Expanding the Spectrum of Genetic Alterations in Pseudomyogenic Hemangioendothelioma With Recurrent Novel ACTB-FOSB Gene Fusions

Utility of an anchored multiplex polymerase chain reaction-based fusion assay for diagnosis of soft tissue tumors in cytology

BACKGROUND

Fine-needle aspiration specimens from soft tissue tumors are complicated by lack of tissue architecture and limited material for ancillary testing. There are little data on the feasibility of next-generation sequencing techniques for fusion detection on soft tissue cytology specimens. This study explored the role of an anchored multiplex polymerase chain reaction (PCR)-based gene fusion assay in aiding the diagnosis of mesenchymal neoplasms on cytology samples.

METHODS

The laboratory information system was queried for cytology specimens that had undergone testing by anchored multiplex PCR. After exclusion of epithelial and hematolymphoid neoplasms, clinical and pathologic information was collected on the remaining cases.

RESULTS

There were 1609 cytology specimens tested with anchored multiplex PCR. Of these, 48 (3%) were cytology specimens from mesenchymal tumors. Anchored multiplex PCR was positive for a reportable fusion transcript in 14 of 48 cases (29%); there was no fusion detected in 32 cases (67%), and there was insufficient tissue for analysis in two cases (4%). The detectable fusion partners included ALK (n = 4), STAT6 (n = 4), EWSR1 (n = 3), and one each of SS18, YAP1, and PHF1. Of the cases in which a fusion partner was detected, eight of 14 were disease-defining on cytology preparation, and six of 14 provided molecular confirmation of a metastatic focus of a previously diagnosed tumor.

CONCLUSIONS

The anchored, multiplex PCR-based gene fusion assay is a powerful orthogonal tool in helping diagnose mesenchymal neoplasms on cytology specimens. The material obtained for cytologic analysis yields sufficient quality/quantity of tissue in the majority of cases tested.

Lack of FOS/FOSB Gene Rearrangements in Ischemic Fasciitis Indicates Distinct Pathogenesis from Proliferative Fasciitis and Proliferative Myositis

Ischemic fasciitis is a pseudosarcomatous fibroblastic/myofibroblastic proliferation that shares several overlapping morphological features with proliferative fasciitis and proliferative myositis. Prompted by a recent study that demonstrated FOS gene rearrangements in proliferative fasciitis and proliferative myositis, suggesting that these lesions likely represent examples of "transient neoplasia," we examined a cohort of ischemic fasciitis for similar events. Nine cases of ischemic fasciitis were retrieved from our institutional archives for diagnosis verification, immunostaining for FOSB, and fluorescence in situ hybridization using validated FOS and FOSB break-apart probes. Additionally, RNAseq was performed on a subset of cases. In our cohort, eight out of nine cases of ischemic fasciitis were positive for FOSB IHC, but FISH studies were consistently negative for FOSB and FOS gene rearrangements in all cases. Additionally, RNA sequencing did not detect any gene fusions. These findings suggest that the pathogenesis of ischemic fasciitis is distinct from that of proliferative fasciitis and proliferative myositis.

Update on immunohistochemistry in bone and soft tissue tumors: Cost-effectively replacing molecular testing with immunohistochemistry

Soft tissue tumors form part of a challenging domain in diagnostic pathology owing to their comparative rarity, astonishing histologic diversity, and overlap between entities. Many of these tumors are now known to be defined by highly recurrent, or, in some instances, unique molecular alterations. Insights from gene profiling continue to elucidate the wider molecular landscape of soft tissue tumors; many of these advances have been co-opted by immunohistochemistry (IHC) for diagnostic applications. There now exists a multitude of antibodies serving as surrogate markers of recurrent gene fusions, amplifications, and point mutations, which, in certain settings, can replace the need for more resource and time-intensive cytogenetic and molecular genetic analyses. IHC presents many advantages including rapid turnaround time, cost-effectiveness, and interpretative reproducibility. A sensible application of these immunohistochemical markers complemented by a working knowledge of the molecular pathogenesis of bone and soft tissue tumors permits accurate diagnosis in the majority of cases. In this review, we will outline some of these biomarkers while emphasizing molecular correlates and highlighting interpretative challenges and pitfalls.

Primary vascular tumors of bone: A comprehensive literature review on classification, diagnosis and treatment

Primary vascular tumors of bone are a heterogeneous group of neoplasms, ranging from benign hemangiomas to frankly malignant epithelioid hemangioendotheliomas and angiosarcomas. Over the years, their classification has been a matter of discussion, due to morphologic similarities and uncertainty regarding biologic behavior. Over the past decade, with the development of next-generation sequencing, there has been a significant improvement in the molecular characterization of these lesions. The integration of their morphologic, immunohistochemical and molecular features has led to a better stratification, with important prognostic and therapeutic implications. Nevertheless, primary vascular bone tumors still represent a challenge for medical oncologists. Given their rarity and heterogeneity, in the last few years, there has been no significant progress in medical treatment options, so further research is needed. Here we present a review of the current knowledge regarding primary vascular tumors of the bone, correlating clinicopathologic features with tumor behavior and therapeutic approaches.

Molecularly Confirmed Pseudomyogenic Hemangioendothelioma with Unusual EGFL7::FOSB Fusion in the Head and Neck Region of an Older Patient

Pseudomyogenic hemangioendothelioma (PMHE), a rare vascular neoplasm, was first described in 1992 asa fibroma-like variant of epithelioid sarcoma, and would be termed as epithelioid sarcoma-like hemangioendothelioma a decade later due to its significant histologic overlap with epithelioid sarcoma and diffuse cytokeratin expression. PHME is currently defined as a distinct, potentially intermediate malignant, rarely metastasizing neoplasm with vascular/endothelial differentiation. It is characterized by young age (typically less than 40 years old), extremity location (approximately ~80%), and t(7:19) SERPINE1::FOSB fusion as the most common molecular alteration. Herein, we report a case of a 59-year-old male presenting with multifocal lesions, including in the right temporalis muscle, right frontoparietal calvarium, right pterygoid muscles, and right mandibular condyle. Histologic examination of the right temporal lesion revealed a multinodular biphasic lesion composed of sheets and fascicles of elongated spindle and epithelioid cells infiltrating into the adjacent skeletal muscle. Admixed abundant neutrophilic infiltration is noted; however, areas of necrosis, increased mitosis, nuclear atypia, or rhabdomyoblast-like cells are absent. Immunohistochemical (IHC) staining showed that the tumor cells were diffusely and strongly positive for FOSB, pan-cytokeratin (AE1/AE3), CD31, and ERG. Molecular testing demonstrated a t(9:19) EGFL7::FOSB fusion mRNA. This constellation of morphological, IHC and molecular findings was consistent with a diagnosis of PMHE. This is the first reported case of multifocal PMHE with EGFL7::FOSB fusion in the head and neck area of a patient aged more than 50 years old. Since the differential diagnoses for PMHE includes high-grade malignancies with aggressive clinical behavior, coupled with the rare reports of PMHE in the head and neck region, awareness of this tumor in the head and neck region will avoid the misdiagnosis and overtreatment of this entity.

