USP6gene rearrangement in nodular fasciitis and histological mimics

Physiological roles and therapeutic implications of USP6

Ubiquitin-specific protease 6 (USP6) is a member of deubiquitinating enzyme family, recognized for its essential roles in physiological and pathological processes. USP6 is initially identified as a hominoid-specific enzyme residing on chromosome 17p13. USP6 is involved in regulating cellular functions, signaling pathways, protein degradation, intracellular trafficking, tumorigenesis and immune responses. USP6 is pivotal in signaling pathways, including NF-κB, JAK-STAT, and Wnt, which are fundamental for maintaining cellular homeostasis and mediating stress responses. Dysregulation of USP6 has been implicated in a spectrum of diseases, including bone tumors, breast and colorectal cancers, cranial fasciitis, and neurological disorders such as memory dysfunction. Furthermore, USP6 is involved in emerging therapeutic strategies highlighting its implications for drug development. A number of potential small molecule inhibitors are known to be responsible for suppression of USP6, such as Momelotinib (CYT387), FT385, USP30 Inh-1, -2 and -3, 2,6-Diaminopyridine-3,5-bis(thiocyanate) (PR-619) and so on. This review explores the emerging role of USP6 as a key regulator of cellular signaling pathways, its involvement in disease progression, its physiological functions, and the inhibitors that effectively suppress USP6 activity in detail. The comprehensive study provides insight to enhance our understanding of biological importance and therapeutic interventions of USP6 in drug development.

Nodular Fasciitis of the Nose and External Auditory Canal: Two Rare Case Reports

Nodular fasciitis(NF) is a proliferative disease of fibroblasts and myofibroblasts that generally affects subcutaneous tissue, muscle tissue, and fascia. NF usually occurs in young adults aged 20-40 and is more common in the upper extremities and relatively rare in the region of the head and neck. Here, we report on two patients with NF in the ear and nose. Under general anesthesia, the masses of NF were completely resected along the safety margin. The patients recovered well after surgery and there was no recurrence after more than half a year of follow-up.

Nodular fasciitis: a case series unveiling novel and rare gene fusions, including two cases with aggressive clinical behavior

Nodular fasciitis is a benign myofibroblastic tumor characterized by rapid growth and spontaneous regression. While nodular fasciitis is typically an indolent process, rare cases with benign morphologic features have developed metastases. Conversely, nodular fasciitis with malignant histologic features and benign clinical course have also been reported. In this study, we present seven nodular fasciitis cases with novel USP6 gene fusion partners, in addition to two cases with rare fusions that displayed aggressive clinical behavior. The cohort comprised five females and four males with a median age of 36 years (range 13-59). Tumors were located in the forearm (n = 3), thigh (n = 2), and shoulder, abdominal wall, chest wall, and oral cavity (one each), ranging from 1.4 to 24.0 cm in size (median, 2.2 cm). Except for the clinically aggressive cases, patients presented with painless masses of varying onset from days to months. Of the clinically aggressive cases, one patient presented with a slowly growing subfascial thigh/hip mass over nine years, leading to erosion of the femur and pelvis; the other presented with a painful subfascial thigh mass of several months' duration. Histologically, all cases, including the clinically aggressive ones, showed conventional nodular fasciitis features without nuclear pleomorphism or atypical mitotic figures; one case with aggressive clinical behavior exhibited focal infarction-type necrosis. Break-apart FISH analysis using USP6 flanking probes failed to detect USP6 rearrangement in two cases (false negatives) and was inconclusive in one case. Next-generation RNA sequencing identified USP6 fusions in all cases. The clinically aggressive cases showed fusions with COL1A1 (exon 1) and PPP6R3 (exon 1), while novel fusions were identified in the remaining cases including EIF4A1 (exon 1), FILIP1L (exon 2), NF1 (exon 33), OMD (exon 1), PFN1 (exon 1), RLIM (exon 1), and SETD5 (exon 1). Six patients underwent surgical resection; three were managed conservatively, with two experiencing spontaneous tumor resolution. Of the clinically aggressive cases, one patient had progression of the tumor with erosion of the underlying bone, and the second patient developed local recurrence at 14 months and lung metastasis at 19 months, ultimately dying of disease at 22 months. The remaining patients showed no recurrence or metastasis. Our findings expand the spectrum of USP6 gene fusion partners in nodular fasciitis and, for the first time, report cases with conventional morphology exhibiting aggressive behavior, including death. These observations raise the question of whether a subset of deep lesions with conventional nodular fasciitis histology but unusual clinical features, such as large tumor size, represents malignant nodular fasciitis or alternatively a nodular fasciitis-like myofibroblastic sarcoma.

An Examination of the Diagnostic Utility of Ubiquitin-Specific Peptidase 6 (USP6) Rearrangement in Differentiating Nodular Fasciitis From Inflammatory Myofibroblastic Tumor: A Case Report

Nodular fasciitis (NF) is a benign yet diagnostically challenging mesenchymal myofibroblastic proliferation that often mimics the histological features of inflammatory myofibroblastic tumors (IMTs) and soft tissue sarcomas. The overlap in histopathological appearance, compounded by the variability in immunohistochemical (IHC) staining, frequently leads to diagnostic uncertainty. In this report, we present a case of a rapidly expanding lesion on the left medial mandible, ultimately diagnosed as NF. Molecular analysis through fluorescence in situ hybridization (FISH) identified a ubiquitin-specific peptidase 6 (USP6; 17p13.2) gene rearrangement, a distinctive marker of NF, which played a critical role in confirming the diagnosis. IHC analysis, including negative staining for cytokeratin and ALK, further helped differentiate this benign entity from other IMTs and malignancies, highlighting the importance of combining molecular diagnostics with traditional histopathological techniques to ensure accurate classification and avoid misdiagnosis.

