Identification of SETD2-NF1 fusion gene in a pediatric spindle cell tumor with the chromosomal translocation t(3;17)(p21;q12)

Spindle cell tumors are clinically heterogeneous but morphologically similar neoplasms. The term refers to the tumor cells' long and slender microscopic appearance. Distinct subgroups of spindle cell tumors are characterized by chromosomal translocations and also fusion genes. Other spindle cell tumors exist that have not yet been found to have characteristic, let alone pathognomonic, genetic or pathogenetic features. Continuous examination of spindle cell tumors is likely to reveal other subgroups that may, in the future, be seen to correspond to meaningful clinical differences and may even be therapeutically decisive. We analyzed genetically a pediatric spindle cell tumor. Karyotyping showed the tumor cells to carry a t(3;17)(p21;q12) chromosomal translocation whereas RNA sequencing identified a SETD2-NF1 fusion gene caused by the translocation. RT-PCR together with Sanger sequencing verified the presence of the above-mentioned fusion transcript. Interphase FISH analysis confirmed the existence of the chimeric gene and showed that there was no reciprocal fusion. The fusion transcript codes for a protein in which the last 114 amino acids of SETD2, i.e., the entire Set2 Rpb1 interacting (SRI) domain of SETD2, are replaced by 30 amino acids encoded by the NF1 sequence. The result would be similar to that seen with truncating SETD2 mutations in leukemias. Absence of the SRI domain would result in inability to recruit SETD2 to its target gene locus through binding to the phosphor-C-terminal repeat domain of elongating RNA polymerase II and may affect H3K36 methylation. Alternatively, loss of one of two functional SETD2 alleles might be the crucial tumorigenic factor.

Immunohistochemistry as a Practical Tool in Molecular Pathology

CONTEXT

-Molecular genetics is playing an increasingly important role in patient care and pathology practice. Immunohistochemistry (IHC) is a valuable and practical tool employed by most pathologists on a regular basis.

OBJECTIVE

-To highlight select examples of how IHC may be used in the realm of molecular diagnostics.

DATA SOURCES

-Select sources on IHC relating to tumor subtyping, hereditary cancer screening, and treatment-response prediction are reviewed. These represent some of the areas in which IHC can be employed by anatomic pathologists to optimize patient care and further inform molecular testing.

CONCLUSION

-In the emerging era of personalized medicine, IHC continues to serve a valuable function, complementing and enhancing other molecular techniques.

Fluorescence in situ hybridization in surgical pathology: principles and applications

Identification of recurrent tumour‐specific chromosomal translocations and novel fusion oncogenes has important diagnostic, therapeutic and prognostic implications. Over the past decade, fluorescence in situ hybridization (FISH) analysis of tumour samples has been one of the most rapidly growing areas in genomic medicine and surgical pathology practice. Unlike traditional cytogenetics, FISH affords a rapid analysis of formalin‐fixed, paraffin‐embedded cells within a routine pathology practice workflow. As more diagnostic and treatment decisions are based on results of FISH, demand for the technology will become more widespread. Common FISH‐detected alterations are chromosome deletions, gains, translocations, amplifications and polysomy. These chromosome alterations may have diagnostic and therapeutic implications for many tumour types. Integrating genomic testing into cancer treatment decisions poses many technical challenges, but rapid progress is being made to overcome these challenges in precision medicine. FISH assessment of chromosomal changes relevant to differential diagnosis and cancer treatment decisions has become an important tool for the surgical pathologist. The aim of this review is to provide a theoretical and practical survey of FISH detected translocations with a focus on strategies for clinical application in surgical pathology practice.

Endobronchial solitary fibrous tumor

Solitary fibrous tumor (SFT) is a mesenchymal neoplasm that appears primarily in the pleura and rarely in intrapulmonary or endobronchial topography. The authors report the case of a 47-year-old woman who presented obstructive respiratory symptoms for 4 years. The chest computed tomography and bronchoscopy showed an obstructive polypoid lesion located between the trachea and the left main bronchus associated with distal atelectasis of the left lung. A resection of the lesion was performed and, macroscopically, the mass was oval, encapsulated, and firm, measuring 2.3 × 1.7 × 1.5 cm. Histology revealed low-grade mesenchymal spindle cell neoplasm, with alternating cellularity, myxoid areas, and mature adipose tissue outbreaks, as well as blood vessels with irregular walls. The immunohistochemical study was positive for CD34, CD99, and BCL2. The diagnosis was SFT in an unusual topography. The patient’s symptoms remitted after tumor excision, and no systemic problems were evident. SFTs primarily affect adults and often follow a benign course; however, their behavior is unpredictable. The presence of necrosis and mitotic activity may portend a poor prognosis. Endobronchial SFTs are rare but should be evaluated and monitored similar to SFTs at other sites, with a long-term follow-up.