A cutaneous epithelioid vascular tumor harboring a TPM3::ALK fusion

Substantial progress has been made in understanding the molecular pathways associated with vascular tumors over the last two decades. In addition to mutations and copy number aberrations, fusions have emerged as significant contributors to the pathogenesis of a notable subset of vascular tumors. In this report, we present a case of an unusual intradermal vascular tumor with epithelioid cytomorphology. Immunohistochemistry revealed diffuse positivity for CD31, ERG and Factor VIII, supporting its endothelial lineage. RNA sequencing (ArcherFusion Plex) revealed the presence of an in-frame fusion between the genes TPM3 Exon 8 and ALK Exon 20. Immunohistochemistry confirmed ALK expression by the endothelial cells. To our knowledge, this is the first documented case of a vascular tumor harboring an ALK fusion. It may fall within the spectrum of epithelioid hemangiomas; nevertheless, we cannot definitively exclude the possibility of it being a distinct and potentially unique benign entity on its own.

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.

NR1D1::MAML1 epithelioid and spindle cell sarcoma mimicking pseudomyogenic hemangioendothelioma in core biopsy: A case report and review of the literature

Epithelioid and spindle cell sarcomas with NR1D1::MAML1/2 gene fusions are rare and emerging entities. Only six cases of NR1D1-rearranged mesenchymal tumors have previously been reported in the literature; they are often characterized by an epithelioid morphology, at least focal pseudogland formation, prominent cytoplasmic vacuoles, and focal to diffuse immunohistochemical expression of keratin. We herein report the first case of an NR1D1::MAML1 epithelioid and spindle cell sarcoma with dual immunohistochemical expression of ERG and FOSB, mimicking a pseudomyogenic hemangioendothelioma (PHE) on core biopsy. The sarcoma arose in the left forearm of a 64-year-old man. Initial biopsy showed a mesenchymal neoplasm composed of epithelioid and spindle cells dispersed in myxoid stroma with scattered stromal neutrophils. The morphologic features, combined with the dual immunohistochemical expression of ERG and FOSB, initially mimicked PHE, representing an important potential diagnostic pitfall. The patient subsequently underwent a radical resection, which showed a much more diffuse epithelioid appearance with nested architecture and pseudogland formation. Next-generation sequencing was performed on the resection specimen, which revealed an NR1D1::MAML1 gene fusion, confirming the final diagnosis. Given the fully malignant potential of this tumor, knowledge and recognition of this rare entity are essential to ensure proper management, prevent misdiagnosis, and further characterize the clinical course of this emerging entity. Comprehensive molecular testing can help to identify these rare tumors and exclude the possibility of epithelioid mimics, including PHE.

New kids on the block: FOS and FOSB gene

FOS and FOSB proto-oncogens are involved in a wide variety of tumourigenic processes. FOS and FOSB gene rearrangements are observed in epithelioid haemangioma, pseudomyogenic haemangioendothelioma, osteoid osteoma/osteoblastoma/cementoblastoma and proliferative myositis/fasciitis. In this review, we provide an overview of FOS and FOSB, including their functions and the differences between lesions with known FOS/FOSB gene rearrangements. Additionally, we discuss the use of FOS/FOSB immunohistochemistry as a diagnostic tool for these lesions.

Cutaneous vascular tumors: an updated review

Cutaneous vascular tumors constitute a heterogeneous group of entities that share overlapping morphologic and immunohistochemical features, which can be diagnostically challenging for pathologists and dermatopathologists. Our understanding and knowledge of vascular neoplasms have improved over time, resulting in both a refinement of their classification by the International Society for the Study of Vascular Anomalies (ISSVA) and an improvement in the accurate diagnosis and clinical management of vascular neoplasms. This review article aims to summarize the updated clinical, histopathological, and immunohistochemical characteristics of cutaneous vascular tumors, as well as to highlight their associated genetic mutations. Such entities include infantile hemangioma, congenital hemangioma, tufted angioma, spindle cell hemangioma, epithelioid hemangioma, pyogenic granuloma, Kaposiform hemangioendothelioma, retiform hemangioendothelioma, pseudomyogenic hemangioendothelioma, Kaposi sarcoma, angiosarcoma, and epithelioid hemangioendothelioma.

[Special mesenchymal neoplasms of the head and neck: Update from the 2022 WHO]

Similar to the approach adopted in the classification of tumors in other organs, a new feature of the current Word Health Organization (WHO) classification of the head and neck tumors is the exclusion of soft tissue tumors from single organs and their inclusion in a separate chapter devoted to them. This applies to tumors that are principally ubiquitous but show a predilection for the head and neck sites. The only exception to this rule represents those entities that are almost restricted to a specific head and neck site/organ (such as nasopharyngeal angiofibroma, sinonasal glomangiopericytoma, and biphenotypic sinonasal sarcoma), which remain in their respective organ chapters. Included among soft tissue tumors are some old but still underrecognized entities, such as phosphaturic mesenchymal tumors, and newly defined entities, such as GLI1-altered tumors. The aim of including these entities is to enhance recognition of these uncommon but likely under-recognized entities to better characterize them in the future. This review summarizes the main features of these rare entities and discusses their differential diagnoses.

Benign and low-grade superficial endothelial cell neoplasms in the molecular era

Vascular tumors are the most common mesenchymal neoplasms of the skin and subcutis, and they encompass a heterogeneous group with diverse clinical, histological, and molecular features, as well as biological behavior. Over the past two decades, molecular studies have enabled the identification of pathogenic recurrent genetic alterations that can be used as additional data points to support the correct classification of these lesions. The purpose of this review is to summarize the available data related to superficially located benign and low-grade vascular neoplasms and to highlight recent molecular advances with the role of surrogate immunohistochemistry to target pathogenic proteins as diagnostic biomarkers.

