Cutaneous nodular fasciitis with genetic analysis: a case series

Novel ACTB::FER Promoter Swap Fusion Characterizes Rare Superficial Myoid/Myofibroblastic Tumors

Pediatric fibroblastic, myofibroblastic, and myoid tumors encompass several entities, many with characteristic gene fusions that are now emerging as molecularly defined tumor groups. Here, we present two cases of spindle cell neoplasms with novel ACTB::FER promoter swap fusions. Both tumors presented in the extremities of pediatric patients (9-year-old and 6-year-old females) as superficial skin nodules with slow growth. Histologically, both tumors showed monomorphic spindle cell proliferation in short fascicles, but without significantly increased mitotic activity, high-grade atypia, or necrosis. Both cases showed diffuse positivity for SMA with patchy desmin expression. RNA sequencing confirmed fusion breakpoints, revealing transcriptional upregulation of FER. Neither patient has had evidence of interval growth or recurrence to date. While the biological significance of ACTB::FER fusions remains unclear, their recurrence and the absence of other clear oncogenic drivers suggest a distinct molecular pathway that may define a novel entity. Fusions of ACTB and FER genes with different partners have been observed in rare aggressive mesenchymal tumors; however, the ACTB::FER promoter swap fusion is currently unrecognized in soft tissue tumors. We report the first two cases of soft tissue tumors harboring ACTB::FER fusions and expand the molecular spectrum of mesenchymal tumors with kinase gene alterations. Further, we highlight the importance of target-agnostic approaches for the detection of rare kinase fusions, which may not be included on targeted next-generation sequencing panels.

USP6 rearrangement in pediatric nodular fasciitis

Nodular fasciitis (NF) is a myofibroblastic proliferation that uncommonly presents in pediatric patients. These benign neoplasms can masquerade as more insidious sarcomatous proliferations on both clinical exam and initial histopathologic review, often prompting undue concern in patients, parents and providers. While immunohistochemical analysis of NF can be variable, adding to the diagnostic uncertainty, molecular analysis documenting ubiquitin-specific protease 6 (USP6) gene rearrangement can help confirm the diagnosis as an association between NF and USP6 overexpression was first identified 10 years ago in an analysis that found rearrangements of the involved locus in over 90% of studied samples.¹ In this report, we review one case of nodular fasciitis located on the chin of a 9-year-old in which molecular testing was essential to secure the correct diagnosis, and provide a summary of documented cases of USP6 overexpression in transient pediatric neoplasms. This article is protected by copyright. All rights reserved.

Intraarticular Nodular Fasciitis of the Knee With MHY9-USP6 Fusion: A Case Report

Background. Nodular fasciitis (NF) is a self-limiting, benign mesenchymal neoplasm of fibroblastic/myofibroblastic origin. Due to the fast growth, cellularity, and frequently observed high mitotic count, it is commonly misdiagnosed as a sarcoma, often resulting in overtreatment. Intraarticular examples of NF are extremely rare. Radiologically, NF can mimic fibroma of the tendon sheath, tenosynovial giant cell tumor, and synovial chondromatosis. Histology can vary from hypercellular, mitotically active lesions to fibrotic, less cellular ones, and can, therefore, mimic other benign and low-grade malignant neoplasms. Recently, the MYH9-USP6 fusion has been found in up to 92% of NF. Case Presentation. In this article, we report a case of a 38-year-old patient with an intraarticular lesion, radiologically suspicious of tenosynovial giant cell tumor. Histology demonstrated a spindle cell lesion composed of fibroblasts/myofibroblasts embedded in a highly collagenous/hyalinized stroma, partly arranged in short fascicles. Extravasated erythrocytes and rare mitotic figures were present. Immunohistochemically, tumor cells expressed smooth muscle actin and were negative for desmin, β-catenin, CD34, and SOX10. These findings rendered the diagnosis of NF. Molecular analysis using next-generation sequencing (Archer FusionPlex Sarcoma Panel) revealed gene rearrangement involving USP6 and MYH9 supporting the diagnosis of NF in the knee joint. Conclusions. Radiological and histological features of NF can overlap with other benign and low-grade malignant lesion. Identification of the USP6 gene rearrangements or finding of the MYH9-USP6 fusion, especially in core needle biopsies and in the lesions occurring at unusual sites, can result in adequate therapeutic approach avoiding overtreatment.

