylation profiling, a pathologist aware of this tumor type will search for a fusion involving PATZ1 and EWSR1 or MN1. The physiopathology of neuroepithelial tumor with PATZ1 fusion is not fully understood. The prognosis appears to align with that of intermediate-grade tumors but follow-up data are scarce. The therapeutic management is often similar to that of high-grade neoplasms. Nonetheless, PATZ1 fusion is a potential therapeutic avenue that may lead to personalized and less aggressive treatments.

PATZ1-Rearranged Tumors of the Central Nervous System: Characterization of a Pediatric Series of Seven Cases

PATZ1-rearranged sarcomas are well-recognized tumors as part of the family of round cell sarcoma with EWSR1-non-ETS fusions. Whether PATZ1-rearranged central nervous system (CNS) tumors are a distinct tumor type is debatable. We thoroughly characterized a pediatric series of PATZ1-rearranged CNS tumors by chromosome microarray analysis (CMA), DNA methylation analysis, gene expression profiling and, when frozen tissue is available, optical genome mapping (OGM). The series consisted of 7 cases (M:F=1.3:1, 1-17 years, median 12). On MRI, the tumors were supratentorial in close relation to the lateral ventricles (intraventricular or iuxtaventricular), preferentially located in the occipital lobe. Two major histologic groups were identified: one (4 cases) with an overall glial appearance, indicated as "neuroepithelial" (NET) by analogy with the corresponding methylation class (MC); the other (3 cases) with a predominant spindle cell sarcoma morphology, indicated as "sarcomatous" (SM). A single distinct methylation cluster encompassing both groups was identified by multidimensional scaling analysis. Despite the epigenetic homogeneity, unsupervised clustering analysis of gene expression profiles revealed 2 distinct transcriptional subgroups correlating with the histologic phenotypes. Interestingly, genes implicated in epithelial-mesenchymal transition and extracellular matrix composition were enriched in the subgroup associated to the SM phenotype. The combined use of CMA and OGM enabled the identification of chromosome 22 chromothripsis in all cases suitable for the analyses, explaining the physical association of PATZ1 to EWSR1 or MN1. Six patients are currently disease-free (median follow-up 30 months, range 12-92). One patient of the SM group developed spinal metastases at 26 months from diagnosis and is currently receiving multimodal therapy (42 months). Our data suggest that PATZ1-CNS tumors are defined by chromosome 22 chromothripsis as causative of PATZ1 fusion, show peculiar MRI features (eg, relation to lateral ventricles, supratentorial frequently posterior site), and, although epigenetically homogenous, encompass 2 distinct histologic and transcriptional subgroups.

An Exome Capture-Based RNA-Sequencing Assay for Genome-Wide Identification and Prioritization of Clinically Important Fusions in Pediatric Tumors

This study reports the development of an exome capture-based RNA-sequencing assay to detect recurring and novel fusions in hematologic, solid, and central nervous system tumors. The assay used Twist Comprehensive Exome capture with either fresh or formalin-fixed samples and a bioinformatic platform that provides fusion detection, prioritization, and downstream curation. A minimum of 50 million uniquely mapped reads, a consensus read alignment/fusion calling approach using four callers (Arriba, FusionCatcher, STAR-Fusion, and Dragen), and custom software were used to integrate, annotate, and rank the candidate fusion calls. In an evaluation of 50 samples, the number of calls varied substantially by caller, from a mean of 24.8 with STAR-Fusion to 259.6 with FusionCatcher; only 1.1% of calls were made by all four callers. Therefore a filtering and ranking algorithm was developed based on multiple criteria, including number of supporting reads, calling consensus, genes involved, and cross-reference against databases of known cancer-associated or likely false-positive fusions. This approach was highly effective in pinpointing known clinically relevant fusions, ranking them first in 47 of 50 samples (94%). Detection of pathogenic gene fusions in three diagnostically challenging cases highlights the importance of a genome-wide and nontargeted method for fusion detection in pediatric cancer.

Sarcomas Harboring EWSR1::PATZ1 Fusions: A Clinicopathologic Study of 17 Cases

Soft tissue sarcomas harboring EWSR1::PATZ1 are a recently recognized entity with variable morphology and a heterogeneous immunohistochemical profile. We studied 17 such tumors. The tumors occurred in 12 men and 5 women (median age, 50 years; range, 15-71 years), involved the thoracoabdominal soft tissues (14 cases; 82%), lower extremities (2 cases; 12%), and tongue (1 case; 6%), and ranged from 0.7 to 11.3 cm (median, 4.7 cm). All but 1 patient received complete surgical resection; 7 were also treated with neoadjuvant chemo/radiotherapy. All cases showed typical features of EWSR1::PATZ1 sarcoma, including uniform round to spindled cells, fibromyxoid matrix, fibrous bands, hyalinized vessels, and pseudoalveolar/microcystic spaces. Unusual features, seen in a subset of cases, included degenerative-appearing nuclear atypia, epithelioid cytomorphology, mature fat, abundant rhabdomyoblasts, high mitotic activity, and foci with increased cellularity and nuclear atypia. Positive immunohistochemical results were desmin (16/17, 94%), MyoD1 (13/14, 93%), myogenin (6/14, 43%), GFAP (10/10, 100%), S100 protein (15/17, 88%), SOX10 (7/13, 54%), keratin (10/17, 59%), CD99 (4/11, 36%), H3K27me3 (retained expression 9/9, 100%), p16 (absent expression 1/4, 25%), and p53 (wild type 3/3, 100%). Fusion events included EWSR1 exon 8::PATZ1 exon 1 (14/17, 82%), EWSR1 exon 9::PATZ1 exon 1 (2/17, 12%), and EWSR1 exon 7::PATZ1 exon 1 (1/17, 6%). No evaluated tumor had alterations of CDKN2A/B and/or TP53, or MDM2 amplification. Clinical follow-up (16 patients: median, 13.5 months; range, 1-77 months) showed distant metastases in 3 patients (1/3 at time of presentation) and no local recurrences. At the time of last follow-up, 14 patients were disease free, 1 was alive with disease, 1 was dead of disease (at 13 months), and 1 had an indeterminant pulmonary nodule. We conclude that the morphologic spectrum of EWSR1::PATZ1 is broader than has been previously appreciated. Although more long-term follow-up is needed, the prognosis of these very rare sarcomas may be more favorable than previously reported.