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.

Distinctive Nested Glomoid Neoplasm: Clinicopathologic Analysis of 20 Cases of a Mesenchymal Neoplasm With Frequent GLI1 Alterations and Indolent Behavior

Recently, it has been recognized that a subset of primary soft tissue neoplasms with GLI1 gene alterations exhibit nested architecture and can mimic glomus tumors or well-differentiated neuroendocrine tumors. Here, we report a series of 20 such neoplasms, which we have provisionally termed "distinctive nested glomoid neoplasm." Eleven patients (55%) were female and 9 were male. The median age at presentation was 41.5 years (range: congenital to 74 y). The anatomic distribution was wide, with body sites including the trunk (7 tumors), lower extremity (5), tongue (4), upper extremity (3), and neck (1). Excluding tumors of the tongue, 10 tumors (62%) arose in deep soft tissue and 6 (38%) arose primarily in the subcutis. Tumor size ranged from 0.9 to 11.1 cm (median: 3 cm). Distinctive nested glomoid neoplasms are composed of nests of round-to-ovoid cells with scant, palely eosinophilic cytoplasm and monomorphic nuclei with vesicular chromatin and small nucleoli. The nests are invested by prominent capillary networks, and they are situated within large lobules separated by irregular, thick fibrous septa. Among 18 tumors for which adjacent non-neoplastic tissue could be assessed, perivascular proliferation of tumor cells was identified in 16 tumors (89%). Microcystic architecture was present at least focally in 8 tumors (40%), and myxoid stroma was identified at least focally in 5 (25%). Seven tumors (35%) showed clear cell features. By immunohistochemistry, some tumors expressed MDM2 (7/15; 47%), S100 (5 of 19; 26%), STAT6 (2 of 5; 20%), and AE1/AE3 (1/5; 20%). Tumors rarely expressed pan-keratin (1/10; 10%) or CAM5.2 (1/10), and all tumors were negative for β-catenin (12 tumors), chromogranin (12), synaptophysin (11), epithelial membrane antigen (10), desmin (10), smooth muscle actin (9), INSM1 (7), and CD34 (6). GLI1 break-apart fluorescence in situ hybridization was performed on 7 tumors, and next-generation sequencing was performed on 15 tumors (10 DNA sequencing only, 1 RNA sequencing only, 4 both DNA and RNA sequencing). Sixteen tumors, including all 15 tested by next-generation sequencing and an additional case tested by fluorescence in situ hybridization only, were found to harbor GLI1 gene alterations: 10 harbored GLI1 gene rearrangements (3 ACTB :: GLI1 , 2 PTCH1 :: GLI1 , 1 HNRNPA1 :: GLI1 , 1 NEAT1 :: GLI1 , 1 TXNIP :: GLI1 , 2 undetermined fusion partners), and 6 harbored GLI1 amplification. Clinical follow-up was available for 10 patients (50%; range: 3 mo to 10 y; median: 6.4 y), including 8 with >1 year of follow-up. Three patients (30%) experienced local recurrence (at intervals of 3 mo to 10 y). None developed distant metastases or died of disease as yet. Overall, our findings support the notion that a subset of GLI1 -altered soft tissue neoplasms are indolent, morphologically distinctive nested glomoid neoplasms that should not be classified as sarcomas.

Identification of stable housekeeping genes for induced pluripotent stem cells and -derived endothelial cells for drug testing

There are no validated housekeeping genes in induced pluripotent stem cells (iPSC) and derived endothelial iPSC (iPSC-EC). Thus a comparison of gene expression levels is less reliable, especially during drug treatments. Here, we utilized transcriptome sequencing data of iPSC and iPSC-EC with or without CRISPR-Cas9 induced translocation to identify a panel of 15 candidate housekeeping genes. For comparison, five commonly used housekeeping genes (B2M, GAPDH, GUSB, HMBS, and HPRT1) were included in the study. The panel of 20 candidate genes were investigated for their stability as reference genes. This panel was analyzed and ranked based on stability using five algorithms, delta-Ct, bestkeeper, geNorm, Normfinder, and Reffinder. Based on the comprehensive ranking of Reffinder, the stability of the top two genes—RPL36AL and TMBIM6, and the bottom two genes—UBA1 and B2M, were further studied in iPSC-EC with and without genetic manipulation, and after treatment with telatinib. Using quantitative reverse-transcriptase polymerase chain reaction (qRT-PCR), it was shown that gene expression of the top two housekeeping genes, RPL36AL and TMBIM6, remained stable during drug treatment. We identified a panel of housekeeping genes that could be utilized in various conditions using iPSC and iPSC-derived endothelial cells as well as genetically modified iPSC for drug treatment.

Management of Vascular Sarcoma

Vascular sarcomas encompass 3 well-defined sarcoma types: hemangioendothelioma, Kaposi sarcoma, and angiosarcoma. These distinct types are exceedingly rare and very different in terms of clinical behavior, biological features, and treatment approach. Because of this rarity and heterogeneity, it is crucial that vascular sarcomas are treated in sarcoma reference centers or networks, in order to ensure optimal management. The diversity of vascular sarcomas also needs to be taken into account in the design of clinical trials, in order to produce meaningful results that can be consistently translated into everyday clinical practice.

Immunohistochemistry for FOSB and FOS is a Useful Ancillary Tool in the Diagnosis of Epithelioid Hemangioma but There are Pitfalls in Interpretation Including Expression in Other Vascular Lesions

Introduction: Epithelioid hemangioma is a benign vascular neoplasm associated with FOS and/or FOSB protein overexpression detected by immunohistochemistry (IHC). Methods: The aim of our study was to determine the co-expression or independent IHC expression of FOS and FOSB in a cohort of epithelioid hemangiomas. We also included two cohorts of other vascular lesions: papillary endothelial hyperplasia and lobular capillary hemangioma / pyogenic granuloma. Results: We identified 50 cases of epithelioid hemangioma, 84% of which were cutaneous and the remaining involved other anatomic locations. Over two thirds of all cases expressed FOSB (68%; 34/50) while FOS immunoreactivity was identified in 46% of all cases. Co-expression of FOSB and FOS occurred in 37% of cases while 76% of all cases stained for at least one of the antibodies. Fifty-eight percent (n = 14/24) and 33% (8/24) of all cases of papillary endothelial hyperplasia expressed FOS and FOSB, respectively. Thirty-two per cent of lobular capillary hemangiomas (n = 8/25) were positive for either FOS or FOSB. Conclusion: In summary, we present the largest cohort of epithelioid hemangiomas assessed with both FOS and FOSB and demonstrated that the use of both antibodies increases the detection rate of these proliferations by 10%. Nonetheless, the use of thresholds may not be appropriate, as only a subset of lesional endothelial cells label with FOS/FOSB. Over half of all cases of papillary endothelial hyperplasia and a third of lobular capillary hemangiomas also displayed immunoreactivity with FOS and/or FOSB.