Large Nodular Fasciitis of the Shoulder Presenting as Soft Tissue Sarcoma: A Case Report

CASE

This case report describes a patient who presented with clinical and radiographic features of a soft tissue sarcoma of the shoulder. Despite having a painless and relatively large mass, a biopsy and resection revealed nodular fasciitis (NF).

CONCLUSION

This is an unusual case of a painless 10 cm mass that histopathologically was diagnosed as NF in the upper extremity with proximity to the axillary nerve and posterior humeral circumflex vessels. The USP6 rearrangement was helpful in confirming the diagnosis. Careful clinical, radiographic, and pathologic correlation is necessary in diagnosing these relatively rare tumors. In cases where there are discordant findings, molecular markers can be very helpful.

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.

Unravelling the USP6 gene: an update

Ubiquitin-specific protease 6 (USP6) rearrangements have been identified in aneurysmal bone cyst, nodular fasciitis, myositis ossificans, fibro-osseous pseudotumour of digits and cellular fibroma of tendon sheath. These entities show clinical as well as histological overlap, suggesting they are all clonal neoplastic belonging to the same biological spectrum and referred to as 'USP6-associated neoplasms'. They all show a characteristic gene fusion formed by juxtaposition of the USP6 coding sequences to the promoter regions of several partner genes, leading to USP6 transcriptional upregulation.

An update on selected cutaneous (myo) fibroblastic mesenchymal tumors

Cutaneous (myo)fibroblastic tumors constitute a group of tumors with overlapping clinicopathological features and variable biologic behavior. In the present review we focus on the histomorphology, immunohistochemical profile and molecular background of the following entities: dermatofibrosarcoma protuberans (DFSP), CD34-positive fibroblastic tumor (SCD34FT), myxoinflammatory sarcoma (MIFS), low-grade myofibroblastic sarcoma, solitary fibrous tumor and nodular fasciitis. Although some of these entities typically arise in deep-seated locations, they may occasionally present as cutaneous/superficial tumors and might be challenging to recognize. This review covers in depth the latest advances in molecular diagnostics and immunohistochemical markers that have significantly facilitated the correct classification and diagnosis of these neoplasms.

Case report: Large-size intramuscular nodular fasciitis, a challenging histopathologic diagnosis confirmed by molecular detection of USP6 gene rearrangement: Case report and literature review

The intramuscular subtype of nodular fasciitis (NF) is rare with lesions normally not more than 2 cm in size and characterized by pseudosarcomatous morphology. We report a case of a 27-year-old man with a large-size intramuscular NF. The patient came for treatment complaining of an increasingly enlarged mass in the left upper arm for 4 months. Magnetic resonance imaging (MRI) confirmed the presence of a well-defined tumor measuring 5 cm within the outer edge of the middle humerus. Microscopically, the neoplasm was rich in fibroblasts and myofibroblasts in an interlaced pattern with high mitotic index and evident multinuclear giant cells. Erythrocyte extravasation was easily seen in the stroma. The tumor border was infiltrative. Immunohistochemically, the tumor cells were positive for smooth muscle actin (SMA) and negative for cytokeratin, desmin, H-Caldesmon, CD34, S100, ALK, and β-catenin. Fibrosarcoma was highly suspected by histopathological and immunohistochemical examination. Molecular detection demonstrated evidence of ubiquitin-specific peptidase 6 (USP6) gene rearrangement in this tumor. Based on the findings, the tumor was diagnosed as intramuscular NF. At 56 months after the initial surgery, the patient had recovered with no evidence of recurrence or metastasis. Large-size intramuscular NF is very rare and easily overdiagnosed as malignant tumor due to its obvious pseudosarcomatoid pathological features. USP6 gene rearrangement detection can effectively avoid this major misdiagnosis.

Molecular testing of soft tissue tumors

BACKGROUND

The diagnosis of soft tissue tumors is challenging, especially when the evaluable material procured is limited. As a result, diagnostic ancillary testing is frequently needed. Moreover, there is a trend in soft tissue pathology toward increasing use of molecular results for tumor classification and prognostication. Hence, diagnosing newer tumor entities such as CIC-rearranged sarcoma explicitly requires molecular testing. Molecular testing can be accomplished by in situ hybridization, polymerase chain reaction, as well as next generation sequencing, and more recently such testing can even be accomplished leveraging an immunohistochemical proxy.

CONCLUSION

This review evaluates the role of different molecular tests in characterizing soft tissue tumors belonging to various cytomorphologic categories that have been sampled by small biopsy and cytologic techniques.

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.