TERT promoter mutations and prognosis in solitary fibrous tumor

Solitary fibrous tumor is a mesenchymal neoplasm exhibiting a broad spectrum of biological behavior and harboring the NAB2-STAT6 fusion. Clinicopathologic parameters are currently used in risk-prediction models for solitary fibrous tumor, but the molecular determinants of malignancy in solitary fibrous tumors remain unknown. We proposed that the activation of telomere maintenance pathways confers a perpetual malignant phenotype to these tumors. Therefore, we investigated telomerase reverse transcriptase (TERT) reactivation induced by promoter mutations as a potential molecular mechanism for aggressive clinical behavior in solitary fibrous tumor. The retrospective study included tumor samples from 94 patients with solitary fibrous tumor (31 thoracic and 63 extra-thoracic). Follow-up information was available for 68 patients (median, 46 months). TERT promoter mutation analysis was performed by PCR and Sanger sequencing, and TERT mRNA expression was assessed by real-time quantitative reverse transcription PCR. Patients were stratified into clinicopathologic subgroups (high-risk (n=20), moderate-risk (n=28), and low-risk (n=46)) according to the risk-stratification model proposed by Demicco et al. TERT promoter mutations were identified in 26 of 94 (28%) solitary fibrous tumors: -124C>T in 23 tumors (88%), -124C>A in 1 tumor (4%), and -146C>T in 2 tumors (8%). Real-time quantitative reverse transcription PCR revealed that TERT mRNA expression was higher in all solitary fibrous tumors with the mutant TERT promoter than those with the wild-type TERT promoter. TERT promoter mutations were strongly associated with high-risk clinicopathologic characteristics and outcome. An adverse event (relapse, death) occurred in 16 of 68 (24%) patients, 12 with solitary fibrous tumors with TERT promoter mutations and 4 with the wild-type TERT promoter. TERT promoter mutations were strongly associated with older age (P=0.006), larger tumor size (P=0.000002), higher risk classifications (P=2.9 × 10^-9), and a worse event-free survival (P=0.0082). Thus, TERT promoter mutations in solitary fibrous tumor influence gene expression and are associated with adverse patient outcome. Integrating TERT promoter mutational status with existing multivariable risk-prediction models might improve risk prediction in patients with solitary fibrous tumor.

Pancreatic solitary fibrous tumor causing ectopic adrenocorticotropic hormone syndrome

Solitary fibrous tumors occasionally present with hypoglycemia because of the excessive release of insulin-like growth factor II. We report the first case of pancreatic solitary fibrous tumor causing ectopic adrenocorticotropic hormone syndrome. An 82-year-old Japanese man presented with lower limb edema, uncontrolled hypertension, hypokalemia, and baseline hypercortisolism. Distal pancreatectomy was performed after the clinical diagnosis of a neuroendocrine tumor with ectopic secretion of adrenocorticotropic hormone. On histological examination, the tumor showed spindle cells in a fascicular arrangement. The diagnosis of the solitary fibrous tumor was confirmed by the identification of the NAB2-STAT6 fusion gene and positive immuno-histochemical staining for STAT6 and CD34. Using quantitative real-time polymerase chain reaction, mRNA that encoded proopiomelanocortin, precursor of adrenocorticotropic hormone, was detected. Proopiomelanocortin production through the demethylation of the promoter region Domain IV was detected. Pancreatic solitary fibrous tumors represent a new cause of ectopic adrenocorticotropic hormone syndrome.

Solitary fibrous tumour of the genitourinary tract: a clinicopathological study of 11 cases and their association with the NAB2-STAT6 fusion gene

AIMS

To characterise clinicopathological features and clinical outcomes of the genitourinary tract solitary fibrous tumours, incorporating NAB2-STAT6 gene fusion status.

METHODS

The presence of the molecular hallmark NAB2-STAT6 gene fusion and for the defining fusion partner product STAT6 was assessed in 11 cases of the genitourinary tract solitary fibrous tumours. NAB2-STAT6 gene fusion analysis was performed using a break-apart fluorescence in situ hybridisation (FISH) probe using a probe cocktail with Bacterial artificial chromosome (BAC) clones for STAT6 and NAB2.

RESULTS

Eleven solitary fibrous tumours were diagnosed in eight male patients and three female patients with a mean age of 46 years (range: 11-64 years). Four of the tumours had malignant histological features, and three were considered moderate risk for metastasis. With a mean follow-up time of 61 months, 1 recurred locally and 2 presented at distant metastatic sites. Using a break-apart FISH probe cocktail, we found the NAB2-STAT6 gene fusion and nuclear STAT6 expression in 58% and 91% of cases, respectively. However, the NAB2-STAT6 fusion status was not correlated with STAT6 expression or useful in discriminating between malignant histological features or subsequent clinical outcomes in the genitourinary solitary fibrous tumours.

CONCLUSIONS

A subset of solitary fibrous tumours of the genitourinary tract behaved aggressively. Using a break-apart FISH probe cocktail, we found the NAB2-STAT6 gene fusion in 64% of cases. However, the NAB2-STAT6 fusion status was not correlated with STAT6 expression or useful in discriminating between low-risk or high-risk tumours and subsequent clinical outcomes.

Solitary Fibrous Tumor of the Sigmoid Colon Masquerading as an Adnexal Neoplasm

Solitary fibrous tumor is a rare, benign spindle cell neoplasm that was first described in the thoracic pleura. This tumor is now known to occur at many extrapleural sites. There are established criteria for the diagnosis of malignant solitary fibrous tumor including ≥4 mitotic figures per 10 high-power fields, increased cellularity, cytologic atypia, infiltrative margins, and/or necrosis. Although all solitary fibrous tumors have the potential to recur or metastasize, those with malignant histologic features tend to behave more aggressively. We report a case of solitary fibrous tumor, with malignant histologic features, in a 21-year-old woman which arose from the serosal surface of the sigmoid colon.