Malignant Superficial Mesenchymal Tumors in Children

Malignant superficial mesenchymal tumors are a very diverse group of neoplasms with few clinical and radiological discriminatory factors. Hence, some of these cancers are rarely suspected based on clinical and radiological grounds, others may be easily misdiagnosed, and the histological analysis of a biopsy or resection is central in the diagnostic process. In children, the age at presentation is a major element of the differential diagnosis. Some tumors have a very distinct epidemiology, while others may be seen at any age. More recently, the advances in molecular biology have greatly improved the diagnosis of mesenchymal tumors and new entities are still being described. In the present review, we provide an overview of the diversity of malignant superficial mesenchymal tumors in children, including new and/or rare entities. We discuss the important diagnostic features, be they clinical, histological, or molecular. Special attention was given to the genetic features of these tumors, particularly when they were helpful for the diagnosis or treatment.

Update from the 5th Edition of the World Health Organization Classification of Head and Neck Tumors: Soft Tissue Tumors

The fifth (5th) edition of the World Health Organization (WHO) Classification of Head and Neck Tumors introduces a new chapter dedicated to soft tissue neoplasms commonly affecting the head and neck. While the diversity, rarity, and wide anatomic range of soft tissue tumors precludes a discussion of all entities that may be found in the head and neck, the addition of this new chapter to the head and neck "blue book" aims to provide a more comprehensive and uniform reference text, including updated diagnostic criteria, of mesenchymal tumor types frequently (or exclusively) arising at head and neck sites. Since publication of the previous edition in 2017, there have been numerous advances in our understanding of the pathogenesis of many soft tissue tumors which have facilitated refinements in tumor classification, identification of novel entities, development of diagnostic markers, and improved prognostication. This review will provide a focused discussion of the soft tissue tumors included in the 5th edition WHO Head and Neck classification, with an emphasis on updates.

Pseudomyogenic hemangioendothelioma with bone and soft tissue involvement with favorable response to pamidronate: a case report and systematic review of the literature

Pseudomyogenic hemangioendothelioma (PMH) can be a challenge for diagnosis and might be confused with other tumors, such as epithelioid sarcoma. Here we present a case and a systematic review of the literature to identify and discuss PMH treatment in primary bone involvement. A 25-year-old woman was referred for bone pain (10/10) in the left lower limb. Magnetic resonance imaging (MRI) showed multiple bone lesions (left femur, tibia, patella, ankle, and foot) with well-defined borders without signs of local aggressiveness. Positron Emission Tomography-Computed Tomography (PET-CT) showed multiple metabolic musculoskeletal lesions in the left lower limb. A CT scan-guided biopsy was performed. Histological and immunohistochemical findings confirmed the diagnosis of PMH. After treatment with intravenous pamidronate (90 mg/monthly), the patient had clinical improvement, mild pain 2/10 without the use of non-steroidal anti-inflammatory drugs or opiates. Follow-up was assessed by MRI and PET-CT. PET-CT showed metabolic resolution of most of the bone and muscular lesions and a significant improvement of the femoral lesion. MRI showed that the lesions in the left femur, tibia, and foot had a marked decrease in size without intravenous post-contrast enhancement and smaller lesions had disappeared. After a 3-year follow-up, PET-CT showed no metabolically active images. Literature review identified 31 records including 58 clinical cases of PMH with primary bone involvement and treatment description for qualitative analysis. Most lesions (69%) were treated by local excision or curettage. In addition, amputations were performed in a significant percentage of cases (20.7%). In the last years, mTOR inhibitors (n = 7) and anti-resorptive treatments (n = 4) were considered as alternative treatment options, especially in multifocal lesions.

Diagnosis of soft tissue tumors using immunohistochemistry as a surrogate for recurrent fusion oncoproteins

Soft tissue neoplasms encompass a broad spectrum of clinicopathologic manifestations. In a subset of soft tissue tumors, spanning a wide range of clinical behavior from indolent to highly aggressive, recurrent genetic translocations yield oncogenic fusion proteins that drive neoplastic growth. Beyond functioning as primary mechanisms of tumorigenesis, recurrent translocations represent key diagnostic features insofar as the presence of a particular oncogenic gene fusion generally points to specific tumor entities. In addition to more direct methods for identifying recurrent translocations, such as conventional cytogenetics or fluorescence in situ hybridization, immunohistochemistry for a component of the fusion oncoprotein increasingly is being used as a surrogate marker, exploiting the tendency of these fusion components to be distinctively overexpressed by translocation-bearing tumor cells. Diagnostic immunohistochemistry can also be used to identify the characteristic gene expression changes that occur downstream of oncogenic fusions. Here, we review the use of immunohistochemistry to detect surrogate markers of recurrent translocations in soft tissue tumors, focusing on the practical applications and limitations of this diagnostic approach.

