The fifth edition of the World Health Organization Classification of Tumors of Soft Tissue and Bone was published in early 2020. The revisions reflect a consensus among an international expert ...editorial board composed of soft tissue and bone pathologists, geneticists, a medical oncologist, surgeon, and radiologist. The changes in the soft tissue tumor chapter notably include diverse, recently described tumor types (eg, atypical spindle cell/pleomorphic lipomatous tumor, angiofibroma of soft tissue, and CIC-rearranged sarcoma), new clinically significant prognostic information for a variety of existing entities (eg, dedifferentiated liposarcoma and solitary fibrous tumor), and a plethora of novel genetic alterations, some of practical diagnostic relevance (eg, NAB2-STAT6 in solitary fibrous tumor, FOSB rearrangements in epithelioid hemangioma and pseudomyogenic hemangioendothelioma, and SUZ12 or EED mutations in malignant peripheral nerve sheath tumor, leading to loss of H3K27 trimethylation). In this review, we highlight the major changes to the soft tissue chapter in the 2020 World Health Organization Classification, as well as the new chapter on undifferentiated small round cell sarcomas, with a focus on updates in diagnostic categories, prognostication, and novel markers. Recent discoveries in molecular genetics are also discussed, particularly those of immediate utility in differential diagnosis, including protein correlates detectable using immunohistochemistry.
Diagnosing soft tissue tumors is challenging, even on ample incisional biopsies or resection specimens. There are more than 100 distinct types of soft tissue neoplasms, including more than 80 benign ...and intermediate mesenchymal tumors and around 40 soft tissue sarcomas. Accurate diagnosis relies first upon recognition of characteristic histologic and cytologic features, including architecture, stromal characteristics, vascular patterns, and dominant cytology; these features may not be represented or apparent in limited core needle biopsy or fine needle aspiration specimens. Once a differential diagnosis is established, application of immunohistochemistry and cytogenetic or molecular diagnostic assays (especially fluorescence in situ hybridization) is used in an attempt to reach a specific diagnosis. In recent years, the diagnostic armamentarium for soft tissue tumors has expanded dramatically, following the discovery of molecular alterations that underlie the pathogenesis of soft tissue tumors. These include new diagnostic immunohistochemical markers that serve as useful surrogates for molecular genetic alterations. Availability of such markers has improved our ability to render accurate and specific diagnoses based on limited biopsy samples. In this review, examples of recently developed markers for the diagnosis of selected soft tissue tumor types will be discussed, including solitary fibrous tumor (STAT6), malignant peripheral nerve sheath tumor (H3K27me3), epithelioid hemangioendothelioma (CAMTA1), dedifferentiated liposarcoma (MDM2), and CIC-DUX4 sarcoma (WT1 and ETV4).
Angiotensin converting enzyme 2 (ACE2) is the putative functional receptor for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Current literature on the abundance and distribution of ...ACE2 protein in the human respiratory tract is controversial. We examined the effect of age and lung injury on ACE2 protein expression in rodent and non-human primate (NHP) models. We also examined ACE2 expression in human tissues with and without coronavirus disease 19 (COVID-19). ACE2 expression was detected at very low levels in preterm, but was absent in full-term and adult NHP lung homogenates. This pattern of ACE2 expression contrasted with that of transmembrane protease serine type 2 (TMPRSS2), which was significantly increased in full-term newborn and adult NHP lungs compared to preterm NHP lungs. ACE2 expression was not detected in NHP lungs with cigarette smoke-induced airway disease or bronchopulmonary dysplasia. Murine lungs lacked basal ACE2 immunoreactivity, but responded to hyperoxia, bacterial infection, and allergen exposure with new ACE2 expression in bronchial epithelial cells. In human specimens, robust ACE2 immunoreactivity was detected in ciliated epithelial cells in paranasal sinus specimens, while ACE2 expression was detected only in rare type 2 alveolar epithelial cells in control lungs. In autopsy specimens from patients with COVID-19 pneumonia, ACE2 was detected in rare ciliated epithelial and endothelial cells in the trachea, but not in the lung. There was robust expression of ACE2 expression in F344/N rat nasal mucosa and lung specimens, which authentically recapitulated the ACE2 expression pattern in human paranasal sinus specimens. Thus, ACE2 protein expression demonstrates a significant gradient between upper and lower respiratory tract in humans and is scarce in the lung. This pattern of ACE2 expression supports the notion of sinonasal epithelium being the main entry site for SARS-CoV-2 but raises further questions on the pathogenesis and cellular targets of SARS-CoV-2 in COVID-19 pneumonia.