Refinements in Sarcoma Classification in the Current 2013 World Health Organization Classification of Tumours of Soft Tissue and Bone

The fourth edition of the World Health Organization (WHO) Classification of Tumours of Soft Tissue and Bone was published in February 2013. The 2013 WHO volume provides an updated classification scheme and reproducible diagnostic criteria, which are based on recent clinicopathologic studies and genetic and molecular data that facilitated refined definition of established tumor types, recognition of novel entities, and the development of novel diagnostic markers. This article reviews updates and changes in the classification of bone and soft tissue tumors from the 2002 volume.

Hemangiopericytomas in the Central Nervous System: A Multicenter Study of Korean Cases with Validation of the Usage of STAT6 Immunohistochemistry for Diagnosis of Disease

BACKGROUND

Hemangiopericytoma (HPC) in the central nervous system (CNS) is a rare disease with distinctive biological/clinical characteristics compared with meningioma.

METHODS

Cases of HPCs of the CNS were collected, and clinicopathological records were retrospectively reviewed and analyzed. Immunohistochemistry (IHC) for proliferative markers (Ki-67, PHH3) and STAT6 were performed.

RESULTS

A total of 140 cases were collected, with mean follow-up of 77 months (median 58.8 months; range 0.53-540.5 months). 1-, 5-, 10-, and 20-year survival rates were 99.1, 94.0, 74.4, and 61.9 %, respectively. Thirty-nine patients (27.9 %) had recurrent disease. Mean and median times to recurrence were 62.9 and 47.3 months with 1-, 5-, 10-, and 20-year recurrence-free survival rates of 98.3, 78.3, 50.1, and 11.0 %, respectively. Thirteen patients (9.3 %) developed extracranial metastases. No adjuvant radiation therapy, higher histologic grades, failure of gross-total resection, and cases with gamma-knife surgery (GKS) were factors associated with shorter disease-free survival (log-rank test, p = 0.02, 0.00, 0.02, 0.00), among which high histologic grade and cases with GKS were significant in multivariable analysis. Strong nuclear STAT6 expression was noted in HPCs in 62 cases of HPCs (60/62, 96.8 %), whereas diffuse weak positivity was demonstrated in all meningioma cases.

CONCLUSIONS

The survival rate in patients with HPC of the CNS is comparable to that of previously reported series. Recurrence remains a critical clinical issue of the disease. Identification of NAB2-STAT6 fusion transcript with surrogate IHC marker is a valuable diagnostic tool in the differential diagnosis of the disease.

Practical Issues for Retroperitoneal Sarcoma

Background

Retroperitoneal sarcoma is rare. Using initial specimens on biopsy, a definitive diagnosis of histological subtypes is ideal but not always achievable.

Methods

A retrospective institutional review was performed for all cases of adult retroperitoneal sarcoma from 1996 to 2015. A review of the literature was also performed related to the distribution of retroperitoneal sarcoma subtypes. A meta-analysis was performed.

Results

Liposarcoma is the most common subtype (45%), followed by leiomyosarcoma (21%), not otherwise specified (8%), and undifferentiated pleomorphic sarcoma (6%) by literature review. Data from Moffitt Cancer Center demonstrate the same general distribution for subtypes of retroperitoneal sarcoma. A pathology-based algorithm for the diagnosis of retroperitoneal sarcoma is illustrated, and common pitfalls in the pathology of retroperitoneal sarcoma are discussed.

Conclusions

An informative diagnosis of retroperitoneal sarcoma via specimens on biopsy is achievable and meaningful to guide effective therapy. A practical and multidisciplinary algorithm focused on the histopathology is helpful for the management of retroperitoneal sarcoma.

The NAB2-STAT6 gene fusion in solitary fibrous tumor can be reliably detected by anchored multiplexed PCR for targeted next-generation sequencing

Solitary fibrous tumor (SFT) is a mesenchymal tumor of fibroblastic origin, which can affect any region of the body. 10-15% of SFTs metastasize and metastatic tumors are uniformly lethal with no effective therapies. The behavior of SFT is difficult to predict based on morphology. Recently, an intrachromosomal gene fusion between NAB2 and STAT6 was identified as the defining driving genetic event of SFT and different fusion types correlated with tumor histology and behavior. Due to the proximity of NAB2 and STAT6 on chromosome 12, this fusion may be missed by fluorescence in-situ hybridization. We evaluated 12 SFTs from 10 patients. All tumors showed strong nuclear staining for STAT6 by immunohistochemistry (IHC). The same formalin-fixed, paraffin-embedded blocks for IHC were used for gene fusion detection by a next-generation sequencing (NGS)-based assay. Targeted RNA fusion sequencing for gene fusions was performed using the Universal RNA Fusion Detection Kit, the Archer(™) FusionPlex(™) Sarcoma Panel and the Ion Torrent PGM, and data were analyzed using the Archer Analysis Pipeline 3.3. All tumors were positive for NAB2-STAT6 fusion. Six types of fusions were detected: NAB2ex4-STAT6ex2, NAB2ex2-STAT6ex5, NAB2ex6-STAT6ex16, NAB2ex6-STAT6ex17, NAB2ex3-STAT6ex18 and NAB2intron6-STAT6Ex17. The NGS findings were confirmed by RT-PCR followed by Sanger sequencing. No STAT6 fusion was detected in selected morphologic mimics of SFT. The assay also allows for detection of novel fusions and can detect NAB2-STAT6 fusions at a single-base resolution.