Laryngeal Manifestation of Nodular Fasciitis: A Case Report and Literature Review

Nodular fasciitis (NF) are non-neoplastic, fibroblastic lesions, typically located on the trunk and the extremities. The incidence of NF in the head and neck region is 13%-20%. However, a manifestation in the larynx of adult patients is extremely rare. Therefore, the occurrence of NF in this region can lead to diagnostic challenges and a high risk of misdiagnosis as well as potential mishandling when not aware of its possible laryngeal manifestation. Following emergency admission of a 41-year-old woman to the emergency department (ED) due to progressive dyspnea and inspiratory stridor a transnasal flexible laryngeal endoscopic examination revealed a left-lateral, subglottic mass. A subsequently performed CT demonstrated a 2.2 cm x 1.5 cm sized lesion of the subglottic larynx with profound stenosis of the lumen (Myer-Cotton grade III), no extraluminal extension, and no distant metastases. Histopathological processing of a tissue sample obtained by microlaryngoscopy and translaryngeal tracheoscopy revealed a spindle-cell lesion with immunohistochemical and molecular-pathogenic profile of NF. After tumor debulking and steroid infiltration (triamcinolone), a regrowth tendency quickly became apparent, which is why a tracheostomy had to be performed. Laryngectomy was rejected by the patient. After multiple transoral tumor reduction attempts, radiotherapy was performed according to an interdisciplinary tumor board decision to limit regrowth tendency. Subsequently, a substantial reduction of the tumor volume could be seen, although a discreet stenosis of the subglottic tracheal lumen persists in the follow-up. Laryngeal NF poses several challenges due to its rare occurrence in this location. This case report emphasizes the knowledge of this differential diagnosis and also depicts an interdisciplinary therapeutic approach aiming for function-preserving treatment of this benign but potentially relapsing pathology.

Inflammatory Myofibroblastic Tumor 12 Years After Treatment for Synovial Sarcoma: A Case Report

Inflammatory myofibroblastic tumors (IMTs) are mesenchymal neoplasms most seen in the abdominopelvic region, lung, and retroperitoneum; and less commonly seen in virtually any other site. We report a case of two lower limb masses consistent with diagnosis of IMTs. This is a 39-year-old woman with a history of right lower extremity popliteal fossa synovial sarcoma diagnosed 12 years prior and treated with chemotherapy, surgery, and radiation. She presented with two new - one anterior and one posterior - right thigh masses. Biopsies of the lesions demonstrated low-grade inflammatory spindle cell lesions at both sites. Wide resection was performed for both masses and further characterization of the surgical specimens was most consistent with IMT. At follow-up, the patient is well with no signs of recurrence 19 and 7 months postoperative to the resection of the anterior and posterior thigh masses, respectively. This case represents the first reported IMTs occurring as late as 12 years after primary cancer treatment, and the first occurring after synovial sarcoma.

Nodular fasciitis of the breast: clinicopathologic and molecular characterization with identification of novel USP6 fusion partners

Nodular fasciitis is a benign, self-limited, pseudosarcomatous neoplasm that can mimic malignancy due to its rapid growth, cellularity, and mitotic activity. Involvement of the breast is rare and diagnosis on biopsy can be challenging. In this largest series to date, we examined the clinicopathologic and molecular characteristics of 12 cases of nodular fasciitis involving the breast/axilla. All patients were female, with a median age of 32 years (range 15-61). The lesions were 0.4 to 5.8 cm in size (median 0.8). All cases presented as palpable masses, and two patients had overlying skin retraction. Microscopically, lesions were relatively well-circumscribed nodular masses of bland myofibroblastic spindle cells within a variably myxoid stroma. Infiltrative growth into adipose tissue or breast epithelium was frequent. Mitotic figures were present in all cases, ranging from 1 to 12 per 10 high-power fields (median 3). Immunohistochemically, all cases expressed smooth muscle actin and were negative for pan-cytokeratin, p63, desmin, CD34, and nuclear beta-catenin. Targeted RNA sequencing performed on 11 cases identified USP6 gene fusions in eight; one additional case was positive by break-apart fluorescence in situ hybridization. The common MYH9-USP6 rearrangement was detected in four cases; another case had a rare alternative fusion with CTNNB1. Three cases harbored novel USP6 gene fusions involving NACA, SLFN11, or LDHA. All fusions juxtaposed the promoter region of the 5' partner gene with the full-length coding sequence of USP6. Outcome data were available for eight patients; none developed recurrence or metastasis. Five patients elected for observation without immediate excision, and self-resolution of the lesions was reported in three cases. Albeit uncommon, nodular fasciitis should be considered in the differential diagnosis of breast spindle cell lesions. A broad immunohistochemical panel to exclude histologic mimics, including metaplastic carcinoma, is important. Confirmatory detection of USP6 rearrangements can aid in classification, with potential therapeutic implications.

Nodular fasciitis of the breast: Report of two cases illustrating the diagnostic implications for USP6 gene rearrangement and brief review of the literature

Nodular fasciitis is a benign, self-limited, pseudosarcomatous neoplasm that is cytogenetically characterized by recurrent USP6 gene rearrangement. Involvement of the breast by nodular fasciitis is very rare with only a few documented cases. It can clinically, radiologically and histologically mimic a malignancy, posing significant diagnostic challenges to clinicians, radiologists, and pathologists. In this study, we report 2 cases of nodular fasciitis occurring in the female breast, reviewing the literature and emphasizing the application of fluorescence in situ hybridization analysis of USP6 gene rearrangement in its diagnosis and differential diagnosis.