The diagnosis of malignant peripheral nerve sheath tumor is challenging, particularly in the sporadic setting. Inactivation of the polycomb repressive complex 2 (PRC2), resulting from inactivating ...mutations of its constituents SUZ12 or EED1, has recently been identified in 70-90% of malignant peripheral nerve sheath tumors. Homozygous PRC2 inactivation results in loss of histone H3K27 trimethylation (H3K27me3). PRC2 inactivation promotes tumor progression and may render patients sensitive to epigenetic-based targeted therapies. H3K27me3 loss has not yet been validated as a diagnostic marker. We evaluated immunohistochemistry for H3K27me3 in 100 malignant peripheral nerve sheath tumors (70 sporadic, 10 neurofibromatosis type 1-associated, 10 radiation-associated, 10 epithelioid) and 200 other spindle cell neoplasms representing potential mimics (20 each monophasic synovial sarcoma, leiomyosarcoma, dedifferentiated liposarcoma, malignant solitary fibrous tumor, low-grade fibromyxoid sarcoma, cellular schwannoma, spindle cell melanoma, unclassified postradiation sarcoma; 10 each atypical neurofibroma, spindle cell rhabdomyosarcoma, gastrointestinal stromal tumor, fibrosarcomatous dermatofibrosarcoma protuberans). In total, 51 (51%) malignant peripheral nerve sheath tumors, including 34 (49%) sporadic, 7 (70%) neurofibromatosis type 1-associated, and 10 (100%) radiation-associated, but no epithelioid malignant peripheral nerve sheath tumors, were negative for H3K27me3. An additional 6 (6%) tumors showed heterogeneous H3K27me3 expression. Among the 90 sporadic, neurofibromatosis type 1-associated, and radiation-associated malignant peripheral nerve sheath tumors, complete H3K27me3 loss was observed in 29% of low-grade, 59% of intermediate-grade, and 83% of high-grade tumors (low vs intermediate/high grade, P=0.0003). Among other tumor types, 4 (20%) unclassified postradiation sarcomas were negative for H3K27me3, whereas all other neoplasms were positive. Loss of H3K27me3 is highly specific for malignant peripheral nerve sheath tumor (although only modestly more sensitive than S-100 protein and SOX10) and may be a useful diagnostic marker. Our findings suggest that PRC2 inactivation in malignant peripheral nerve sheath tumor may occur during progression to higher grades.
Pleomorphic sarcomas are a heterogeneous group of mesenchymal neoplasms with widely varied clinical behavior but overlapping histologic appearances. The following guidelines are helpful when ...approaching the diagnosis of a pleomorphic sarcoma. (1) Be aware of the relative incidence of the various sarcoma types: several pleomorphic sarcomas are relatively common (e.g., dedifferentiated liposarcoma and undifferentiated pleomorphic sarcoma), whereas others are exceptionally rare. (2) Pay attention to anatomic location: some pleomorphic sarcomas have a predilection for somatic soft tissues, especially the thigh (e.g., undifferentiated pleomorphic sarcoma, pleomorphic liposarcoma, pleomorphic rhabdomyosarcoma), whereas other pleomorphic sarcomas most often arise in the retroperitoneum (e.g., dedifferentiated liposarcoma). (3) Carefully sample the resection specimen, paying particular attention to areas with differences in gross appearances (e.g., fleshy, fibrous, mucoid, or gritty). (4) Search for histologic clues (i.e., myxoid stroma, lipoblasts, and osteoid matrix, in order to diagnose myxofibrosarcoma, pleomorphic liposarcoma, and extraskeletal osteosarcoma, respectively); these critical diagnostic features may be limited in extent. (5) Apply immunohistochemistry judiciously, after generating a differential diagnosis; always exclude metastatic sarcomatoid carcinoma and melanoma before diagnosing a pleomorphic sarcoma. This review will present an approach to the diagnosis of pleomorphic sarcomas, emphasizing differential diagnosis and the application of ancillary studies (immunohistochemistry and FISH), when relevant.