Diagnostically Challenging "Fatty" Retroperitoneal Tumors

A variety of benign and malignant retroperitoneal mesenchymal lesions may have a component of adipose tissue, including entities such as lipoma, myolipoma, angiomyolipoma, solitary fibrous tumor, genital stromal tumors, and well-differentiated/dedifferentiated liposarcoma. Although definitive diagnosis is usually straightforward on the complete resection specimen, it is often more difficult to workup these lesions on small biopsy samples. This review focuses on challenging diagnostic scenarios of retroperitoneal lesions with a "fatty" component and provides major differential diagnoses for commonly encountered morphologic patterns, clinicopathologic features of the various entities, and strategy for use of ancillary techniques, such as immunohistochemistry and cytogenetic studies.

Solitary Fibrous Tumor/Hemangiopericytoma Dichotomy Revisited: A Restless Family of Neoplasms in the CNS

Solitary fibrous tumor (SFT) and hemangiopericytoma (HPC) both entered the literature as separate entities in the early to mid 1900s. In contrast to their central nervous system (CNS) counterparts, there has been a tendency to consider these 2 entities as 1 since the early 1990s, as soft tissue SFT gradually included the tumors previously diagnosed as HPC. The most recent World Health Organization (WHO) classification of the tumors of soft tissue considered the term HPC obsolete, and places all such tumors within the extrapleural SFT category. In contrast, CNS SFT and HPC continue to be regarded as different entities in the latest version of the WHO CNS tumor classification. A change in this approach is currently being considered for the upcoming revision of the WHO scheme, but it is not quite clear whether such a change will be as drastic as the one adopted by the soft tissue and bone tumor working group. This article focuses on the historical evolution of these 2 labels as primary CNS neoplasms, and reviews their differences and similarities in terms of clinical, pathologic, and molecular features.

Update on Select Benign Mesenchymal and Meningothelial Sinonasal Tract Lesions

Several benign and malignant mesenchymal and meningothelial lesions may preferentially affect or extend into the sinonasal tract. Glomangiopericytoma (GPC, formerly sinonasal-type hemangiopericytoma) is a specific tumor with a predilection to the sinonasal tract. Sinonasal tract polyps with stromal atypia (antrochoanal polyp) demonstrate unique histologic findings in the sinonasal tract. Juvenile nasopharyngeal angiofibroma (JNA) arises from specialized tissue in this location. Meningioma may develop as direct extension from its intracranial counterpart or as an ectopic tumor. Selected benign mesenchymal tumors may arise in the sinonasal tract and pose a unique differential diagnostic consideration, such as solitary fibrous tumor and GPC or lobular capillary hemangioma and JNA. Although benign and malignant vascular, fibrous, fatty, skeletal muscle, and nerve sheath tumors may occur in this location, this paper focuses on a highly select group of rare benign sinonasal tract tumors with their clinicopathological and molecular findings, and differential diagnosis.

NAB2-STAT6 Gene Fusion in Meningeal Hemangiopericytoma and Solitary Fibrous Tumor

Meningeal solitary fibrous tumor (SFT) and hemangiopericytoma (HPC) are considered to be distinct entities in the WHO Classification of CNS Tumours (2007). They harbor NAB2-STAT6 fusions similar to their soft tissue counterparts, supporting the view that they are part of a tumor continuum. We examined 30 meningeal-based tumors originally diagnosed as either SFT or HPC. These showed a spectrum of morphologic features and were diagnosed as SFTs, malignant SFTs, HPCs, or tumors with "intermediate" features. All of the tumors showed nuclear expression of STAT6. SFTs consistently expressed diffuse CD34, while HPCs and intermediate tumors had heterogeneous staining. NAB2-STAT6 fusions were identified in 20 cases, including 7 with exon 4-exon 3, 9 with exon 6-exon 17, and 4 with exon 6-exon 18 fusions. NAB2 exon 4-STAT6 exon 3 fusion correlated with classic SFT morphology and older age and showed a trend toward less mitotic activity; there was also a trend toward more aggressive behavior in tumors lacking NAB2 exon 4-STAT6 exon 3. Thus, despite their clinical and morphologic differences, meningeal-based SFTs, HPCs, and tumors with intermediate features, similar to their soft tissue counterparts, form a histopathologic spectrum unified by STAT6 immunoexpression and NAB2-STAT6 fusion.