Recurrent FOS rearrangement in proliferative fasciitis/proliferative myositis

Proliferative fasciitis (PF) and proliferative myositis (PM) are rare benign soft tissue lesions, usually affecting the extremities of middle-aged or older adults. Presenting as poorly circumscribed masses, they histologically show bland spindle cell proliferation in a myxoid to fibrous background and a hallmark component of large epithelioid "ganglion-like" cells in various numbers, which may lead to their misdiagnosis as sarcoma. PF/PM has been long considered as reactive, akin to nodular fasciitis; however, its pathogenesis has remained unknown. In this study, we analyzed the FOS status in 6 PF/PMs (5 PFs and 1 PM). Five PF/PMs occurred in adults, all showing diffuse strong expression of c-FOS primarily in the epithelioid cells, whereas spindle cell components were largely negative. Using fluorescence in situ hybridization (FISH), all 5 c-FOS-immunopositive tumors showed evidence of FOS gene rearrangement in the epithelioid cells. RNA sequencing in 1 case detected a FOS-VIM fusion transcript, which was subsequently validated by reverse transcriptase-polymerase chain reaction, Sanger sequencing, and VIM FISH. The one pediatric PF case lacked c-FOS expression and FOS rearrangement. c-FOS immunohistochemistry was negative in 45 cases of selected mesenchymal tumor types with epithelioid components that may histologically mimic PF/PM, including pleomorphic sarcoma with epithelioid features and epithelioid sarcoma. Recurrent FOS rearrangement and c-FOS overexpression in PF/PM suggested these lesions to be neoplastic. FOS abnormality was largely restricted to the epithelioid cell population, clarifying the histological composition of at least 2 different cell types. c-FOS immunohistochemistry may serve as a useful adjunct to accurately distinguish PF/PM from mimics.

Nodular fasciitis: a comprehensive, time-correlated investigation of 17 cases

The self-limited nature of nodular fasciitis (NF) is well-known but its precise mechanism has not yet been clarified. We observed that "young" NF (preoperative duration <1 month) consistently contains a higher percentage (~80%) of USP6 break-apart FISH signals than "old" NF (preoperative duration >3 months) (~20%). Thus, we hypothesized that our original observation may reflect a connection with the self-limited nature of NF. Seventeen cases with reliable data concerning the onset were selected, thus approximating the lifetime of each tumor. Besides the USP6 interphase FISH examination, we also checked the most common MYH9-USP6 fusion using RT-PCR. Because of the known pathways of the tumorigenesis of NF, the mRNA level of USP6, TRAIL, IFN-beta, JAK1, STAT1, STAT3, JUN, and CDKN2A was measured using qRT-PCR. Regarding proteins, USP6, p16, p27, TRAIL, and IFN-beta were examined using immunohistochemistry. Targeted gene panel next-generation sequencing (NGS) of three cases was additionally performed. We found a strong negative correlation (p = 0.000) between the lifetime and percentage of USP6 break-apart signals and a strong positive relationship (p = 0.000) between USP6 break-apart signals and mitotic counts. Results of immunostainings, along with qRT-PCR results, favored the previously-suggested USP6-induced negative feedback mechanism through activation of TRAIL and IFN-beta, likely resulting in apoptosis and senescence of tumor cells harboring USP6 fusions. Targeted-NGS resulted in the detection of several variants, but no additional recurrent changes in the pathogenesis of these tumors. We revealed on a cellular level the USP6-induced negative feedback mechanism. In conclusion, we emphasize that in "old" NF, the percentage of USP6 break-apart FISH signals can be as low as 14-27% which can be very important from a differential diagnostic point of view. We emphasize that a careful examination and interpretation of the NGS data is needed before clinical decision-making on treatment.

An aggressive nodular fasciitis lesion protruding from the palm (USP6 gene fusion helps differentiate from sarcomas): A case report

Nodular fasciitis is a mesenchymal lesion, which has been viewed as a reactive process. The MYH9-USP6 fusion gene was recently detected in nodular fasciitis, and nodular fasciitis is now considered to be a self-limiting neoplastic process. Recently, a case of nodular fasciitis that recurred a number of times and metastasized to soft tissues was reported, and the features of aggressive cases of nodular fasciitis are currently under investigation. Here, a case of locally aggressive nodular fasciitis is presented, in which the lesion grew rapidly and caused ulnar nerve palsy. The lesion was locally controlled via marginal excision, and no metastasis was identified at 24 months post-operation. Histologically, the lesion was consistent with nodular fasciitis, and the detection of the MYH9-USP6 fusion gene supported the diagnosis. Although most nodular fasciitis lesions are benign, some may be locally aggressive or even metastasize. In the case outlined in the present study, marginal excision was sufficient to locally control the lesion.

Papillary thyroid carcinoma with prominent myofibroblastic stromal component: clinicopathologic, immunohistochemical and next-generation sequencing study of seven cases