Gene partners of the EWSR1 fusion may represent molecularly distinct entities

EWSR1 fusions are highly promiscuous and are associated with unique malignancies, clinical phenotypes, and molecular subtypes. However, rare fusion partners (RFP) of EWSR1 has not been well described. Here, we conducted a cross-sectional, retrospective study of 1,140 unique tumors harboring EWSR1 fusions. We identified 64 unique fusion partners. RFPs were identified more often in adults than children. Alterations in cell cycle control and DNA damage response genes as driving the differences between fusion partners. Potentially clinically actionable genomic variants were more prevalent in tumors harboring RFP than common fusions. While the data presented here is limited, tumors harboring RFP of EWSR1 may represent molecularly distinct entities and may benefit from further molecular testing to identify targeted therapeutic options.

Emerging mesenchymal tumour types and biases in the era of ubiquitous sequencing

New tumour types are being described at increasing frequency, and most new tumour types are now identified via retrospective review of next-generation sequencing data. This contrasts with the traditional, morphology-based method of identifying new tumour types, and while the sequencing-based approach has accelerated progress in the field, it has also introduced novel and under-recognised biases. Here, we discuss tumour types identified based on morphology, including superficial CD34-positive fibroblastic tumour, pseudoendocrine sarcoma and cutaneous clear cell tumour with melanocytic differentiation and ACTIN::MITF fusion. We also describe tumour types identified primarily by next-generation sequencing, including epithelioid and spindle cell rhabdomyosarcoma, round cell neoplasms with EWSR1::PATZ1 fusion, cutaneous melanocytic tumour with CRTC1::TRIM11 fusion, clear cell tumour with melanocytic differentiation and MITF::CREM fusion and GLI1-altered mesenchymal neoplasms, including nested glomoid neoplasm.

Neuroepithelial tumor with EWSR1::PATZ1 fusion: A literature review

We present the clinicopathological and molecular genetic characteristics of a neuroepithelial tumor (NET), EWSR1::PATZ1 fusion-positive with a literature review. This fusion has recently been discovered in rare central nervous system tumors and soft tissue sarcomas and was not included in the fifth edition of the WHO classifications. We identified this fusion in 2 NETs. The first case involved a 7-year-old girl and the second case occurred in a 53-year-old man; both presented with headaches and vomiting. The pediatric case initially showed an intermediate grade of the tumor, but upon recurrences, it transformed into a high-grade tumor with 2 relapses in 8.3 years. This case exhibited high mitotic activity (20/10 high-power fields), and a high Ki-67 index (21%). The TERT promoter (TERTp) mutation was present in both initial and recurrent tumors. In contrast, the adult case was a low-grade tumor with no mitotic activity or recurrence over 13.5 months after subtotal resection and gamma knife surgery. Interestingly, the pediatric case demonstrated a longer survival time compared to conventional glioblastoma. The TERTp mutation, similar to being a molecular signature in adult-type glioblastoma, could also be an indicator of high-grade behavior in PATZ1 fusion NET.

The Recent Advances in Molecular Diagnosis of Soft Tissue Tumors

Soft tissue tumors are rare mesenchymal tumors with divergent differentiation. The diagnosis of soft tissue tumors is challenging for pathologists owing to the diversity of tumor types and histological overlap among the tumor entities. Present-day understanding of the molecular pathogenesis of soft tissue tumors has rapidly increased with the development of molecular genetic techniques (e.g., next-generation sequencing). Additionally, immunohistochemical markers that serve as surrogate markers for recurrent translocations in soft tissue tumors have been developed. This review aims to provide an update on recently described molecular findings and relevant novel immunohistochemical markers in selected soft tissue tumors.

Top 10 Histological Mimics of Neuroendocrine Carcinoma You Should Not Miss in the Head and Neck

BACKGROUND

The spectrum of neuroendocrine neoplasia (NEN) of the head and neck region is wide-ranging and diverse, including a variety of diagnoses stretching from benign and low-malignant tumor forms to highly proliferative, poor prognosis neuroendocrine carcinoma (NEC). Moreover, there are several non-neuroendocrine differential diagnoses to keep in mind as well, displaying various degree of morphological and/or immunohistochemical overlap with bona fide neuroendocrine lesions.

METHODS

Review.

RESULTS

While the growth patterns may vary, well-differentiated NEN usually display a stippled "salt and pepper" chromatin, a granular cytoplasm, and unequivocal expression of neuroendocrine markers such as chromogranin A and synaptophysin. However, these features are often less pronounced in NEC, which may cause diagnostic confusion-not the least since several non-NEC head and neck tumors may exhibit morphological similarities and focal neuroendocrine differentiation.

CONCLUSION

As patients with NEC may require specific adjuvant treatment and follow-up, knowledge regarding differential diagnoses and potential pitfalls is therefore clinically relevant. In this review, the top ten morphological and/or immunohistochemical mimics of NEC are detailed in terms of histology, immunohistochemistry, and molecular genetics.

Case report: A unique presentation of a high-grade neuroepithelial tumor with EWSR1::PATZ1 fusion with diagnostic, molecular, and therapeutic insights

Background

EWSR1::PATZ1 fusion tumors are exceedingly rare in the central nervous system with only 14 prior cases documented. PATZ1 fusion neuroepithelial tumors are beginning to be recognized as a distinct molecular class of neoplasms that most often occur in children and young adults. These tumors are polyphenotypic, show diverse morphologic features, may be low- or high-grade, and tend to have an intermediate prognosis.

Case presentation

Herein, we present an unusual case of a high-grade neuroepithelial tumor in a young man with an EWSR1::PATZ1 fusion. This case is unique because the tumor appears to have undergone high-grade transformation from a persistent low-grade glioma, which has yet to be reported. Furthermore, this case is the first to document concurrent RB1 loss, SMAD4 loss, and TP53 inactivation in this tumor type, which correlates with high-grade transformation. Fortunately, this patient is alive 2.5 years after treatment and 18.5 years after initial presentation, which provides a unique window into how these tumors clinically behave over a long follow-up period. Finally, we discuss the altered molecular pathways that are a result of the EWSR1::PATZ1 fusion and discuss potential therapeutic targets.