The Current Status of Solitary Fibrous Tumor: Diagnostic Features, Variants, and Genetics

Solitary fibrous tumor (SFT) is a fibroblastic mesenchymal tumor originally described in the pleura but now shown at almost every anatomic site. Histopathologically, SFT is characteristically a circumscribed neoplasm composed of variably cellular and patternless distributions of bland spindle and ovoid cells within prominent collagenous stroma and shows diffuse expression of CD34, but it has a broad spectrum of both morphology and of biologic behavior. Many different names (particularly hemangiopericytoma) were previously used in the course of our understanding of this neoplasm but are now subsumed under the term "SFT," and the putative cell of origin was debated. However, it is now recognized that SFT is a translocation-associated neoplasm, consistently associated with NAB2-STAT6 gene fusions arising from recurrent intrachromosomal rearrangements on chromosome 12q, and this translocation is a likely major contributor to its pathogenesis. While most SFT with classical morphologic features behave in an indolent manner and those with overtly malignant histologic features tend to be aggressive neoplasms that behave as high-grade sarcomas, the behavior of SFT is unpredictable, and it is important to be aware of the propensity for aggressive behavior in a minority of histologically classical SFT and to ensure adequate clinical follow-up. Surgical excision remains the treatment gold standard; while radiotherapy and conventional chemotherapeutic agents have only shown limited efficacy, further understanding of the molecular events underlying tumorigenesis may allow the development of novel targeted treatments. We review SFT, discussing the morphologic spectrum and variants, including malignant and dedifferentiated subtypes, clinicopathological aspects, recent molecular genetic findings, and the differential diagnosis.

The clinicopathological significance of NAB2-STAT6 gene fusions in 52 cases of intrathoracic solitary fibrous tumors

NAB2-STAT6 gene fusion drives STAT6 nuclear expression and is the pathognomonic hallmark of solitary fibrous tumors (SFTs). However, no study has systematically analyzed the clinicopathological features, STAT6 immunoexpression status, or the fusion variants of NAB2-STAT6 in intrathoracic SFTs. Fifty-two intrathoracic SFTs were retrieved to appraise histopathology, assess STAT6 immunoexpression, and determine NAB2-STAT6 fusion variants by RT-PCR. Location-relevant histologic mimics served as controls. Thirty-one pleura-based, 12 mediastinal/pericardial, and nine intrapulmonary lesions were histologically categorized into eight malignant, eight atypical, and 36 conventional or cellular SFTs, including two fat-forming and two giant cell angiofibroma-like SFTs. STAT6 distinctively decorated the tumoral nuclei in 51 (98%) SFTs. However, no nuclear staining was observed in the histological mimics. NAB2-STAT6 fusion was detected in 34 SFTs. Twenty-nine (85.3%) exhibited the major NAB2ex4-STAT6ex2/3 variant and 5 (14.7%) the minor NAB2ex6-STAT6ex16/17. NAB2ex4-STAT6ex2 was significantly associated with older age (P = 0.01) and pleuropulmonary tumors (P = 0.025). After a median follow-up of 33.9 (range, 0.3-174.6) months, adverse outcomes occurred in one atypical and five malignant SFTs, including two local relapses, one intrapulmonary metastasis, and three extrathoracic metastases. Inferior disease-free survival was univariately associated with atypical/malignant histology (P = 0.001) and a mitosis >4/10 HPFs (P = 0.0012) but was unrelated to fusion variants. In conclusion, the majority of intrathoracic SFTs exhibited STAT6 nuclear staining, and NAB2ex4-STAT6ex2/3 was the predominant fusion type. However, clinical aggressiveness is associated with atypical/malignant histology primarily contributed by increased mitosis but was unrelated to the NAB2-STAT6 fusion variants.

Gene fusions in soft tissue tumors: Recurrent and overlapping pathogenetic themes

Gene fusions have been described in approximately one-third of soft tissue tumors (STT); of the 142 different fusions that have been reported, more than half are recurrent in the same histologic subtype. These gene fusions constitute pivotal driver mutations, and detailed studies of their cellular effects have provided important knowledge about pathogenetic mechanisms in STT. Furthermore, most fusions are strongly associated with a particular histotype, serving as ideal molecular diagnostic markers. In recent years, it has also become apparent that some chimeric proteins, directly or indirectly, constitute excellent treatment targets, making the detection of gene fusions in STT ever more important. Indeed, pharmacological treatment of STT displaying fusions that activate protein kinases, such as ALK and ROS1, or growth factors, such as PDGFB, is already in clinical use. However, the vast majority (52/78) of recurrent gene fusions create structurally altered and/or deregulated transcription factors, and a small but growing subset develops through rearranged chromatin regulators. The present review provides an overview of the spectrum of currently recognized gene fusions in STT, and, on the basis of the protein class involved, the mechanisms by which they exert their oncogenic effect are discussed.