The genetics of vascular tumours: an update

Recent molecular advances have shed significant light on the classification of vascular tumours. Except for haemangiomas, vascular lesions remain difficult to diagnose, owing to their rarity and overlapping clinical, radiographic and histological features across malignancies. In particular, challenges still remain in the differential diagnosis of epithelioid vascular tumours, including epithelioid haemangioma and epithelioid haemangioendothelioma at the benign/low-grade end of the spectrum, and epithelioid angiosarcoma at the high-grade end. Historically, the classification of vascular tumours has been heavily dependent on the clinical setting and histological features, as traditional immunohistochemical markers across the group have often been non-discriminatory. The increased application of next-generation sequencing in clinical practice, in particular targeted RNA sequencing (such as Archer, Illumina), has led to numerous novel discoveries, mainly recurrent gene fusions (e.g. those involving FOS, FOSB, YAP1, and WWTR1), which have resulted in refined tumour classification and improved diagnostic reproducibility for vascular tumours. However, other molecular alterations besides fusions have been discovered in vascular tumours, including somatic mutations (e.g. involving GNA family and IDH genes) in a variety of haemangiomas, as well as copy number alterations in high-grade angiosarcomas (e.g. MYC amplifications). Moreover, the translation of these novel molecular abnormalities into diagnostic ancillary markers, either fluorescence in-situ hybridisation probes or surrogate immunohistochemical markers (FOSB, CAMTA1, YAP1, and MYC), has been remarkable. This review will focus on the latest molecular discoveries covering both benign and malignant vascular tumours, and will provide practical diagnostic algorithms, highlighting frequently encountered pitfalls and challenges in the diagnosis of vascular lesions.

ACTB and GAPDH appear at multiple SDS-PAGE positions, thus not suitable as reference genes for determining protein loading in techniques like Western blotting

We performed polyacrylamide gel electrophoresis of human proteins with sodium dodecyl sulfate, isolated proteins at multiple positions, and then used liquid chromatography and tandem mass spectrometry (LC-MS/MS) to determine the protein identities. Although beta-actin (ACTB) and glyceraldehyde-3-phosphate dehydrogenase (GAPDH) are 41.7 and 36 kDa proteins, respectively, LC-MS/MS identified their peptides at all the positions studied. The National Center for Biotechnology Information (USA) database lists only one ACTB mRNA but five GAPDH mRNAs and one noncoding RNA. The five GAPDH mRNAs encode three protein isoforms, while our bioinformatics analysis identified a 17.6 kDa isoform encoded by the noncoding RNA. All LC-MS/MS-identified GAPDH peptides at all positions studied are unique, but some of the identified ACTB peptides are shared by ACTC1, ACTBL2, POTEF, POTEE, POTEI, and POTEJ. ACTC1 and ACTBL2 belong to the ACT family with significant similarities to ACTB in protein sequence, whereas the four POTEs are ACTB-containing chimeric genes with the C-terminus of their proteins highly similar to the ACTB. These data lead us to conclude that GAPDH and ACTB are poor reference genes for determining the protein loading in such techniques as Western blotting, a leading role these two genes have been playing for decades in biomedical research.

Vascular Tumors of Bone: Updates and Diagnostic Pitfall

Vascular tumors of bone can be diagnostically challenging because of their rarity and histologic overlap with diverse mimics. Vascular tumors of bone can be categorized as benign (hemangioma), intermediate-locally aggressive (epithelioid hemangioma), intermediate-rarely metastasizing (pseudomyogenic hemangioendothelioma), and malignant (epithelioid hemangioendothelioma and angiosarcoma). Recurrent genetic alterations have been described, such as FOSB rearrangements in pseudomyogenic hemangioendothelioma and a subset of epithelioid hemangiomas; CAMTA1 or TFE3 rearrangements in epithelioid hemangioendothelioma. This review discusses the clinical, histologic, and molecular features of vascular tumors of bone, along with diagnostic pitfalls and strategies for avoidance.

A novel SERPINE1-FOSB fusion gene in pseudomyogenic hemangioendothelioma results in activation of intact FOSB and the PI3K-AKT-mTOR signaling pathway and responsiveness to sirolimus

Pseudomyogenic hemangioendothelioma (PHE) is an extremely rare disease that affects mainly the young and more men than women. PHE are multicentric, locally aggressive, have low metastatic potential, and affect multiple tissue planes. Genetic aberrations are frequently detected in PHE and may play important roles in the occurrence, development, and treatment of this disease. In this study, we report a case of PHE with a novel SERPINE1-FOSB fusion gene. The fusion introduced a strong promoter near the coding region of FOSB, resulting in overexpression of intact FOSB. Immunohistochemical analysis showed overexpression of pAKT and mTOR in tumor cells, suggesting activation of the PI3K-AKT-mTOR signaling pathway. The patient responded well to targeted therapy with sirolimus, an mTOR inhibitor. Our study correlated dysregulation of a specific signaling pathway and the effectiveness of a targeted therapy to a specific genetic aberration. This information may be useful for future investigations of targeted therapeutics and provide a potential predictive biomarker for therapeutic effectiveness in PHE cases.

Molecular Diagnostics of Vascular Tumors of the Skin

In this article, the authors have reviewed all the recent news regarding how the discovery of some novel and recurrent molecular and genetic changes has modified the classification of some entities and have addressed to the description of new variants of vascular tumors. And even more important, the authors also reviewed on how these findings, in addition to gain insight into the tumoral biology, portend significant clinical consequences not only regarding to their diagnosis but also to their management and prognosis because some of these mutations are potential targets for treatment. The authors have also highlighted immunohistochemical markers can help us as a surrogate marker of those molecular alterations.

Gene fusions in vascular tumors and their underlying molecular mechanisms

INTRODUCTION

The group of vascular tumors contains many different entities, and is considered difficult by pathologists, as they often have overlapping histological characteristics. Chromosomal translocations have been identified in ~20% of mesenchymal tumors and are considered the drivers of tumor formation. Many translocations have been discovered over the past decade through next-generation sequencing. This technological advancement has also revealed several recurrent gene fusions in vascular tumors.

AREAS COVERED

This review will discuss the various vascular tumors for which recurrent gene fusions have been identified. The gene fusions and the presumed molecular mechanisms underlying tumorigenesis are shown, and potential implications for targeted therapies discussed. The identification of these gene fusions in vascular tumors has improved diagnostic accuracy, especially since several of these fusions can be easily detected using surrogate immunohistochemical markers.

EXPERT OPINION

The identification of gene fusions in a subset of vascular tumors over the past decade has improved diagnostic accuracy, and has provided the pathologists with novel diagnostic tools to accurately diagnose these often difficult tumors. Moreover, the increased understanding of the underlying molecular mechanisms can guide the development of targeted therapeutic strategies.