Conclusion

Awareness of the emerging entity of PATZ1 fusion neuroepithelial tumors is important not only for accurate diagnostic and prognostic purposes but also for predicting response to therapy.

Ewing and Ewing-like sarcomas: A morphological guide through genetically-defined entities

The fifth edition of the World Health Organization classification of soft tissue and bone tumors redefined Ewing sarcoma by fusions between EWSR1/FUS and ETS family of transcription factors, and recognized three tumor groups among Ewing-like sarcoma: CIC-rearranged sarcoma, sarcoma with BCOR genetic alterations, and round cell sarcoma with EWSR1::non-ETS fusions. Although this classification underscores the critical role of molecular genetics in the diagnosis of small round cell sarcoma, each entry is recognized as a specific entity not only because they have different genetics but because their phenotypes are distinct and reasonably robust to support the diagnosis. This review focuses on the morphological aspects of Ewing sarcoma and a subset of Ewing-like sarcomas (CIC-rearranged sarcoma, BCOR-associated sarcoma, and EWSR1::NFATC2 sarcoma) for which phenotypic characteristics have been well established. Classic histological findings, uncommon variations, and recurrent diagnostic pitfalls are addressed, along with the utility of recently developed immunohistochemical markers (NKX2.2, PAX7, ETV4, BCOR, CCNB3, and NKX3.1). Phenotypic expertise would significantly expedite the diagnostic process and complement (or sometimes outperform) genetic testing, even in well-resourced settings. Morphological knowledge plays an even more substantial role in facilities that do not have easy access to molecular testing.

Small round cell sarcomas

Undifferentiated small round cell sarcomas (SRCSs) of bone and soft tissue comprise a heterogeneous group of highly aggressive tumours associated with a poor prognosis, especially in metastatic disease. SRCS entities mainly occur in the third decade of life and can exhibit striking disparities regarding preferentially affected sex and tumour localization. SRCSs comprise new entities defined by specific genetic abnormalities, namely EWSR1-non-ETS fusions, CIC-rearrangements or BCOR genetic alterations, as well as EWSR1-ETS fusions in the prototypic SRCS Ewing sarcoma. These gene fusions mainly encode aberrant oncogenic transcription factors that massively rewire the transcriptome and epigenome of the as yet unknown cell or cells of origin. Additional mutations or copy number variants are rare at diagnosis and, depending on the tumour entity, may involve TP53, CDKN2A and others. Histologically, these lesions consist of small round cells expressing variable levels of CD99 and specific marker proteins, including cyclin B3, ETV4, WT1, NKX3-1 and aggrecan, depending on the entity. Besides locoregional treatment that should follow standard protocols for sarcoma management, (neo)adjuvant treatment is as yet ill-defined but generally follows that of Ewing sarcoma and is associated with adverse effects that might compromise quality of life. Emerging studies on the molecular mechanisms of SRCSs and the development of genetically engineered animal models hold promise for improvements in early detection, disease monitoring, treatment-related toxicity, overall survival and quality of life.

An international working group consensus report for the prioritization of molecular biomarkers for Ewing sarcoma

The advent of dose intensified interval compressed therapy has improved event-free survival for patients with localized Ewing sarcoma (EwS) to 78% at 5 years. However, nearly a quarter of patients with localized tumors and 60–80% of patients with metastatic tumors suffer relapse and die of disease. In addition, those who survive are often left with debilitating late effects. Clinical features aside from stage have proven inadequate to meaningfully classify patients for risk-stratified therapy. Therefore, there is a critical need to develop approaches to risk stratify patients with EwS based on molecular features. Over the past decade, new technology has enabled the study of multiple molecular biomarkers in EwS. Preliminary evidence requiring validation supports copy number changes, and loss of function mutations in tumor suppressor genes as biomarkers of outcome in EwS. Initial studies of circulating tumor DNA demonstrated that diagnostic ctDNA burden and ctDNA clearance during induction are also associated with outcome. In addition, fusion partner should be a pre-requisite for enrollment on EwS clinical trials, and the fusion type and structure require further study to determine prognostic impact. These emerging biomarkers represent a new horizon in our understanding of disease risk and will enable future efforts to develop risk-adapted treatment.

SELNET clinical practice guidelines for bone sarcoma

Bone sarcoma are infrequent diseases, representing < 0.2% of all adult neoplasms. A multidisciplinary management within reference centers for sarcoma, with discussion of the diagnostic and therapeutic strategies within an expert multidisciplinary tumour board, is essential for these patients, given its heterogeneity and low frequency. This approach leads to an improvement in patient's outcome, as demonstrated in several studies. The Sarcoma European Latin-American Network (SELNET), aims to improve clinical outcome in sarcoma care, with a special focus in Latin-American countries. These Clinical Practice Guidelines (CPG) have been developed and agreed by a multidisciplinary expert group (including medical and radiation oncologist, surgical oncologist, orthopaedic surgeons, radiologist, pathologist, molecular biologist and representatives of patients advocacy groups) of the SELNET consortium, and are conceived to provide the standard approach to diagnosis, treatment and follow-up of bone sarcoma patients in the Latin-American context.

Clinical Utility of CDK4/6 Inhibitors in Sarcoma: Successes and Future Challenges

PURPOSE

Soft tissue and bone sarcomas are rare malignancies that exhibit significant pathologic and molecular heterogeneity. Deregulation of the CDKN2A-CCND-CDK4/6-retinoblastoma 1 (Rb) pathway is frequently observed in about 25% of unselected sarcomas and is pathognomonic for specific sarcoma subtypes. This genomic specificity has fueled the clinical evaluation of selective CDK4/6 inhibitors in sarcomas. Here, we highlight successes, opportunities, and future challenges for using CDK4/6 inhibitors to treat sarcoma.