Clinical Characteristics and Outcomes for Solitary Fibrous Tumor (SFT): A Single Center Experience

Solitary fibrous tumor (SFT) is a mesenchymal neoplasm of fibrous origin. The 2013 WHO classification of soft tissue tumors defines malignant forms as hypercellular, mitotically active (>4 mitosis/10 high-power fields), with cytological atypia, tumor necrosis, and/or infiltrative margins. With an IRB-approved protocol, we investigated patient records and clinicopathologic data from our Sarcoma Database to describe the clinical characteristics of both benign and malignant SFT. All pathology specimens were reviewed by two pathologists. Descriptive statistics and univariate/multivariate survival analysis were performed. Patient records and Social Security Death Index were used to evaluate vital status. Of 82 patients, 47 (57%) were women and 73 (89%) were Caucasian. Median age was 62 years (range, 20 to 89). Thirty-two (39%) patients succumbed to the disease. Primary tumor site was lung/pleura in 28 (34%), abdomen/pelvis in 23 (28%), extremity in 13 (16%), and head/neck in 9 (11%) patients. Pathology was described as benign in 42 (51%) and malignant in 40 (49%) patients. Compared to benign SFT, malignant histology is associated with larger tumor size, higher mitotic counts, metastatic disease at diagnosis, and greater use of chemotherapy and radiation therapy. Gender, age, and tumor site were not significantly different between benign and malignant subtypes. By univariate analysis, only benign vs. malignant variant and complete resection positively impacted overall survival (P = 0.02 and P<0.0001, respectively). In the multivariable analysis of overall survival, receiving chemotherapy or not receiving surgery were two variables significantly associated with higher failure rate in overall survival: patients with chemotherapy vs. no chemotherapy (P = 0.003, HR = 4.55, with 95% CI: 1.68–12.34) and patients without surgery vs. with surgery (P = 0.005, HR = 25.49, with 95% CI: 2.62–247.57). Clear survival differences exist between benign and malignant SFT. While surgery appears to be the best treatment option for benign and malignant SFT, better systemic therapies are needed to improve outcomes of patients with metastatic, malignant SFT.

NAB2-STAT6 fusion types account for clinicopathological variations in solitary fibrous tumors

Solitary fibrous tumor (SFT) is characterized by the inv12(q13q13)-derived NAB2-STAT6 fusion, which exhibits variable breakpoints and drives STAT6 nuclear expression. The implications of NAB2-STAT6 fusion variants in pathological features and clinical behavior remain to be characterized in a large cohort of SFTs. We investigated the clinicopathological correlates of this genetic hallmark and analyzed STAT6 immunoexpression in 28 intrathoracic, 37 extrathoracic, and 23 meningeal SFTs. These 88 tumors were designated as histologically nonmalignant in 75 cases and malignant in 13, including 1 dedifferentiated SFT. Eighty cases had formalin-fixed and/or fresh samples to extract assessable RNAs for RT-PCR assay, which revealed NAB2-STAT6 fusion variants comprising 12 types of junction breakpoints in 73 fusion-positive cases, with 65 (89%) falling into 3 major types. The predominant NAB2ex4-STAT6ex2 (n=33) showed constant breakpoints at the ends of involved exons, whereas the NAB2ex6-STAT6ex16 (n=16) and NAB2ex6-STAT6ex17 (n=16) might exhibit variable breakpoints and incorporate NAB2 or STAT6 intronic sequence. Including 73 fusion-positive and 7 CD34-negative SFTs, STAT6 distinctively labeled 87 (99%) SFTs in nuclei, exhibited diffuse reactivity in 73, but did not decorate 98 mimics tested. In seven fusion-negative cases, 6 were STAT6-positive, suggesting rare fusion variants not covered by RT-PCR assay. Regardless of histological subtypes, intrathoracic SFTs affected older patients (P=0.035) and tended to be larger in size (P=0.073). Compared with other variants, NAB2ex4-STAT6ex2/4 fusions were significantly predominant in the SFTs characterised by intrathoracic location (P<0.001), older age (P=0.005), decreased mitoses (P=0.0028), and multifocal or diffuse STAT6 staining (P=0.013), but not found to correlate with disease-free survival. Conclusively, STAT6 nuclear expression was distinctive in the vast majority of SFTs, including all fusion-positive tumors, and exploitable as a robust diagnostics of CD34-negative cases. Despite the associations of NAB2-STAT6 fusion variants with several clincopathological factors, their prognostic relevance should be further validated in large-scale prospective studies of SFTs.

Mesenchymal tumours of the mediastinum--part I

The mediastinum is an anatomically defined space in which organs and major blood vessels reside with surrounding soft tissue elements. The thymus is an important organ in the mediastinum, and many of the masses encountered in the mediastinum are related to this organ. Most neoplasms diagnosed in the mediastinum are epithelial tumours (thymomas and thymic carcinomas), lymphomas or germ cell tumours. In contrast, soft tissue tumours of the mediastinum are rare. In 1963, Pachter and Lattes systematically reviewed soft tissue pathology of the mediastinum, covering the hitherto described [2, 226, 227] In this review, based on the 2013 WHO classification of soft tissue tumours and the 2015 WHO classification of tumours of the lung, pleura, thymus and heart, we provide an updated overview of mesenchymal tumours that may be encountered in the mediastinum.

Soft tissue tumors of the sinonasal tract

Primary soft tissue tumors arising in the sinonasal tract are rare. While many mesenchymal neoplasms have been reported in the nasal cavity, sinuses, and nasopharynx, few are distinctive to this anatomic region. Some tumor types are relatively more common in this area, such as schwannoma and rhabdomyosarcoma. Nasopharyngeal angiofibroma and sinonasal hemangiopericytoma are unique entities of the sinonasal tract, as well as the recently characterized biphenotypic sinonasal sarcoma. This review discusses the clinical, morphologic, and immunohistochemical features and currently known molecular data of the more frequently encountered soft tissue tumors of the sinonasal tract.