Clinical Utility of Anchored Multiplex Solid Fusion Assay for Diagnosis of Bone and Soft Tissue Tumors

Sarcoma diagnosis has become increasingly complex, requiring a combination of morphology, immunohistochemistry, and molecular studies to derive specific diagnoses. We evaluated the role of anchored multiplex polymerase chain reaction-based gene fusion assay in sarcoma diagnostics. Between 2015 and 2018, bone and soft tissue sarcomas with fusion assay results were compared with the histologic diagnosis. Of 143 sarcomas tested for fusions, 43 (30%) had a detectable fusion. In review, they could be classified into 2 main categories: (1) 31 tumors with concordant morphologic and fusion data; and (2) 12 tumors where the fusion panel identified an unexpected rearrangement that played a significant role in classification. The overall concordance of the fusion assay results with morphology/immunohistochemistry or alternate confirmatory molecular studies was 83%. Collectively, anchored multiplex polymerase chain reaction-based solid fusion assay represents a robust means of detecting targeted fusions with known and novel partners. The predictive value of the panel is highest in tumors that show a monomorphic cell population, round cell tumors, as well as tumors rich in inflammatory cells. However, with an increased ability to discover fusions of uncertain significance, it remains essential to emphasize that the diagnosis of bone and soft tissue neoplasms requires the integration of morphology and immunohistochemical profile with these molecular methods, for accurate diagnosis and optimal clinical management of sarcomas.

A unique epithelioid vascular neoplasm of bone characterized by EWSR1/FUS-NFATC1/2 fusions

An increasing number of epithelioid vascular lesions, in particular tumors from the benign and low-grade end of the spectrum, have been characterized by recurrent gene fusions. As a result, the detection of these molecular markers have improved the classification of diagnostically challenging cases. However, despite the significant progress, there are occasional lesions that do not fit in known histologic or molecular groups. Herein, we present five such unclassified epithelioid vascular lesions, which occurred in the bone and showed a distinct morphology composed of alternating vasoformative and solid growth and mild to moderate nuclear pleomorphism. The variegated morphologic appearance resembled that of composite hemangioendothelioma, being distinct from both epithelioid hemangioma and epithelioid hemangioendothelioma, and consistently showed cytologic atypia. Due to their unusual morphologic appearance and negative molecular work-up, targeted transcriptome sequencing was performed in two cases showing the presence of NFATC2 fusions with either EWSR1 or FUS genes. Three additional bone tumors with EWSR1 gene rearrangements were identified by FISH screening of a large cohort of 45 fusion-negative epithelioid vascular neoplasms, one fused to NFATC2 while two others to NFATC1. There were three females and two males, with a wide age range at presentation, mean of 44 years. The lesions occurred in the pelvis, maxillary sinus, and humerus. Two patients presented with polyostotic disease, both located in the pelvic bones. Two patients had available follow-up, one developed two local recurrences in the humerus over a 15-year period, while the other showed no recurrence 4 years subsequent to an en-bloc resection. Tumors were positive for CD31 and ERG, while negative for EMA, CK, synaptophysin, and chromogranin. FISH confirmed this abnormality in all cases, none of them being associated with gene amplifications. Further studies are needed to establish the pathogenetic relationship of this rare molecular subset with other epithelioid vascular tumors and to determine its clinical behavior.

FOS Rearrangement and Expression in Cementoblastoma

Cementoblastomas are rare odontogenic tumors developing in close proximity to the roots of teeth. Due to their striking morphologic resemblance to osteoblastomas of the peripheral skeleton, we set out to determine whether cementoblastomas harbor the same FOS rearrangements with overexpression of c-FOS as has recently been described for osteoblastomas. In total, 16 cementoblastomas were analyzed for FOS expression by immunohistochemistry and for FOS rearrangements by fluorescence in situ hybridization (FISH). We observed strong and diffuse staining of c-FOS in 71% of cementoblastomas and identified a FOS rearrangement in all cases (n=3) applicable for FISH. In the remaining cases, FISH failed due to decalcification. Cementoblastomas harbor similar FOS rearrangements and show overexpression of c-FOS like osteoblastomas, suggesting that both entities might represent parts of the spectrum of the same disease.

Novel GATA6-FOXO1 fusions in a subset of epithelioid hemangioma

The genetic hallmark of epithelioid hemangioma (EH) is the presence of recurrent gene fusions involving FOS and FOSB transcription factors, which occur in one-third of the cases. Certain clinical, pathologic, and genotypic correlations have been described, with FOS-related fusions being more often detected in skeletal and cellular variants of EH, while FOSB gene rearrangements are more commonly associated with atypical histologic features and penile location. These fusions are infrequently detected in the cutaneous or head and neck EH. Overall, two-thirds of EH lack these canonical fusions and remain difficult to classify, especially when associated with atypical features and/or clinical presentations. Triggered by an index case of an intravascular soft tissue EH with a novel GATA6-FOXO1 gene fusion by targeted RNA sequencing (Archer® FusionPlex® Sarcoma Panel), we have investigated 27 additional EH cases negative for FOS and FOSB gene rearrangements for this novel abnormality to determine its recurrent potential, and its association with clinical and pathologic features. Four additional EH cases were found to display GATA6-FOXO1 fusions (18%). There were three females and two males, with a mean age of 32 years old. Three lesions occurred in the head and neck (dura, nasopharyngeal, and cheek), one in the back and one in the leg. Two of these lesions were cutaneous and one was intravascular in the subcutis of the leg. Microscopically, the tumors showed a variegated morphology, with alternating vasoformative and solid components, extravasated red blood cells and mild to moderate cytologic atypia. None showed brisk mitotic activity or necrosis. Tumors were negative for FOS and FOSB by immunohistochemistry. In conclusion, we report a new GATA6-FOXO1 fusion in a subset of EH, with a predilection for skin, and head and neck location. The relationship of this novel molecular subset with the more common FOS/FOSB fusion-positive EH remains to be determined.