MATERIALS AND METHODS

This review summarizes the current evidence for the use of CDK4/6 inhibitors in sarcoma while identifying molecular rationale and predictive biomarkers that provide the foundation for targeting the CDK4/6 pathway in sarcoma. A systematic review was performed of articles indexed in the PubMed database and the National Institutes of Health Clinical Trials Registry (ClinicalTrials.gov). For each sarcoma subtype, we discuss the preclinical rationale, case reports, and available clinical trials data.

RESULTS

Despite promising clinical outcomes in a subset of sarcomas, resistance to CDK4/6 inhibitors results in highly heterogeneous clinical outcomes. Current clinical data support the use of CDK4/6 inhibitors in subsets of sarcoma primarily driven by CDK4/6 deregulation. When dysregulation of the Rb pathway is a secondary driver of sarcoma, combination therapy with CDK4/6 inhibition may be an option. Developing strategies to identify responders and the mechanisms that drive resistance is important to maximize the clinical utility of these drugs in patients with sarcoma. Potential biomarkers that indicate CDK4/6 inhibitor sensitivity in sarcoma include CDK4, CCND, CCNE, RB1, E2F1, and CDKN2A.

CONCLUSION

CDK4/6 inhibitors represent a major breakthrough for targeted cancer treatment. CDK4/6 inhibitor use in sarcoma has led to limited, but significant, early clinical success. Targeted future clinical research will be key to unlocking the potential of CDK4/6 inhibition in sarcoma.

Response to CDK4/6-inhibitors in sarcomas indicate the need for predictive biomarkers specific to this disease.

A novel LARGE1-AFF2 fusion expanding the molecular alterations associated with the methylation class of neuroepithelial tumors with PATZ1 fusions

A novel DNA methylation class of tumor within the central nervous system, the "neuroepithelial tumor (NET), PATZ1 fusion-positive" has recently been identified in the literature, characterized by EWSR1- and MN1-PATZ1 fusions. The cellular origin of this tumor type remains unknown, wavering between glioneuronal or mesenchymal (as round cell sarcomas with EWSR1-PATZ1 of the soft tissue). Because of the low number of reported cases, this tumor type will not be added to the 2021 World Health Organization Classification of Tumors of the Central Nervous System (CNS). Herein, we report one case of a CNS tumor classified by DNA methylation analysis as NET-PATZ1 but harboring a novel LARGE1-AFF2 fusion which has until now never been described in soft tissue or the CNS. We compare its clinical, histopathological, immunophenotypical, and genetic features with those previously described in NET-PATZ1. Interestingly, the current case presented histopathological (astroblastoma-like features, glioneuronal phenotype), clinical (with a favorable course), genetic (1p loss), and epigenetic (DNA-methylation profiling) similarities to previously reported cases of NET-PATZ1. Our results added data suggesting that different histomolecular tumor subtypes seem to be included within the methylation class "NET, PATZ1 fusion-positive", including non PATZ1 fusions, and that further cases are needed to better characterize them.

[Rare kidney tumor - thyroid-like follicular carcinoma]

The literature review provides an analysis of a rare malignant tumor of the kidney: thyroid-like follicular carcinoma of the kidney (TLFCK). In morphology, this tumor is extremely similar to thyroid follicular carcinoma, but the immunophenotype of tumor cells is different. TLFCK has an indolent clinical course, rarely metastasizes, and even the development of metastases does not mean an unfavorable prognosis for the patient. The literature review presents the features of the clinical course of the disease, macroscopic, microscopic, immunohistochemical characteristics of the tumor and typical cytogenetic breakdowns. Particular attention is paid to the issues of differential diagnosis of the tumor with other pathological processes that may microscopically resemble TLFCK.

Advances in the classification of round cell sarcomas

Round cell sarcomas represent a diagnostic challenge for pathologists, owing to the poorly differentiated features of these high-grade tumours. The diagnosis of round cell sarcoma requires large immunohistochemical panels and molecular testing in many cases. This spectrum of malignancies is largely dominated by Ewing sarcomas (ESs), which represent the most common family of these tumours. Nonetheless, new families have been delineated in the past few years, with the addition of two additional families in the 2020 World Health Organization classification of bone and soft tissue tumours, namely sarcomas with CIC rearrangements and sarcomas with BCOR alterations. EWSR1, one of the genes involved in the driver fusion of ESs, is also implicated in the translocation of many other tumours with heterogeneous lineages and variable levels of aggressiveness. Round cell sarcomas associated with fusions inwhichEWSR1is partnered with genes encoding transcription factors distinct from those of the 'Ewing family' represent a heterogeneous group of rare tumours that require further study to determine whether their fusions may or not define a specific subgroup. They include mainly sarcomas with NFATc2 rearrangements and sarcomas with PATZ1 rearrangements. At this point, PATZ1 fusions seem to be associated with tumours of high clinical and morphological heterogeneity. Molecular studies have also helped in the identification of more consistent biomarkers that give tremendous help to pathologists in triaging, if not diagnosing, these tumours in practice. This review compiles the latest accumulated evidence regarding round cell sarcomas, and discusses the areas that are still under investigation.

PATZ1 fusions define a novel molecularly distinct neuroepithelial tumor entity with a broad histological spectrum

Large-scale molecular profiling studies in recent years have shown that central nervous system (CNS) tumors display a much greater heterogeneity in terms of molecularly distinct entities, cellular origins and genetic drivers than anticipated from histological assessment. DNA methylation profiling has emerged as a useful tool for robust tumor classification, providing new insights into these heterogeneous molecular classes. This is particularly true for rare CNS tumors with a broad morphological spectrum, which are not possible to assign as separate entities based on histological similarity alone. Here, we describe a molecularly distinct subset of predominantly pediatric CNS neoplasms (n = 60) that harbor PATZ1 fusions. The original histological diagnoses of these tumors covered a wide spectrum of tumor types and malignancy grades. While the single most common diagnosis was glioblastoma (GBM), clinical data of the PATZ1-fused tumors showed a better prognosis than typical GBM, despite frequent relapses. RNA sequencing revealed recurrent MN1:PATZ1 or EWSR1:PATZ1 fusions related to (often extensive) copy number variations on chromosome 22, where PATZ1 and the two fusion partners are located. These fusions have individually been reported in a number of glial/glioneuronal tumors, as well as extracranial sarcomas. We show here that they are more common than previously acknowledged, and together define a biologically distinct CNS tumor type with high expression of neural development markers such as PAX2, GATA2 and IGF2. Drug screening performed on the MN1:PATZ1 fusion-bearing KS-1 brain tumor cell line revealed preliminary candidates for further study. In summary, PATZ1 fusions define a molecular class of histologically polyphenotypic neuroepithelial tumors, which show an intermediate prognosis under current treatment regimens.