Papillary thyroid carcinoma with desmoid-type fibromatosis or nodular fasciitis-like stroma is an extremely unusual and poorly understood subtype of papillary thyroid cancer. Although prior studies have demonstrated alterations in the Wnt/β-catenin signaling pathway in some of these tumors, controversy still exists regarding the nature of the stromal spindle component. We have studied seven cases of papillary thyroid carcinoma with prominent myofibroblastic stroma, including six men and one woman aged 20-65 years (mean age = 44). All cases displayed areas consistent with conventional papillary thyroid carcinoma embedded in abundant myofibroblastic-like stroma. The myofibroblastic stroma in six cases resembled desmoid-type fibromatosis and in one case it more closely resembled nodular fasciitis. By immunohistochemical staining, the stromal spindle component showed positivity for SMA and low MIB1 proliferation index in all cases, and there was at least patchy strong nuclear positivity for beta-catenin in six/seven cases. Stains for cytokeratin AE1/AE3 and PAX8 were positive in the epithelial elements but negative in the stromal component. Next-generation sequencing was performed on six of seven cases. CTNNB1 gene mutations were identified in six/seven cases. The epithelial component showed BRAF mutations in two cases and an NRAS mutation in one case. The case with fasciitis-like stroma was negative for beta-catenin by sequencing and immunostaining as well as negative for USP6 gene rearrangement. Our findings indicate that papillary thyroid carcinoma with prominent myofibroblastic stroma may represent more than one category of lesions.

Clinicopathological and radiological characterization of myofibroblastoma of breast: A single institutional case review

Myofibroblastoma is a rare type of benign myofibroblastic neoplasm in the breast. It is clinically presented as a well-circumscribed mass, usually small in size (usually less than 4.0 cm), and can mostly be cured by local excision. Rare cases of giant myofibroblastoma greater than 10 cm have been reported, but also follow a benign clinical course. Histologically, breast myofibroblastoma is featured by bland fascicles of spindle cells intermixed with thick hyalinized collagen bundles. Mast cells are frequently found within the stroma. However, a wide spectrum of morphological variants can occur in myofibroblatoma, making its diagnosis challenging sometimes. Differential diagnosis of myofibroblastoma with other spindle cell lesions in the breast, either benign or malignant, is also important in practice. In this study, we collected 15 cases of breast myofibroblastoma diagnosed in our institution during a 20 year period. The sizes of these cases range from 0.4 cm to 35.2 cm (mean is 3.7 cm). To our knowledge, the case of giant breast myofibroblastoma we presented here is the largest one reported to date. The histological examination of the cases show great morphological variations. Besides the classical type, features of cellular, collagenized, palisading, epithelioid, myxoid, myoid, solitary fibrous tumor-like are also identified in the case series. Immunohistochemical staining patterns as well as clinical features of the cases are also summarized and compared. All cases in this study show no recurrence on follow-up. In addition, cases that are important differential diagnosis for breast myofibroblastoma are also studied. Their key histological characteristics are compared with myofibroblastoma, and their immunohistochemical and molecular features are discussed.

Novel EIF5A-USP6 Gene Fusion in Nodular Fasciitis Associated With Unusual Pathologic Features: A Report of a Case and Review of the Literature

Nodular fasciitis (NF) is a benign self-limiting soft tissue lesion that has long been considered a reactive process. Recently, however, the USP6 gene rearrangement has been discovered, and the neoplastic nature of this tumor was suggested. Since then, many fusion partners of the USP6 gene have been reported, with the MYH9 gene as the most common. In this article, we describe a case of NF with a novel EIF5A-USP6 gene fusion associated with unusual pathological features. A 41-year-old healthy woman with a painful, rapidly growing subcutaneous mass on the left forearm with a size of 0.8 cm is presented. A soft tissue fragment measuring 1 cm was surgically excised. Owing to positive surgical margins, re-excision was performed, yielding another 2-cm fragment. The lesion was extensively histologically investigated. Immunohistochemical and molecular-genetic analysis, namely fluorescence in situ hybridization, next-generation sequencing, and reverse transcriptase-polymerase chain reaction, were also performed. Histology revealed a dermally located, mitotically active myofibroblastic proliferation with myxoid areas that ulcerated the overlying epidermis. One atypical mitotic figure was also found. The lesion showed positive immunohistochemical staining with smooth muscle actin, whereas S100 protein and CD34 stains were negative. Using fluorescence in situ hybridization, the USP6 gene rearrangement was detected and subsequent analysis using the Archer fusionPlex Sarcoma kit revealed a novel EIF5A-USP6 gene fusion. In the appropriate clinicopathological context, the detection of USP6 gene rearrangement is extremely useful when diagnosing NF, significantly reducing the risk of misdiagnosis and inappropriate overtreatment.

Pathologic evaluation of the solid variant of aneurysmal bone cysts with USP6 rearrangement with an emphasis on the frequent diagnostic pitfalls

The solid variant of aneurysmal bone cyst (SVABC) is very uncommon and frequently misdiagnosed. We reevaluated and summarized the clinicopathologic features of 17 SVABCs and further discussed the use of this nomenclature to differ SVABCs from extragnathic giant cell reparative granuloma (GCRG) in the setting of the USP6 rearrangement era. The immunohistochemical markers included α-SMA, SATB2, AE1/AE3, Ki67, S100, CD68 and P63. USP-6 status was detected by fluorescence in situ hybridization using a break-apart probe. The 17 patients with SVABCs comprised 10 males and 7 females ranging in age from 4 to 70 years. The involved locations included the long bone (n = 11), hand (n = 4), rib (n = 1) and vertebra (n = 1). The lesions were characterized by proliferated spindle cells with scattered giant cells and hemorrhages with variable positive α-SMA, SATB2, CD68 and Ki-67 expression. All patients had USP6 rearrangements without H3F3A glycine 34 mutations. Our study reveals that SVABC shares similar clinical and histologic features with other bone lesions, which may lead to an erroneous diagnosis. The presence of an USP-6 rearrangement contributes to the diagnosis SVABC; SVABC and most of the previously documented extragnathic GCRGs may be considered within the umbrella of primary aneurysmal bone cysts.