Diagnostic Immunohistochemistry of Soft Tissue and Bone Tumors: An Update on Biomarkers That Correlate with Molecular Alterations

The diagnosis of benign and malignant soft tissue and bone neoplasms is a challenging area of surgical pathology, due to the large number, rarity, and histologic diversity of tumor types. In recent years, diagnosis and classification has been aided substantially by our growing understanding of recurrent molecular alterations in these neoplasms. Concurrently, the role of diagnostic immunohistochemistry has also expanded, with the development of numerous biomarkers based on underlying molecular events. Such biomarkers allow us to infer the presence of these events and can therefore substitute for other ancillary molecular genetic techniques (e.g., fluorescence in situ hybridization, polymerase chain reaction, and next-generation sequencing). In this review, we discuss a range of biomarkers currently available for these neoplasms, highlighting the accuracy, staining characteristics, and interpretation pitfalls of each antibody. These include immunohistochemical antibodies that represent reliable surrogates for the detection of gene fusions (e.g., STAT6, CAMTA1, FOSB, DDIT3) and more recently described breakpoint-specific antibodies (e.g., SS18-SSX, PAX3/7-FOXO1). Additionally, discussed are markers that correlate with the presence of gene amplifications (e.g., MDM2, CDK4), deletions (e.g., SMARCB1, SMARCA4), single nucleotide variants (e.g., G34W, K36M), aberrant methylation (H3K27me3), and increased expression as discovered through gene expression profiling (e.g., MUC4, DOG1, ETV4, NKX2.2, NKX3.1).

Pediatric fibromyxoid soft tissue tumor with PLAG1 fusion: A novel entity?

The classification of undifferentiated soft tissue tumors continues to evolve with the expanded application of molecular analysis in clinical practice. We report three cases of a unique soft tissue tumor in young children (5 months to 2 years old) displaying a purely fibromyxoid histology, with positive staining for desmin and CD34. In two cases, RNA sequencing detected a YWHAZ-PLAG1 gene fusion, while in the third case, a previously unreported EEF1A1-PLAG1 fusion was identified. PLAG1 fusions have been reported in several pathologic entities including pleomorphic adenoma, myoepithelial tumors of skin and soft tissue, and lipoblastoma, the latter occurring preferentially in young children. In these tumors, expression of a full length PLAG1 protein comes under the control of the constitutively active promoter of the partner gene in the fusion, and the current cases conform to that model. Overexpression of PLAG1 was confirmed by diffusely positive immunostaining for PLAG1 in all three cases. Our findings raise the possibility of a novel fibromyxoid neoplasm in childhood associated with these rare PLAG1 fusion variants. The only other report of a PLAG1-YWHAZ fusion occurred in a pediatric tumor diagnosed as a "fibroblastic lipoblastoma." This finding raises the possibility of a relationship with our three cases, even though our cases lacked any fat component. Further studies with regard to a shared pathogenesis are required.

Cancer type-specific alterations in actin genes: Worth a closer look?

Actins form a strongly conserved family of proteins that are central to the functioning of the actin cytoskeleton partaking in natural processes such as cell division, adhesion, contraction and migration. These processes, however, also occur during the various phases of cancer progression. Yet, surprisingly, alterations in the six human actin genes in cancer studies have received little attention and the focus was mostly on deregulated expression levels of actins and even more so of actin-binding or regulatory proteins. Starting from the early mutation work in the 1980s, we propose based on reviewing literature and data from patient cancer genomes that alterations in actin genes are different in distinct cancer subtypes, suggesting some specificity. These actin gene alterations include (missense) mutations, gene fusions and copy number alterations (deletions and amplifications) and we illustrate their occurrence for a limited number of examples including actin mutations in lymphoid cancers and nonmelanoma skin cancer and actin gene copy number alterations for breast, prostate and liver cancers. A challenge in the future will be to further sort out the specificity per actin gene, alteration type and cancer subtype. Even more challenging is (experimentally) distinguishing between cause and consequence: which alterations are passengers and which are involved in tumor progression of particular cancer subtypes?

Pseudomyogenic Hemangioendothelioma Involving the Esophagus

Herein, we describe the case of a 20-year-old woman who presented with dysphagia of 2 months’ duration associated with vomiting, moderate abdominal pain, decreased oral intake, and significant weight loss. During the past 3 years, the patient experienced intermittent mild abdominal pain with infrequent vomiting. Endoscopy at Jordan University Hospital showed a mass in the esophagus, and endoscopic biopsies were performed. The preliminary histopathological report excluded malignancy. Two days after endoscopy, the patient presented to the emergency department complaining of severely worsening pain and total dysphagia. The pain persisted despite intravenous paracetamol administration, which was concerning for esophageal perforation; therefore, an urgent surgical intervention was performed. The mass was removed surgically, along with a para-esophageal lymph node. The final histopathological results of the endoscopic and resected specimens supported the diagnosis of pseudomyogenic hemangioendothelioma (PMHE). This is the first case reporting esophageal involvement of PMHE.

Pseudomyogenic hemangioendothelioma of bone with rare WWTR1-FOSB fusion gene: Case report and literature review

Pseudomyogenic hemangioendothelioma rarely arises in bone. WWTR1-FOSB fusion gene is rarely reported in PMHE of bone. Currently, fusion genes can be used as diagnostic markers in PMHE; however, their prognostic and therapeutic significance is unclear.

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.

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.

EWSR1-SMAD3 rearranged fibroblastic tumor: Case series and review

We report the largest series to date (N = 6) of EWSR1-SMAD3 rearranged fibroblastic tumor. Initially described in 2018, the tumor features a marked female predominance (F:M, 5:1, mean age 44-years, median age 45.5 years; range 27-57), with most cases (5/6, 83%) arising in acral locations (4 on foot/toe, 1 on hand). One case presented on the lower extremity. The lesions presented as nodules and were composed of short, variably cellular, intersecting fascicles of uniform spindled cells in a collagenous to myxoid stroma. In four cases, the tumor abutted the epidermis without a grenz zone. In one case, there was an abrupt transition to a central, acellular hyalinized area. Two other cases had admixed smaller collagenous areas, reminiscent of collagen rosettes. One had a concentric arrangement of tumor cells around blood vessels. Mitotic activity was low (<1/10 HPFs). All were positive for ERG by immunohistochemistry and negative for CD34 (6/6). An EWSR1-SMAD3 fusion was identified in three cases tested by next-generation sequencing (3/3). Rearrangement of EWSR1 by fluorescence in situ hybridization was showed in 1/1 case. Our series reaffirms prior findings and expands the known histopathologic spectrum of this emerging entity.