USP6-Associated Neoplasms: A Rapidly Expanding Family of Lesions

Nearly 20 years ago, the first description of a translocation involving chromosome 17 on which USP6 resides was described. Since then, not only the culprit gene but also many fusion partners, leading to transcriptional activation of USP6, have been detected. The first neoplasm known to harbor USP6 rearrangements was aneurysmal bone cyst. Since then, other entities like nodular fasciitis, myositis ossificans, fibro-osseous pseudotumor of digits, and a subgroup of fibromas of tendon sheath, probably representing tenosynovial nodular fasciitis, have been added to the list of USP6-rearranged lesions. Remarkably, all of them share clinical as well as morphological characteristics, and authors have suggested that these entities actually belong to the same spectrum. This review summarizes the current knowledge regarding USP6-rearranged lesions and further elaborates on how these neoplasms relate to one another. We propose to call these lesions UAN (Usp6-associated neoplasm).

Imaging findings of radiologically misdiagnosed nodular fasciitis

BACKGROUND

Nodular fasciitis rarely occurs in young adults and children; it usually resembles other tumors, even malignancy.

PURPOSE

To review the imaging findings of six cases of nodular fasciitis misdiagnosed radiologically.

MATERIAL AND METHODS

The clinical and radiologic features of six cases of histologically proven but radiologically misdiagnosed nodular fasciitis were reviewed retrospectively. Two cases underwent both plain and enhanced computed tomography (CT) scans and the other four had both regular and enhanced magnetic resonance (MR) scans.

RESULTS

All six patients were young (five children and one young adult). A rapid growing mass, pain or painless, was the most frequent presentation. Most masses were oval, well-defined, and homogeneous, with an average diameter of 2.2 cm. Five were found in superficial fascia with a broad base. Two cyst-like masses showed hypodensity relative to muscle on plain CT and without enhancement. Compared to muscle, these masses showed isointensity (n = 3) or slight hyperintensity (n = 1) on T1-weighted imaging, hyperintensity on T2-weighted imaging (n = 4), with homogeneous notable enhancement (n = 3) or mild enhancement (n = 1). Five (83.3%) were found with a "fascial tail" sign characterized as thickening of adjacent fascial layer with notable enhancement. One mass showed an "inverted target" sign.

CONCLUSION

Nodular fasciitis in young adults and children is usually superficial, rapid growing, well-defined, and homogeneous, frequently with a "fascial tail" sign. Radiologically, it can resemble a benign cyst and might be easily misdiagnosed. Therefore, nodular fasciitis should be remembered in the differential diagnosis for superficial soft tissue tumor found in young adult and children.

Nodular fasciitis: A rapidly enlarging destructive periorbital mass in an infant

Purpose

To review the clinical and histopathological features of nodular fasciitis, a rare benign periorbital tumor that mimics orbital malignancy, by presenting a case involving an infant with marked orbital wall erosion requiring repair.

Observations

A 9-month-old boy developed a rapidly growing periorbital mass concerning for a soft tissue malignancy. Computerized tomography (CT) scans showed bony erosion of the lateral orbital wall. Incisional biopsy revealed nodular fasciitis. USP6 gene rearrangement was negative. The tumor was completely excised and the underlying orbital wall defect was repaired with polydioxanone (PDS) plate.

Conclusions and Importance

Nodular fasciitis is a benign periorbital tumor that presents like malignancies and warrants prompt investigations, especially in children. Orbital wall erosion is rare and can be repaired to yield good functional and cosmetic outcome.

[Morphological spectrum of USP6 rearranged lesions]

USP6, also known as Tre-2 and TRE17, is an ubiquitase-specific proteinase that was identified more than two decades ago as a potential oncogene when it exhibited transforming properties upon overexpression in NIH 3T3 cells. Until recently, however, little was known about the function and the oncogenetic activation of USP6. The identification of rearrangements of the USP6 gene in aneurysmal bone cyst and in nodular fasciitis has not only led to a better understanding of the pathogenesis of these entities, but is also a useful tool in their diagnosis and differential diagnostic delineation from morphological mimics. In this review, the clinical, pathomorphological, and molecular genetic aspects of aneurysmal bone cyst and of nodular fasciitis, as well as from related lesions, are presented and discussed.