From the ashes of "Ewing-like" sarcoma: A contemporary update of the classification, immunohistochemistry, and molecular genetics of round cell sarcomas

Round cell sarcomas include a diverse group of bone and soft tissue tumors, which comprise well-defined entities as well as several nascent categories presented in the 2020 World Health Organization classification. The morphologic overlap yet disparate nosology, prognostic implications, and management strategies places a high value on ancillary testing, including a strategic immunohistochemical approach and directed confirmation by cytogenetic and molecular genetic methods. We review the diagnostic categories that have emerged from the former wastebasket "undifferentiated round cell sarcoma" ("Ewing-like" sarcomas), with an emphasis on algorithmic exclusion of nonsarcomatous entities, diagnostic stratification of well-defined entities (Ewing sarcoma, rhabdomyosarcomas, poorly differentiated synovial sarcoma), and a discussion of the new categories with novel genetic alterations (CIC-rearranged sarcomas, sarcomas with BCOR genetic alterations, and round cell sarcomas with EWSR1-non-ETS fusions).

EWSR1-PATZ1 fusion renal cell carcinoma: a recurrent gene fusion characterizing thyroid-like follicular renal cell carcinoma

Thyroid-like follicular renal cell carcinoma is an uncommon kidney tumor with no distinct molecular alteration described to date. This cohort of eight women with mean and median ages of 45 and 46 years, respectively (range 19-65 years), had unencapsulated, well-circumscribed tumors composed of tightly packed anastomosing follicle-like cysts filled with eosinophilic colloid-like material and lined by cuboidal cells with high nuclear to cytoplasmic ratios, oval to elongated nuclei with perpendicular arrangement toward the lumens, and prominent nuclear overlapping. The stroma between these was minimal with the exception of two tumors. Calcifications and necrosis were absent. Immunohistochemically, the tumors were positive for KRT19 (7/7), PAX8 (5/5), cyclin D1 (6/6), KRT7 (5/7), and AMACR (1/5; focal, weak), and were negative for WT1, TTF1 (transcription termination factor-1), and thyroglobulin. In three of three tumors tested molecularly, EWSR1-PATZ1 fusion was identified by RNA sequencing and confirmed by RT-PCR and Sanger sequencing. Over a follow-up period of 1-7 years, no evidence of recurrence or metastasis has been detected. The EWSR1-PATZ1 fusion has been recognized as a recurrent alteration in a subset of round to spindle cell sarcomas with EWSR1-non-ETS fusions (EWSR1-PATZ1 sarcoma) and in several central nervous system tumors. The finding of an EWSR1-PATZ1 fusion in all three of the thyroid-like follicular renal cell carcinomas for which sufficient tissue was available for genomic profiling provides the first distinct molecular abnormality in thyroid-like follicular renal cell carcinomas, supporting its designation as a distinct diagnostic entity.

Undifferentiated sarcoma of bone with a round to epithelioid cell phenotype harboring a novel EWSR1-SSX2 fusion identified by RNA-based next-generation sequencing

Due to the increased application of RNA-based next-generation sequencing techniques on bone and soft tissue round cell sarcomas new fusions are frequently found, thereby expanding the molecular landscape of these tumors. In this report, we describe and discuss the finding of an undifferentiated sarcoma of the bone with a round to epithelioid cell phenotype harboring a novel EWSR1-SSX2 fusion. Treatment of this new bone tumor entity according to the Euro Ewing 2012 protocol led to complete pathologic response.

Cytology of Undifferentiated Round-Cell Sarcomas of Bone and Soft Tissue: Ewing Sarcoma or Not Ewing Sarcoma , That Is the Question

BACKGROUND

Undifferentiated round-cell sarcomas (URCSs) of soft tissue and bone are a group of clinically heterogeneous tumors. Diagnosis of these malignancies is based mainly on recurrent genetic alterations. The most common and the best known representative of this group is Ewing sarcoma (ES) which is characterized by gene fusions including EWSR1 or FUS and ETS transcription factors family. Other newly described entities are CIC-rearranged sarcoma, sarcoma with BCOR genetic alterations, and round-cell sarcoma with EWSR1-non-ETS fusions. All these novel tumors are known as Ewing-like sarcomas.

SUMMARY

It is believed that morphologic features of ES and Ewing-like sarcomas vary only slightly or even that cytomorphology is not relevant. But differences are usually obvious, and some cytologic findings, such as spindle cells, connective tissue fragments, or myxoid stroma, are typical for Ewing-like sarcomas but not for ES. Each of these entities is also characterized by different immunoprofiles. The aim of this review was to summarize cytomorphologic and immunohistochemical features of URCS and compare them with other small round-cell tumors. Key Messages: Cytology can be successfully used in URCS diagnosis as a complementary tool for core-needle biopsy or even alone in selected cases, especially in recurrent and metastatic tumors. Knowing the morphologic and immunohistochemical differences between URCS is essential to provide appropriate ancillary studies and make a definitive diagnosis.

Mediastinal neuroblastoma, ganglioneuroblastoma, and ganglioneuroma: Pathology review and diagnostic approach

Neuroblastic tumors are a group of tumors of the sympathetic ganglia and adrenal medulla that derive from primordial neural crest cells. These tumors include neuroblastoma, intermixed ganglioneuroblastoma, nodular ganglioneuroblastoma, and ganglioneuroma. Neuroblastomas are the most common extracranial solid tumor arising in childhood and may occur in different anatomic sites. Neuroblastic tumors are common mesenchymal tumors of the mediastinum. Herein, we describe advances in our understanding of neuroblastic tumor biology. Pathologists should be aware of diagnostic challenges associated with these tumors to ensure correct histologic diagnosis and appropriate clinical management. We describe updated mediastinal neuroblastic tumor pathology, focusing on morphological, immunohistochemical, and molecular features and differential diagnoses.