The application of current classification systems in pediatric cytopathology: Perspectives from the pediatric cytopathology symposium at the 20th International Congress of Cytology 2019

A pediatric cytopathology symposium was held at the recent 20th International Congress of Cytology, which convened in Sydney, Australia, in May 2019. This educational event brought together cytopathologists from different countries and different institutions to discuss some of the practical considerations when applying current diagnostic classification systems to cytopathology specimens from young (pediatric and adolescent) patients. Within the past decade, various classification systems have been developed to create more standardized terminology for cytopathology specimen reporting among institutions, which can lead to improved management guidelines based on evidence-based medicine. It is well known that a majority of the peer-reviewed publications in cytopathology discussing the usefulness of these classification schemes predominantly contain case cohorts of adult patients. Although pediatric cases are not excluded from following these diagnostic guidelines, there is less of an emphasis on this age group with respect to unique findings and management differences. Thus, discussing the role of these guidelines and their applications in pediatric cases at an international educational gathering can not only be beneficial in educating the cytopathology community about the value of applying these classification systems to pediatric populations, but also can raise awareness of unique entities in cytologic specimens obtained from young patients.

Nodular fasciitis of the face: A case report

INTRODUCTION

Nodular fasciitis is a benign, self-limited myofibroblastic proliferative lesion of unclear aetiology. It is often misdiagnosed as a malignant tumour due to its rapid growth, high cellularity, and increased mitotic activity.

PRESENTATION OF CASE

We present a case of nodular fasciitis occurring in the medial canthus of a 64-year-old, otherwise healthy, woman. The patient complained of rapidly enlarging asymptomatic subcutaneous nodule over a period of 6 months. The tumour was firm, smooth, and indolent with limited mobility. Fine-needle aspiration was inconclusive, and the CT scan showed a well-defined nodular lesion in the soft tissue without bone destruction. A local excision was performed, and the histopathological findings were consistent with nodular fasciitis.

DISCUSSION

Although rare, nodular fasciitis is the most common pseudosarcoma of soft tissues. Local excision is recommended; however, the tumour often regresses spontaneously, and recurrence is exceedingly rare.

CONCLUSION

Awareness of nodular fasciitis and its benign nature is essential to avoid misdiagnosis and subsequent inappropriate aggressive treatment of the patient.

Giant Protruding Nodular Fasciitis of the Anterior Chest Wall Clinically Mimicking a Soft Tissue Sarcoma

Nodular fasciitis (NF) is a benign reactive proliferation of myofibroblasts that predominantly occurs subcutaneously. Commonly, it presents as a rapidly growing swelling in 4-8 weeks. NF mostly occurs in adults aged 20-50 years and usually has a diameter < 3‐4 cm. Giant NF with a diameter > 4 cm is rare. Owing to its rapidly growing nature, a precise clinical diagnosis is difficult; it is frequently misdiagnosed as an aggressive or malignant tumor. Herein, we present the case of a 15-year-old male who presented with a large protruding mass on the anterior chest wall. The tumor appeared clinically malignant as it was protruding and had doubled in size within a few weeks, reaching approximately 8 × 6 cm. Furthermore, the tumor separated and fell off spontaneously due to its large size. As the remaining tumor continued to grow rapidly, surgery was performed. Following wide tumor resection, no recurrence, metastases, or other complications were noted 1 year postsurgery. NF was diagnosed after pathological evaluation, including immunohistochemical analysis, molecular genetic testing, and cytogenetic testing via fluorescence in situ hybridization analysis. Knowledge of the atypical clinical course and a combination of histopathological examinations are necessary to accurately diagnose NF.

[Pediatric tumors with spindle cell morphology]

Spindle cell tumors in childhood are rare lesions with a heterogeneous morphological picture and clinical course, ranging from benign lesions to fully malignant tumors. The clinical assessment of these tumors is often challenging since some of them show fast growth dynamics but are utterly benign, while a subset of slow-growing tumors can represent malignant entities. Due to the rarity of these tumors as well as the overlapping morphology and the often uncharacteristic immunohistochemical profiles, the pathologic diagnosis is often also difficult. This review gives an overview of some of the more common pediatric spindle cell tumors. In addition to the morphological features and immunohistochemical aspects, specific molecular changes are discussed. Here, some of the newly described translocations that may imply therapeutic options, are presented.

Clinical Utility of In Situ Hybridization Assays in Head and Neck Neoplasms

Head and neck pathology present a unique set of challenges including the morphological diversity of the neoplasms and presentation of metastases of unknown primary origin. The detection of human papillomavirus and Epstein-Barr virus associated with squamous cell carcinoma and newer entities like HPV-related carcinoma with adenoid cystic like features have critical prognostic and management implications. In salivary gland neoplasms, differential diagnoses can be broad and include non-neoplastic conditions as well as benign and malignant neoplasms. The detection of specific gene rearrangements can be immensely helpful in reaching the diagnosis in pleomorphic adenoma, mucoepidermoid carcinoma, secretory carcinoma, hyalinizing clear cell carcinoma and adenoid cystic carcinoma. Furthermore, molecular techniques are essential in diagnosis of small round blue cell neoplasms and spindle cell neoplasms including Ewing sarcoma, rhabdomyosarcoma, synovial sarcoma, biphenotypic sinonasal sarcoma, dermatofibrosarcoma protuberans, nodular fasciitis and inflammatory myofibroblastic tumor. The detection of genetic rearrangements is also important in lymphomas particularly in identifying 'double-hit' and 'triple-hit' lymphomas in diffuse large B cell lymphoma. This article reviews the use of in situ hybridization in the diagnosis of these neoplasms.