•Pleomorphic sarcomas are a heterogeneous group of tumors with widely varied metastatic potential.•A careful search for histologic clues (myxoid stroma, lipoblasts, osteoid matrix) is mandatory.•Metastatic sarcomatoid carcinoma and melanoma should always be excluded.•Immunohistochemistry or FISH for MDM2 aids in the diagnosis of dedifferentiated liposarcoma.
Sarcomas include diverse mesenchymal neoplasms with widely varied prognosis, clinical behavior, and treatment. Owing to their rarity and histologic overlap, accurate diagnosis of sarcomas can be ...challenging. Our approach has evolved dramatically in the past few decades, where novel insights into the molecular pathogenetic basis for sarcomas has dramatically (re)shaped contemporary diagnosis, building on a largely morphology- and clinical presentation-based strategy. Examples include the introduction of novel immunohistochemical markers that serve as surrogates for molecular genetic alterations and identification of characteristic molecular alterations. Accordingly, cytogenetic and molecular genetic analyses, such as conventional karyotyping, fluorescence in situ hybridization, reverse transcription-polymerase chain reaction, and targeted sequencing, have increasingly been incorporated into the routine diagnostic work-up of these neoplasms. For those sarcomas with complex cytogenetic changes that lack specific alterations, additional testing is often directed toward identifying lines of differentiation and excluding pathognomonic (cyto-)genetic alterations. Although some gene rearrangements are diagnostic of particular sarcoma types, certain fusion partners, most notably EWSR1, are not tumor specific (and may, in fact, also be found in benign tumors). Correlation with clinical, radiologic, morphologic, and immunohistochemical findings is particularly important in tumors with such rearrangements to establish the correct diagnosis, acknowledging the inherent limitations of diagnostic tests. The recognition of sarcomas occurring in cancer predisposition syndromes is critical, with implications not only for the index patient but also potentially for family members, including the need for genetic counseling and sometimes particular types of surveillance. Together, contemporary sarcoma evaluation involves combining the initial morphologic evaluation with diagnostically relevant cytogenetic, molecular, and immunohistochemical testing methods.
Synovial sarcoma (SS), an aggressive soft tissue sarcoma with a predilection for the extremities of young adults, harbors the pathognomonic t(X;18)(p11;q11) translocation, resulting in SS18-SSX ...rearrangements. SS includes monophasic, biphasic, and poorly differentiated variants, which show considerable histologic overlap with a range of other tumor types, making the diagnosis challenging on limited biopsies. Immunohistochemistry (IHC) is routinely used in the differential diagnosis; however, presently available markers lack specificity. Thus, cytogenetic or molecular genetic techniques are often employed to confirm the diagnosis. Here, we report the development and characterization of 2 novel antibodiesan SS18-SSX fusion-specific antibody (E9X9V, designed to the breakpoint) as well as an SSX-specific antibody (E5A2C, designed to the SSX C-terminus). We validated the selectivity and specificity of the antibodies using immunoblotting, immunoprecipitation, and chromatin immunoprecipitation followed by next-generation sequencing in SS cell lines and demonstrated that both antibodies capture SS18-SSX on chromatin at established target sites (eg, TLE1 and BCL2) genome-wide. Using IHC in whole sections from 400 tumors including 100 genetically confirmed cases of SS and 300 histologic mimics, the SS18-SSX fusion-specific antibody revealed strong diffuse nuclear staining in 95 of 100 (95%) SS cases, whereas none of the 300 control tumors showed any staining. The SSX antibody showed strong diffuse nuclear staining in all 100 (100%) SS cases; 13 (4%) of the 300 other tumors were also positive, 5 of which displayed >50% nuclear staining. In summary, a novel SS18-SSX fusion-specific antibody is highly sensitive (95%) and specific (100%) for SS, and an antibody to the SSX C-terminus is also highly sensitive (100%), but slightly less specific (96%). IHC using the SS18-SSX antibody could replace molecular genetic or cytogenetic testing in most cases, and these reagents together will also provide the research community with valuable tools for further biochemical and genomic interrogation of the SS18-SSX fusion protein.