EWSR1-The Most Common Rearranged Gene in Soft Tissue Lesions, Which Also Occurs in Different Bone Lesions: An Updated Review

EWSR1 belongs to the FET family of RNA-binding proteins including also Fused in Sarcoma (FUS), and TATA-box binding protein Associated Factor 15 (TAF15). As consequence of the multifunctional role of EWSR1 leading to a high frequency of transcription of the chromosomal region where the gene is located, EWSR1 is exposed to aberrations such as rearrangements. Consecutive binding to other genes leads to chimeric proteins inducing oncogenesis. The other TET family members are homologous. With the advent of widely used modern molecular techniques during the last decades, it has become obvious that EWSR1 is involved in the development of diverse benign and malignant tumors with mesenchymal, neuroectodermal, and epithelial/myoepithelial features. As oncogenic transformation mediated by EWSR1-fusion proteins leads to such diverse tumor types, there must be a selection on the multipotent stem cell level. In this review, we will focus on the wide variety of soft tissue and bone entities, including benign and malignant lesions, harboring EWSR1 rearrangement. Fusion gene analysis is the diagnostic gold standard in most of these tumors. We present clinicopathologic, immunohistochemical, and molecular features and discuss differential diagnoses.

Multiscale-omic assessment of EWSR1-NFATc2 fusion positive sarcomas identifies the mTOR pathway as a potential therapeutic target

Sarcomas harboring EWSR1-NFATc2 fusions have historically been categorized and treated as Ewing sarcoma. Emerging evidence suggests unique molecular characteristics and chemotherapy sensitivities in EWSR1-NFATc2 fusion positive sarcomas. Comprehensive genomic profiles of 1024 EWSR1 fusion positive sarcomas, including 14 EWSR1-NFATc2 fusions, were identified in the FoundationCore® database. Additional data from the Gene Expression Omnibus, the Genomics of Drug Sensitivity in Cancer and The Cancer Genome Atlas datasets were included for analysis. EWSR1-NFATc2 fusion positive sarcomas were genomically distinct from traditional Ewing sarcoma and demonstrated upregulation of the mTOR pathway. We also present a case of a 58-year-old male patient with metastatic EWSR1-NFATc2 fusion positive sarcoma who achieved 47 months of disease stabilization when treated with combination mTOR and VEGF inhibition. EWSR1-NFATc2 fusion positive sarcomas are molecularly distinct entities with overactive mTOR signaling; which may be therapeutically targetable. These findings support the use of precision medicine in the Ewing family of tumors.

The 2020 WHO Classification of Tumors of Bone: An Updated Review

Bone tumors are a rare and heterogeneous group of neoplasms that occur in the bone. The diversity and considerable morphologic overlap of bone tumors with other mesenchymal and nonmesenchymal bone lesions can complicate diagnosis. Accurate histologic diagnosis is crucial for appropriate management and prognostication. Since the publication of the fourth edition of the World Health Organization (WHO) classification of tumors of soft tissue and bone in 2013, significant advances have been made in our understanding of bone tumor molecular biology, classification, prognostication, and treatment. Detection of tumor-specific molecular alterations can facilitate the accurate diagnosis of histologically challenging cases. The fifth edition of the 2020 WHO classification of tumors of soft tissue and bone tumors provides an updated classification scheme and essential diagnostic criteria for bone tumors. Herein, we summarize these updates, focusing on major changes in each category of bone tumor, the newly described tumor entities and subtypes of existing tumor types, and newly described molecular and genetic data.

Ewing Sarcoma-Diagnosis, Treatment, Clinical Challenges and Future Perspectives

Ewing sarcoma, a highly aggressive bone and soft-tissue cancer, is considered a prime example of the paradigms of a translocation-positive sarcoma: a genetically rather simple disease with a specific and neomorphic-potential therapeutic target, whose oncogenic role was irrefutably defined decades ago. This is a disease that by definition has micrometastatic disease at diagnosis and a dismal prognosis for patients with macrometastatic or recurrent disease. International collaborations have defined the current standard of care in prospective studies, delivering multiple cycles of systemic therapy combined with local treatment; both are associated with significant morbidity that may result in strong psychological and physical burden for survivors. Nevertheless, the combination of non-directed chemotherapeutics and ever-evolving local modalities nowadays achieve a realistic chance of cure for the majority of patients with Ewing sarcoma. In this review, we focus on the current standard of diagnosis and treatment while attempting to answer some of the most pressing questions in clinical practice. In addition, this review provides scientific answers to clinical phenomena and occasionally defines the resulting translational studies needed to overcome the hurdle of treatment-associated morbidities and, most importantly, non-survival.

[New in the current WHO classification (2020) for soft tissue sarcomas]

The current WHO classification for tumors of soft tissue and bone includes numerous new entities, most often defined by novel molecular findings. In this article, we present translocation-positive tumors to broaden the spectrum of monomorphic mesenchymal neoplasias. The undifferentiated small round cell sarcomas are now assembled in their own separate chapter to underline their occurrence in both soft tissue and bone, emphasizing their morphologic, molecular, and biologic differences. Another interesting new group are tumors with GLI1 activation, which, however, have not yet been included into the WHO classification. NTRK-driven tumors present with a potential therapeutic target for several established inhibitors. Finally, there have been novel findings in rhabdomyosarcomas allowing more precise subtyping associated with different biological behavior.