Intra-articular nodular fasciitis of the proximal interphalangeal joint of a finger: A case report

Nodular fasciitis is a benign reactive lesion, often mistaken for a soft-tissue sarcoma in clinical practice. Involvement of the finger is rare, and a finger joint even rarer. We report on the clinical, radiological and histological features of intra-articular nodular fasciitis in a 52-year-old man, originating from the proximal interphalangeal joint of the right ring finger, with cortical erosion of adjacent bone. The discussion is focused on the tumor diagnosis and therapeutic approach, the differential diagnosis and the importance of immunohistochemical staining to establish the final diagnosis.

Molecular Analysis of Gene Fusions in Bone and Soft Tissue Tumors by Anchored Multiplex PCR-Based Targeted Next-Generation Sequencing

Molecular assays for translocation detection in bone and soft tissue tumors have gradually been incorporated into routine diagnostics. However, conventional methods such as fluorescence in situ hybridization (FISH) and reverse transcriptase-PCR come with several drawbacks. In this study, the applicability of a novel technique termed anchored multiplex PCR (AMP) for next-generation sequencing (NGS), using the Archer FusionPlex Sarcoma kit, aimed at 26 genes, was evaluated and compared with FISH and reverse transcriptase-PCR. In case of discrepant results, further analysis occurred with a third independent technique. Eighty-one samples were subjected to AMP-based targeted NGS, and 86% (n = 70) were successfully conducted and were either fusion positive (n = 48) or fusion negative, but met all criteria for good quality (n = 22). A concordance of 90% was found between NGS and conventional techniques. AMP-based targeted NGS showed superior results, as in four cases reverse transcriptase-PCR and FISH were false negative. Moreover, because the assay targets one partner of a gene fusion, novel or rare fusion partners can be identified. Indeed, it revealed COL1A1 and SEC31A as novel fusion partners for USP6 in nodular fasciitis. Despite the fact that fusions involving genes outside the selectively captured region cannot be detected and false-negative results due to poor quality samples can be encountered, this method has demonstrated excellent diagnostic utility for translocation detection in sarcomas.

Pseudosarcomatous myofibroblastic proliferations of the genitourinary tract are genetically different from nodular fasciitis and lack USP6, ROS1 and ETV6 gene rearrangements

AIMS

Pseudosarcomatous myofibroblastic proliferations of the genitourinary tract have a debatable relationship with inflammatory myofibroblastic tumour (generally lacking ALK rearrangement); however, they share several overlapping features with nodular fasciitis of soft tissue. As rearrangement of the USP6 gene has been recently recognised as a recurrent alteration in soft tissue nodular fasciitis, and several other alternative gene fusions have been recently recognised in inflammatory myofibroblastic tumour, the aim of this study was to investigate whether USP6, ROS1 or ETV6 rearrangements were present in these lesions (12 cases).

METHODS AND RESULTS

Fluorescence in-situ hybridisation analysis was performed by the use of bacterial artificial chromosome-derived break-apart probes against USP6, ROS1, and ETV6. Two cases with adequate genetic material from recent paraffin tissue blocks were also tested by use of a solid tumour gene fusion detection assay via next-generation sequencing, targeting >50 known genes involved in recurrent fusions. None of the genitourinary pseudosarcomatous myofibroblastic proliferations was found to harbour USP6 (0/12), ROS1 (0/8) or ETV6 (0/7) rearrangements, and no gene fusions were detected in two cases studied by sequencing.

CONCLUSIONS

Despite overlap in histological and immunohistochemical features between pseudosarcomatous myofibroblastic proliferation and nodular fasciitis, these tumours lack the recently recognised USP6 rearrangements that occur in nodular fasciitis, as well as alternative fusions found in ALK-negative inflammatory myofibroblastic tumours. At present, this diagnosis remains based primarily on clinical, histological and immunohistochemical features.

Nodular fasciitis involving the palm

Nodular fasciitis (NF) is a self-limiting fibrous neoplasm that can be mistaken for a soft tissue sarcoma. It is characterised by rapid growth, slight pain and local tenderness. Although it is frequently found in the forearm, a lesion distal to the wrist is quite rare. We present two unusual cases of NF involving the palm, supported by detecting ubiquitin specific protease 6 gene rearrangement. The first patient had non-intraneural NF presenting as peripheral neuropathy affecting the digital nerve while the second patient suffered from painless, non-tender NF in the palm, which had not regressed spontaneously during the five months prior to surgery.

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.

Benign Fibromyxoid Lesion of the Breast: A Distinct Entity From Benign Spindle Cell Tumors of the Mammary Stroma?