CT and MRI of superficial solid tumors

Superficial solid masses are common conditions in clinical practice, however, some of which can be easily diagnosed and others would be difficult. Although imaging of superficial masses is not always characteristic, it would be helpful to give a definitive diagnosis or narrow a differential diagnosis. Crossing-section imaging can depicture the masses directly, find some pathognomonic signs and demonstrate their relationship with adjacent structures, which can provide decision support for clinician's reference. Computed tomography (CT) can be used to detect calcifications and bone erosion which could not be seen on radiographs. Magnetic resonance imaging (MRI) is the preferred way for evaluating soft tissue lesions and provides information on hemorrhage, necrosis, edema, cystic and myxoid degeneration, and fibrosis. Other advantages of MRI are its superior soft tissue resolution and any profile imaging, which can aid the assessment of extension and adjacent infiltration. Positron emission tomography (PET)/CT and PET/MRI have been increasingly used in bone and soft tissue sarcomas and provides advantages in the initial tumor staging, tumor grading, therapy assessment, and recurrence detection. Therefore, imaging examination can play an important role in treatment decision making for superficial solid tumors. Here we review the important conditions presenting as superficial mass and show the imaging of typical cases diagnosed in our hospital.

Nonepithelial Tumors and Tumor-like Lesions of the Skin and Subcutis in Children

This overview of mesenchymal tumors presenting in the skin and/or subcutis in children brings together the range of neoplasms and hamartomas which are seen in this age-group. It is not surprising from the perspective of the pediatric or general surgical pathologist that vascular anomalies, including true neoplasms and vascular malformations, are the common phenotypic category. Since there is considerable morphologic overlap among these lesions, clinicopathologic correlation may be more important than for many of the other mesenchymal tumors. The skin and subcutis are the most common sites of clinical presentation for the infantile myofibroma which is the most common of fibrous mesenchymal tumors in children. Several of the other mesenchymal tumors are more common adults-like dermatofibrosarcoma protuberans, but nonetheless have an important presence in children, even as a congenital neoplasm. A lipomatous tumor in a young child should be considered as a possible manifestation of an overgrowth syndrome.

USP6 activation in nodular fasciitis by promoter-swapping gene fusions

Nodular fasciitis is a self-limited myofibroblastic lesion that can be misdiagnosed as a sarcoma as a result of its rapid growth, cellularity, and sometimes prominent mitotic activity. A recurrent translocation t(17;22) has been identified in nodular fasciitis, fusing the coding region of USP6 to the promoter region of MYH9, and resulting in increased USP6 expression. A subset of cases show USP6 rearrangement without the typical fusion variants by RT-PCR, or any MYH9 rearrangement by FISH. We sought to further characterize such tumors using molecular diagnostic assays. A novel RT-PCR assay was designed to detect the two known MYH9-USP6 fusion types in formalin-fixed paraffin-embedded and frozen tissue, and a break-apart FISH assay was designed to detect USP6 rearrangement. Twenty-six cases of nodular fasciitis diagnosed between 2002 and 2013 were retrieved from the pathology files of our institutions and were confirmed to be positive by FISH and/or RT-PCR. Seven samples showed USP6 rearrangement by FISH but were negative for MYH9-USP6 fusion by RT-PCR; these cases were subjected to a next-generation sequencing assay utilizing anchored multiplex PCR technology. This assay targets a single partner gene associated with fusions in bone and soft tissue tumors for agnostic detection of gene fusion partners. Novel fusion partners were identified in all seven cases and confirmed by RT-PCR. Structurally, all fusions consisted of the juxtaposition of the entire coding region of USP6 with the promoter of the partner gene, driving increased USP6 expression. This study confirms the neoplastic nature of nodular fasciitis, defines additional pathogenic fusion partners, and adds to the growing body of literature on USP6-associated neoplasia. Given the diagnostic challenges of these tumors, molecular assays can be useful ancillary tools; however, the prevalence of promoter swapping must be recognized when interpreting results.