EWSR1-PATZ1-rearranged sarcoma: a report of nine cases of spindle and round cell neoplasms with predilection for thoracoabdominal soft tissues and frequent expression of neural and skeletal muscle markers

The knowledge of clinical features and, particularly, histopathological spectrum of EWSR1-PATZ1-rearranged spindle and round cell sarcomas (EPS) remains limited. For this reason, we report the largest clinicopathological study of EPS to date. Nine cases were collected, consisting of four males and five females ranging in age from 10 to 81 years (average: 49 years). Five tumors occurred in abdominal wall soft tissues, three in the thorax, and one in the back of the neck. Tumor sizes ranged from 2.5 to 18 cm (average 6.6 cm). Five patients had follow-up with an average of 38 months (range: 18-60 months). Two patients had no recurrence or metastasis 19 months after diagnosis. Four patients developed multifocal pleural or pulmonary metastasis and were treated variably by surgery, radiotherapy, and chemotherapy. The latter seemed to have little to no clinical benefit. One of the four patients was free of disease 60 months after diagnosis, two patients were alive with disease at 18 and 60 months, respectively. Morphologically, low, intermediate, and high-grade sarcomas composed of a variable mixture of spindled, ovoid, epithelioid, and round cells were seen. The architectural and stromal features also varied, resulting in a broad morphologic spectrum. Immunohistochemically, the following markers were most consistently expressed: S100-protein (7/9 cases), GFAP (7/8), MyoD1 (8/9), Pax-7 (4/5), desmin (7/9), and AE1/3 (4/9). By next-generation sequencing, all cases revealed EWSR1-PATZ1 gene fusion. In addition, 3/6 cases tested harbored CDKN2A deletion, while CDKN2B deletion and TP53 mutation were detected in one case each. Our findings confirm that EPS is a clinicopathologic entity, albeit with a broad morphologic spectrum. The uneventful outcome in some of our cases indicates that a subset of EPS might follow a more indolent clinical course than previously appreciated. Additional studies are needed to validate whether any morphological and/or molecular attributes have a prognostic impact.

Versatile knowledge guided network inference method for prioritizing key regulatory factors in multi-omics data

Constantly decreasing costs of high-throughput profiling on many molecular levels generate vast amounts of multi-omics data. Studying one biomedical question on two or more omic levels provides deeper insights into underlying molecular processes or disease pathophysiology. For the majority of multi-omics data projects, the data analysis is performed level-wise, followed by a combined interpretation of results. Hence the full potential of integrated data analysis is not leveraged yet, presumably due to the complexity of the data and the lacking toolsets. We propose a versatile approach, to perform a multi-level fully integrated analysis: The Knowledge guIded Multi-Omics Network inference approach, KiMONo (https://github.com/cellmapslab/kimono). KiMONo performs network inference by using statistical models for combining omics measurements coupled to a powerful knowledge-guided strategy exploiting prior information from existing biological sources. Within the resulting multimodal network, nodes represent features of all input types e.g. variants and genes while edges refer to knowledge-supported and statistically derived associations. In a comprehensive evaluation, we show that our method is robust to noise and exemplify the general applicability to the full spectrum of multi-omics data, demonstrating that KiMONo is a powerful approach towards leveraging the full potential of data sets for detecting biomarker candidates.

The 2020 WHO Classification: What's New in Soft Tissue Tumor Pathology?

The fifth edition of the World Health Organization Classification of Tumors of Soft Tissue and Bone was published in early 2020. The revisions reflect a consensus among an international expert editorial board composed of soft tissue and bone pathologists, geneticists, a medical oncologist, surgeon, and radiologist. The changes in the soft tissue tumor chapter notably include diverse, recently described tumor types (eg, atypical spindle cell/pleomorphic lipomatous tumor, angiofibroma of soft tissue, and CIC-rearranged sarcoma), new clinically significant prognostic information for a variety of existing entities (eg, dedifferentiated liposarcoma and solitary fibrous tumor), and a plethora of novel genetic alterations, some of practical diagnostic relevance (eg, NAB2-STAT6 in solitary fibrous tumor, FOSB rearrangements in epithelioid hemangioma and pseudomyogenic hemangioendothelioma, and SUZ12 or EED mutations in malignant peripheral nerve sheath tumor, leading to loss of H3K27 trimethylation). In this review, we highlight the major changes to the soft tissue chapter in the 2020 World Health Organization Classification, as well as the new chapter on undifferentiated small round cell sarcomas, with a focus on updates in diagnostic categories, prognostication, and novel markers. Recent discoveries in molecular genetics are also discussed, particularly those of immediate utility in differential diagnosis, including protein correlates detectable using immunohistochemistry.

Round Cell Sarcoma with EWSR1-PATZ1 Fusion in the Face of a Five-Year-Old Boy: Report of a Case with Unusual Histologic Features

Round cell sarcomas with EWSR1-PATZ1 fusion are rare polyphenotypic sarcomas that typically show both neural and myogenic differentiation on immunohistochemistry. The histology features lobular admixture of cellular fascicles of relatively monotonous spindle cells and small blue round cells separated by fibrotic stroma. The clinical behavior of EWSR1-PATZ1 sarcoma is uncertain currently with mixed outcomes reported even in cases with metastases. We herein report an additional case of EWSR1-PATZ1 fusion-related round cell sarcoma in the face of a 5-year-old boy with unusual histologic features of pale zones, rosette/gland-like structures and expression of epithelial markers. Fluorescent in-situ hybridization study (FISH) using EWSR1 breakapart probes was negative and molecular study with RNA sequencing was required to confirm the diagnosis. These findings highlight the diagnostic challenge and potential pitfall of FISH study in EWSR1-PATZ1 sarcoma. Further studies are required to increase the understanding of their behavior, morphologic spectrum and molecular features that will help devise new treatment strategies to these rare tumours.

Updates from the 2020 World Health Organization Classification of Soft Tissue and Bone Tumours

The fifth edition of the World Health Organization (WHO) classification of soft tissue and bone tumours was published in May 2020. This 'Blue Book', which is also available digitally for the first time, incorporates an array of new information on these tumours, amassed in the 7 years since the previous edition. Major advances in molecular characterisation have driven further refinements in classification and the development of ancillary diagnostic tests, and have improved our understanding of disease pathogenesis. Several new entities are also included. This review summarises the main changes introduced in the 2020 WHO classification for each subcategory of soft tissue and bone tumours.