Solitary fibrous tumor (SFT) is composed of spindled to ovoid cells in a patternless architecture with prominent stromal collagen and hemangiopericytoma-like vessels. Some tumors show ...hypercellularity, nuclear atypia, and significant mitotic activity; the latter feature in particular often portends an aggressive clinical course. SFT can sometimes be difficult to distinguish from other benign mesenchymal tumors and sarcomas. The most characteristic (albeit nonspecific) immunohistochemical finding in SFT is CD34 expression. A NAB2–STAT6 gene fusion, resulting in a chimeric protein in which a repressor domain of NGFI-A binding protein 2 (EGR1 binding protein 2) (NAB2) is replaced with a carboxy-terminal transactivation domain from signal transducer and activator of transcription 6, interleukin-4 induced (STAT6), was recently identified as a consistent finding in SFT. However, as these genes are located in close proximity on 12q13, this fusion can only rarely be detected by conventional chromosomal banding or fluorescence in situ hybridization analysis. Nuclear expression of the carboxy terminal part of STAT6 is a consistent finding in SFT of the meninges (so-called ‘meningeal hemangiopericytoma'). We investigated STAT6 expression by immunohistochemistry in SFTs and other soft tissue tumors arising outside the central nervous system to validate the diagnostic utility of this novel marker. Whole-tissue sections of 231 tumors were evaluated, including 60 cases of SFT as well as other benign and malignant mesenchymal neoplasms and sarcomatoid mesotheliomas. Fifty-nine of 60 SFT cases (98%) showed nuclear expression of STAT6, which was usually diffuse and intense. All other tumor types were negative for STAT6, except for three dedifferentiated liposarcomas and one deep fibrous histiocytoma, which showed weak staining. In conclusion, STAT6 is a highly sensitive and almost perfectly specific immunohistochemical marker for SFT and can be helpful to distinguish this tumor type from histologic mimics.
In the 2018 World Health Organization Classification of Skin Tumors, a wide range of predominantly benign mesenchymal neoplasms are included in the fibroblastic, myofibroblastic, and ...“fibrohistiocytic” categories. By far the most common of these tumors is dermatofibroma (fibrous histiocytoma). There are many histologic variants of dermatofibroma, some of which (cellular, aneurysmal, and atypical) are associated with a higher risk of local recurrence; these variants may be mistaken for more aggressive tumor types, including sarcomas. Furthermore, distinguishing among the fibrous and “fibrohistiocytic” tumors can be a diagnostic challenge, given their sometimes-similar histologic appearances and confusing nomenclature. Immunohistochemistry and molecular genetic assays play a relatively limited role in the diagnosis of these tumor types, with notable exceptions (i.e., epithelioid fibrous histiocytoma and dermatofibrosarcoma protuberans). Proper recognition of dermatofibrosarcoma protuberans is critical, since this tumor type is associated with locally aggressive behavior; transformation to the fibrosarcomatous variant brings metastatic potential. In recent years, understanding of the molecular pathogenetic basis for cutaneous mesenchymal neoplasms has increased dramatically, with the discovery of gene rearrangements in some of these tumor types. In this review, the histologic features of the most common fibrous and “fibrohistiocytic” cutaneous mesenchymal neoplasms will be discussed, as well as recently identified molecular genetic alterations.
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