A GRIA2 and PAX8-positive renal solitary fibrous tumor with NAB2-STAT6 gene fusion

Solitary fibrous tumor (SFT) is a rare neoplasm composed of mesenchymal-derived spindle cells. Although SFT occurs anywhere in the body, they most frequently affects the thoracic region. Here, we reported an extremely rare case of an extrathoracic SFT occurring primarily in the kidney. To our knowledge, little information has been described on the immunohistochemistry (IHC) and genetics of renal SFT.

A 41-year old Japanese female came to our hospital for further examination of a left kidney mass detected incidentally with ultrasound. Extensive investigation of the tumor, including physical, laboratory, and image examinations led to a clinical diagnosis of renal cancer (cT1aN0M0), which were in most parts imbedded in the lower polar parenchyma. The patient underwent laparoscopic radical nephrectomy. The mass was diagnosed pathologically as SFT originating from the kidney, but not as renal carcinoma. Microscopically, the tumor was composed of spindle-shape cells distributed variably in dense collagenous stroma and had a focal hemangiopericytomatous staghorn-like vascular pattern. Mitotic figures, atypical structures, necrosis and hemorrhage were not identified. No adjuvant therapies were given postoperatively. The patient has been free of tumor recurrence for 25 months since the surgery. IHC revealed that the tumor diffusely expressed CD34, CD99, Bcl2, PAX8, NAB2, STAT6, and GRIA2. The tumor stained negatively for desmin, S-100, c-Kit, CK-AE1/AE3, CDK4 and MDM2. A NAB2-SATA6 gene fusion was detected in tumor cells by reverse transcription-polymerase chain reaction, direct sequencing, and an in situ proximity ligation brightfield assay. The gene fusion occurred as an 831 bp truncation of exon 2 in NAB2 connected to the beginning of exon 3 in STAT6. We have reported a case of GRIA2 and PAX8-positive SFT occurring primarily in the kidney with such NAB2-STAT6 gene fusion for the first time. Diffuse expression of PAX8 in the tumor might present with a renal origin. Reportedly, benign histology of SFT cannot necessarily predict favorable clinical prognosis. Genetic alterations recently identified in SFT could possibly refer to risk stratification for tumor recurrence. However, malignant preponderance of extrathoracic SFT over thoracic SFT remains unexplained so far. Long-term follow-up after surgery should be performed in the present case.

Retroperitoneal solitary fibrous tumor: surgery as first line therapy

BACKGROUND

Solitary fibrous tumors (SFT) of the retroperitoneum are rare spindle cell neoplasms, with a paucity of data on treatment outcomes. We hypothesized that surgical excision offered acceptable outcomes in SFTs.

METHODS

The National Cancer Database (NCDB) was used to identify patients with SFT from 2004 to 2011. Primary outcome measures were 30 day mortality and overall survival. Descriptive analyses were performed. Furthermore, a systematic review of published literature was conducted after creating a pre-specified search strategy.

RESULTS

Of 51 patients in the NCDB, 58.8 % (n = 30) were males, with a median age 60 years (IQR 49-72 years). Median tumor size was 16 cm (IQR 11-21 cm). Surgical resection was performed in 92.2 % (n = 47) with 63.8 % (n = 30) having a margin negative resection. Peri-operative mortality was 2.1 % (n = 1). Of survival outcomes available for 18 patients, the median OS was 51.1 months. From the systematic review, we identified 8 studies, with 24 patients. Median age and tumor size was similar to the NCDB [47.5 years (IQR 39-66.5 years), 12 cm (IQR 7-17 cm)]. Majority [91.7 % (n = 22)] underwent surgical excision alone while one received adjuvant chemotherapy and none received radiation. After median follow up of 54 months (IQR 28-144 months), 79.2 % (n = 19) were alive without disease. Three patients (12.5 %) died of disease, one was alive with disease and one was lost to follow up. Recurrence was reported in 16.7 % (n = 4) of patients.

CONCLUSION

Complete surgical excision is a viable treatment modality for retroperitoneal SFT leading to long term survival. Low recurrence rates would argue against the need for routine adjuvant radiation or chemotherapy.

Solitary fibrous tumors: loss of chimeric protein expression and genomic instability mark dedifferentiation

Solitary fibrous tumors, which are characterized by their broad morphological spectrum and unpredictable behavior, are rare mesenchymal neoplasias that are currently divided into three main variants that have the NAB2-STAT6 gene fusion as their unifying molecular lesion: usual, malignant and dedifferentiated solitary fibrous tumors. The aims of this study were to validate molecular and immunohistochemical/biochemical approaches to diagnose the range of solitary fibrous tumors by focusing on the dedifferentiated variant, and to reveal the genetic events associated with dedifferentiation by integrating the findings of array comparative genomic hybridization. We studied 29 usual, malignant and dedifferentiated solitary fibrous tumors from 24 patients (including paired samples from five patients whose tumors progressed to the dedifferentiated form) by means of STAT6 immunohistochemistry and (when frozen material was available) reverse-transcriptase polymerase chain reaction and biochemistry. In addition, the array comparative genomic hybridization findings were used to profile 12 tumors from nine patients. The NAB2/STAT6 fusion was detected in all of the tumors, but immunohistochemistry and western blotting indicated that chimeric protein expression was atypical or absent in 9 out of 11 dedifferentiated tumors. The comparative genomic hybridization results revealed that the usual and malignant solitary fibrous tumors had a simple profile, whereas the genome of the dedifferentiated tumors was complex and unstable, and suggested that 13q and 17p deletions and TP53 mutations may be present in malignant lesions before the full expression of a dedifferentiated phenotype. Solitary fibrous tumor dedifferentiation is associated with the loss of chimeric oncoprotein expression, genomic instability, and cell decommitment and reprogramming. The assessment of dedifferentiated solitary fibrous tumors is based on the presence of the fusion transcripts and, in principle, negative STAT6 immunohistochemistry should not rule out a diagnosis of solitary fibrous tumor.