Myxoid lesions of the breast can be diagnostically challenging entities. We report 4 cases of CD34+ fibromyxoid lesion that have been previously diagnosed as “benign myxoid lesion,” “nodular mucinosis,” or “mammary myofibroblastoma, myxoid type” on the basis of CD34-positivity. The lesions were microscopically well circumscribed and composed of a paucicellular spindle cell proliferation in a background of myxoid stroma. No epithelial component was identified. The spindle cells showed immunohistochemical reactivity for CD34 and smooth muscle actin. Based on morphologic and immunohistochemical similarities between these cases and myxoid myofibroblastoma, we compared 4 myxoid lesions with cases of typical myofibroblastoma, utilizing retinoblastoma (Rb) antibody and fluorescent in situ hybridization for 13q14 gene rearrangement (encoding the Rb gene). The myxoid lesions showed retention of Rb protein by immunohistochemistry, whereas Rb expression was lost in cases of myofibroblastoma. We identified loss of 13q14 in 3 of 4 cases of myofibroblastoma. Notably, 13q14 gene rearrangement was not observed in any of the myxoid lesions. Our data show that there is at least a subset of CD34+ fibromyxoid lesions that, despite overlapping morphologic and immunohistochemical phenotype and proposed common histogenesis with myofibroblastomas, is genetically distinct from the latter based on Rb analysis.

Differential Diagnosis of Benign Spindle Cell Lesions

Spindle cell lesions of the breast cover a wide spectrum of diseases ranging from reactive tumor-like lesions to high-grade malignant tumors. The recognition of the benign spindle cell tumor-like lesions (nodular fasciitis; reactive spindle cell nodule after biopsy, inflammatory pseudotumor/inflammatory myofibroblastic tumor; fascicular variant of pseudoangiomatous stromal hyperplasia) and tumors (myofibroblastoma, benign fibroblastic spindle cell tumor, leiomyoma, schwannoma, spindle cell lipoma, solitary fibrous tumor, myxoma) is crucial to avoid confusion with morphologically similar but more aggressive bland-appearing spindle cell tumors, such as desmoid-type fibromatosis, low-grade (fibromatosis-like) spindle cell carcinoma, low-grade fibrosarcoma/myofibroblastic sarcoma and dermatofibrosarcoma protuberans.

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.

An update on mesenchymal tumours of the orbit with an emphasis on the value of molecular/cytogenetic testing

Mesenchymal tumours of the orbit are uncommon. Beyond childhood primary sarcomas are extremely rare and the literature is limited to case reports and short case series. However there is a diverse assortment of benign and malignant soft tissue tumours that may involve the orbit. Techniques to identify tumour specific cytogenetic or molecular genetic abnormalities often resulting in over- expressed proteins are becoming an increasingly important ancillary technique for these tumours. This review focuses on 3 specific areas: 1. Orbital mesenchymal tumours where cytogenetics are important to reach the correct diagnosis. The majority of these are chromosomal translocations that often result in a fusion gene and protein product; 2. Orbital mesenchymal tumours where cytogenetics are important to identify patients who will do well versus those with a poorer prognosis. This is turn helps with therapeutic options. In some tumours e.g. synovial sarcoma the chromosomal translocations can occur with 2 different regions resulting in different fusion products that carry a different prognosis. Alternatively whilst the majority of alveolar rhadomyosarcomas are fusion positive a minority are fusion negative with a better prognosis; 3. Orbital mesenchymal tumours where the identification of specific cytogenetic abnormalities has resulted in overexpression of specfic proteins which are diagnostically useful biomarkers for immunohistochemistry.

USP6 gene rearrangement differentiates primary paranasal sinus solid aneurysmal bone cyst from other giant cell-rich lesions: report of a rare case

Aneurysmal bone cysts (ABCs) mostly occur in the metaphysis of long bones. Primary paranasal ABCs are extremely rare, and most reported cases reveal typical histopathological features including cystic space with fibrous septa and hemorrhage. Solid-variant ABCs or solid ABCs lacking cyst formation may be histologically indistinguishable from giant cell reparative granulomas, giant cell tumor of bone, and brown tumor. Here we report the case of a 24-year-old woman with a paranasal mass diagnosed as USP6-rearranged solid ABC, mimicking giant cell reparative granuloma, giant cell tumor of bone, and brown tumor. For paranasal sinus bone or soft tissue tumors containing numerous giant cells, molecular analysis including the USP6 gene may serve as a useful diagnostic tool to distinguish solid ABCs from other giant cell-rich lesions.

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.

Cutaneous/subcutaneous mesenchymal proliferations of the breast

Cutaneous mesenchymal "spindle cell" lesions arising in the vicinity of the breast represent a complex clinical and diagnostic scenario which may overlap histopathologically and immunohistochemically with mammary spindle cell proliferations, potentially impacting management and overall prognostication. In this review, we suggest a pattern-based approach to assist in the evaluation of these lesions. A comprehensive description of each entity is accompanied by a cutaneous and mammary differential diagnosis.

Nodular Fasciitis Complicating a Staged Surgical Excision of Dermatofibrosarcoma Protuberans

Dermatofibrosarcoma protuberans (DFSP) is an unusual spindle cell tumor with a high rate of local recurrence with traditional excision. Fortunately, Mohs micrographic surgery yields excellent cure rates for this neoplasm due to contiguous tumor spread and meticulous tumor mapping and margin analysis. We present the unique case of a patient treated with a modified Mohs technique with an analysis of the final margin with permanent sections, who developed a spindle cell neoplasm in the margins of her second stage excision consistent with nodular fasciitis. Distinguishing residual DFSP from a benign reactive process was an essential and challenging component of this patient's management.