The 2020 WHO Classification of Tumors of Soft Tissue: Selected Changes and New Entities

Soft tissue tumors are a relatively rare and diagnostically challenging group of neoplasms that can have varying lines of differentiation. Accurate diagnosis is important for appropriate treatment and prognostication. In the 8 years since the publication of the 4th Edition of World Health Organization (WHO) classification of soft tissue tumors, significant advances have been made in our understanding of soft tissue tumor molecular biology and diagnostic criteria. The 5th Edition of the 2020 WHO classification of tumors of soft tissue and bone incorporated these changes. Classification of tumors, in general, but particularly in soft tissue tumors, is increasingly based on the molecular characteristics of tumor types. Understanding tumor molecular genetics improves diagnostic accuracy for tumors that have been difficult to classify on the basis of morphology alone, or that have overlapping morphologic features. In many large hospitals in the United States and Europe, molecular tests on soft tissue tumors are a routine part of diagnosis. Therefore, surgical pathologists should be familiar with newly emerging molecular genetic techniques in clinical settings. In the near future, molecular tests, particularly in soft tissue tumor diagnosis, will become as routine during diagnosis as immunohistochemistry is currently. This new edition provides an updated classification scheme and essential diagnostic criteria for soft tissue tumors. Newly recognized entities and subtypes of existing tumor types, several reclassified tumors, and newly defined molecular and genetic data have been incorporated. Herein, we summarize the updates in the WHO 5th Edition, focusing on major changes in each category of soft tissue tumor, and the newly described tumor entities and subtypes.

Molecular Approaches to Diagnosis in Ewing Sarcoma: Fluorescence In Situ Hybridization (FISH)

The differential diagnosis of small round cell tumors (SRCT) crucially relies on the synoptic evaluation of morphology, immunohistochemical patterns, and molecular features. Though the implementation of broad RNA sequencing in diagnostic molecular pathology routines has substantially changed the standards of molecular affirmation of diagnoses, fluorescence in situ hybridization (FISH) on formalin-fixed, paraffin-embedded (FFPE) tissue sections is still an elementary tool to provide a rapid molecular corroboration of diagnoses, essentially required for therapeutic decisions. We discuss here the major FISH approaches currently employed in diagnostic molecular pathology, addressing classic Ewing sarcoma and differential diagnoses among SRCT which cannot sufficiently be ruled out by immunohistochemistry. This chapter will approach technical issues but particularly strategies and pitfalls in the interpretation of FISH patterns.

The spectrum of rare central nervous system (CNS) tumors with EWSR1-non-ETS fusions: experience from three pediatric institutions with review of the literature

The group of CNS mesenchymal (non‐meningothelial) and primary glial/neuronal tumors in association with EWSR1‐non‐ETS rearrangements comprises a growing spectrum of entities, mostly reported in isolation with incomplete molecular profiling. Archival files from three pediatric institutions were queried for unusual cases of pediatric (≤21 years) CNS EWSR1‐rearranged tumors confirmed by at least one molecular technique. Extra‐axial tumors and cases with a diagnosis of Ewing sarcoma (EWSR1‐ETS family fusions) were excluded. Additional studies, including anchored multiplex‐PCR with next‐generation sequencing and DNA methylation profiling, were performed as needed to determine fusion partner status and brain tumor methylation class, respectively. Five cases (median 17 years) were identified (M:F of 3:2). Location was parenchymal (n = 3) and undetermined (n = 2) with topographic distributions including posterior fossa (n = 1), frontal (n = 1), temporal (n = 1), parietal (n = 1) and occipital (n = 1) lobes. Final designation with fusion findings included desmoplastic small round cell tumor (EWSR1‐WT1; n = 1) and tumors of uncertain histogenesis (EWSR1‐CREM, n = 1; EWSR1‐CREB1, n = 1; EWSR1‐PLAGL1, n = 1; and EWSR1‐PATZ1, n = 1). Tumors showed a wide spectrum of morphology and biologic behavior. For EWSR1‐CREM, EWSR1‐PLAGL1 and EWSR1‐PATZ1 tumors, no significant methylation scores were reached in the known brain tumor classes. Available outcome (4/5) was reported as favorable (n = 2) and unfavorable (n = 2) with a median follow‐up of 30 months. In conclusion, we describe five primary EWSR1‐non‐ETS fused CNS tumors exhibiting morphologic and biologic heterogeneity and we highlight the clinical importance of determining specific fusion partners to improve diagnostic accuracy, treatment and monitoring. Larger prospective clinicopathological and molecular studies are needed to determine the prognostic implications of histotypes, anatomical location, fusion partners, breakpoints and methylation profiles in patients with these rare tumors.

The 2020 WHO Classification of Soft Tissue Tumours: news and perspectives

Mesenchymal tumours represent one of the most challenging field of diagnostic pathology and refinement of classification schemes plays a key role in improving the quality of pathologic diagnosis and, as a consequence, of therapeutic options. The recent publication of the new WHO classification of Soft Tissue Tumours and Bone represents a major step toward improved standardization of diagnosis. Importantly, the 2020 WHO classification has been opened to expert clinicians that have further contributed to underline the key value of pathologic diagnosis as a rationale for proper treatment. Several relevant advances have been introduced. In the attempt to improve the prediction of clinical behaviour of solitary fibrous tumour, a risk assessment scheme has been implemented. NTRK-rearranged soft tissue tumours are now listed as an "emerging entity" also in consideration of the recent therapeutic developments in terms of NTRK inhibition. This decision has been source of a passionate debate regarding the definition of "tumour entity" as well as the consequences of a "pathology agnostic" approach to precision oncology. In consideration of their distinct clinicopathologic features, undifferentiated round cell sarcomas are now kept separate from Ewing sarcoma and subclassified, according to the underlying gene rearrangements, into three main subgroups (CIC, BCLR and not ETS fused sarcomas) Importantly, In order to avoid potential confusion, tumour entities such as gastrointestinal stroma tumours are addressed homogenously across the different WHO fascicles. Pathologic diagnosis represents the integration of morphologic, immunohistochemical and molecular characteristics and is a key element of clinical decision making. The WHO classification is as a key instrument to promote multidisciplinarity, stimulating pathologists, geneticists and clinicians to join efforts aimed to translate novel pathologic findings into more effective treatments.