The emerging complexity of gene fusions in cancer

Structural chromosome rearrangements may result in the exchange of coding or regulatory DNA sequences between genes. Many such gene fusions are strong driver mutations in neoplasia and have provided fundamental insights into the disease mechanisms that are involved in tumorigenesis. The close association between the type of gene fusion and the tumour phenotype makes gene fusions ideal for diagnostic purposes, enabling the subclassification of otherwise seemingly identical disease entities. In addition, many gene fusions add important information for risk stratification, and increasing numbers of chimeric proteins encoded by the gene fusions serve as specific targets for treatment, resulting in dramatically improved patient outcomes. In this Timeline article, we describe the spectrum of gene fusions in cancer and how the methods to identify them have evolved, and also discuss conceptual implications of current, sequencing-based approaches for detection.

RNA sequencing of sarcomas with simple karyotypes: identification and enrichment of fusion transcripts

Gene fusions are neoplasia-associated mutations arising from structural chromosomal rearrangements. They have a strong impact on tumor development and constitute important diagnostic markers. Malignant soft tissue tumors (sarcomas) constitute a heterogeneous group of neoplasms with >50 distinct subtypes, each of which is rare. In addition, there is considerable morphologic overlap between sarcomas and benign lesions. Several subtypes display distinct gene fusions, serving as excellent biomarkers. The development of methods for deep sequencing of the complete transcriptome (RNA-Seq) has substantially improved the possibilities for detecting gene fusions. With the aim of identifying new gene fusions of biological and clinical relevance, eight sarcomas with simple karyotypes, ie, only one or a few structural rearrangements, were subjected to massively parallel paired-end sequencing of mRNA. Three different algorithms were used to identify fusion transcripts from RNA-Seq data. Three novel (KIAA2026-NUDT11, CCBL1-ARL1, and AFF3-PHF1) and two previously known fusions (FUS-CREB3L2 and HAS2-PLAG1) were found and could be verified by other methods. These findings show that RNA-Seq is a powerful tool for detecting gene fusions in sarcomas but also suggest that it is advisable to use more than one algorithm to analyze the output data as only two of the confirmed fusions were reported by more than one of the gene fusion detection software programs. For all of the confirmed gene fusions, at least one of the genes mapped to a chromosome band implicated by the karyotype, suggesting that sarcomas with simple karyotypes constitute an excellent resource for identifying novel gene fusions.

Genital soft tissue tumors

Mesenchymal neoplasms of the vulvovaginal and inguinoscrotal regions are among the most diagnostically challenging specimens in the pathology laboratory owing largely to their unique intersection between general soft tissue tumors and relatively genital-specific mesenchymal tumors. Genital stromal tumors are a unique subset of soft tissue tumors encountered at this location, and this group includes fibroepithelial stromal polyp, superficial (cervicovaginal) myofibroblastoma, cellular angiofibroma, mammary-type myofibroblastoma, angiomyofibroblastoma and aggressive angiomyxoma. Aside from the striking morphologic and immunophenotypic similarity that is seen with these entities, there is evidence that a subset of genital stromal tumors may be linked genetically. This review will focus on simplifying this group of tumors and provide the pathologist or dermatopathologist with practical management information. Smooth muscle tumors of the external genitalia will also be discussed.

Extensive survey of STAT6 expression in a large series of mesenchymal tumors

OBJECTIVES

Expression of strong nuclear STAT6 is thought to be a specific marker for solitary fibrous tumors (SFTs). Little is known about subtle expression patterns in other mesenchymal lesions.

METHODS

We performed immunohistochemical studies against the C-terminus of STAT6 in tissue microarrays and whole sections, comprising 2366 mesenchymal lesions.

RESULTS

Strong nuclear STAT6 was expressed in 285 of 2,021 tumors, including 206 of 240 SFTs, 49 of 408 well-differentiated/dedifferentiated liposarcomas, eight of 65 unclassified sarcomas, and 14 of 184 desmoid tumors, among others. Expression in SFTs was predominately limited to the nucleus. Other positive tumors typically expressed both nuclear and cytoplasmic STAT6. Complete absence of STAT6 was most common in pleomorphic liposarcoma and alveolar soft part sarcoma (60% and 72% cases negative, respectively).

CONCLUSIONS

Strong nuclear STAT6 is largely specific for SFTs. Physiologic low-level cytoplasmic/nuclear expression is common in mesenchymal neoplasia and is of uncertain significance.