Epithelioid hemangioma of bone harboring FOS and FOSB gene rearrangements: A clinicopathologic and molecular study

The diagnosis of epithelioid hemangioma (EH) remains challenging due to its rarity, worrisome histologic features, and locally aggressive clinical and radiographic presentation. Especially in the bone, EH can be misdiagnosed as a malignant vascular neoplasm due its lytic, often destructive or multifocal growth, as well as atypical morphology. The discovery of recurrent FOS and FOSB gene fusions in the pathogenesis of most EH has strengthened its stand-alone classification, distinct from other malignant epithelioid vascular lesions, such as epithelioid hemangioendothelioma or angiosarcoma. In this study we investigate a group of molecularly confirmed skeletal EH by the presence of FOS or FOSB gene rearrangements to better define its clinical and pathologic characteristics within a homogenous molecular subset. The cohort included 38 patients (25 males, 13 females), with a mean age at diagnosis of 38 years (range, 4-75). Regional, multifocal presentation was noted in 10 cases. Only six cases were correctly recognized as EH by the referring institutions, while most were misdiagnosed as other vascular tumors. Of the 17 patients with follow-up data available, five patients (29%) developed local recurrence after marginal en bloc excision (n = 3) or curettage (n = 2). Local recurrence-free survival rates were 84% at 3 years and 38% at 5 years. No metastasis or disease-related death was identified. Imaging studies exhibited no specific features, showing cortical bone destruction and soft-tissue extension in 14 (38%) cases. FOS gene rearrangements were detected in 28 (74%) of cases, while FOSB rearrangements in 10 (26%) cases. Our results highlight the significant challenges encountered in establishing a correct diagnosis exclusive of the molecular testing, mainly due to its overlap to other malignant epithelioid vascular tumors. Skeletal EH emerges as a genetically defined locally aggressive vascular neoplasm, with a high rate of local recurrence, but lacking the propensity for distant spread.

Vascular tumors of the mediastinum

Vascular tumors represent only a sliver of all tumors affecting the mediastinum, but they pose diagnostic challenges due to significant overlap among entities, ever-evolving classification schemes, and the exquisite rarity of some of the entities not only in the mediastinum but in pathology practice as a whole. Most of the vascular tumors are better known to the practice of soft tissue pathology, from which some of the knowledge of clinical behavior can be extrapolated. For example, the stratification of epithelioid hemangioendothelioma (EHE) into two biologically separate categories has effectively translated from the somatic soft tissues to the thorax. For other entities, the effective validation of soft tissue parameters is hindered by the small numbers of reported mediastinal cases. Many of the treatment modalities have also transferred over, with the key differences resting on the difficulty in achieving complete surgical resection for mediastinal tumors as opposed to those in the somatic soft tissues. Accordingly, systemic drug therapies have emerged as attractive options for some of the mediastinal vascular tumors, such as kaposiform hemangioendothelioma (KHE) and Kaposi sarcoma (KS). The categories presented herein mirror the classification scheme set forth by the 5th Edition WHO Classification of Soft Tissue and Bone Tumors. This review focuses on the biologically aggressive vascular neoplasms while limiting discussion of the benign entities to relevant comparisons in the differential diagnoses. While distinguishing among the benign entities has academic importance, in practice, the stakes are far higher for diagnosing the biologically aggressive tumors given their marked heterogeneity in clinical outcomes. Practical advice for problem areas in pathology will be reviewed alongside tumor pathobiology, including the latest in molecular diagnostics.

Cytomorphological features of an unusual case of a multifocal pseudomyogenic (epithelioid sarcoma-like) hemangioendothelioma in a young adult male

Pseudomyogenic (epithelioid sarcoma-like) hemangioendothelioma (PMHE) is a relatively newly described vascular neoplasm, characterized by distinct clinicopathological and molecular features, with no report on its cytomorphological features on smears, till date. A 17-year-old male presented with multiple nodules on his left upper limb. Radiologic imaging disclosed multiple lesions involving the soft tissues, skin, and bones of his left upper limb. Biopsy of one of the lesions on his left ring finger revealed a cellular tumor comprising plump spindle and epithelioid cells, containing moderate to abundant, eosinophilic cytoplasm; mild nuclear atypia, vesicular nuclear chromatin, discernible nucleoli, infrequent mitotic figures, and interspersed neutrophils. Immunohistochemically, tumor cells were positive for AE1/AE3, CD31, and FLI-1, while negative for CD34 and desmin. INI1/SMARCB1 was retained. MIB1/Ki67 highlighted nearly 3% tumor cells (low). Diagnosis of a PMHE was offered. A simultaneous fine needle aspiration cytology smears of the lesions in his left forearm and ipsilateral axillary region revealed spindle and some polygonal shaped cells, arranged in tight and loose clusters, as well as scattered singly, containing abundant cytoplasm with tapering cell membranes, at places; vesicular nuclear chromatin and characteristic intercellular fibrillary stromal material. The present case constitutes the first report on cytomorphological features of a PMHE, diagnosed on cytology, including its differential diagnoses, immunohistochemical features with molecular updates.

Two Cases of Intraosseous Pseudomyogenic (Epithelioid Sarcoma-Like) Hemangioendothelioma With Unusual Features, Expanding the Clinicopathological Spectrum

Pseudomyogenic (epithelioid sarcoma-like) hemangioendothelioma (PMHE) is a distinct vascular neoplasm mostly observed in the lower extremities of young adults with characteristic histopathological features. In this article, we present 2 unusual cases of PMHE. Case 1: A 28-year-old male presented with pain and stiffness in his left shoulder. Radiologic examination revealed an expansile, lytic, heterogeneously enhancing, destructive lesion in his left scapula, along with multiple avid marrow lesions in his other bones. Biopsy revealed a cellular tumor composed of plump spindly and epithelioid cells arranged in fascicles and focally, in clusters, admixed with neutrophils and histiocytes, with interspersed many osteoclast-like giant cells and reactive bone. Case 2: A 63-year-old male presented pain and swelling in his right middle finger, with no other lesion elsewhere in his body. Radiologic imaging disclosed a 1.5-cm-sized lobulated, expansile, lytic, destructive lesion in the distal end of the third metacarpal bone of his right hand. Biopsy examination revealed a cellular spindle cell tumor, composed of plump spindly cells, arranged in fascicles, including "tadpole-like" or "strap-like" cells and interspersed inflammatory cells. Immunohistochemically, tumor cells in both cases were positive for AE1/AE3, CD31, and Fli1, while negative for desmin and CD34. INI11 was retained. The presented cases of PMHE, occurring at unusual locations, in an older individual in the second case, along with variable histopathological features, noted in the first case, seem to expand the clinicopathological spectrum of these uncommon neoplasms. Differential diagnoses and review of similar cases are presented.