Review Article. neoplasm. Angiomatoid fibrous histiocytoma. The Current Status of Pathology and Genetics

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1 Review Article Angiomatoid Fibrous Histiocytoma The Current Status of Pathology and Genetics Khin Thway, MBBS, BSc, FRCPath; Cyril Fisher, MD, DSc, FRCPath Context. Angiomatoid fibrous histiocytoma (AFH) is a rare soft tissue neoplasm of intermediate biologic potential and uncertain differentiation, most often arising in the superficial extremities of children and young adults. While it has characteristic histologic features of nodular distributions of ovoid and spindle cells with blood-filled cystic cavities and a surrounding dense lymphoplasmacytic infiltrate, there is a significant morphologic spectrum, which coupled with its rarity and lack of specific immunoprofile can make diagnosis challenging. Angiomatoid fibrous histiocytoma is associated with 3 characteristic gene fusions, EWSR1-CREB1 and EWSR1-ATF1, which are also described in other neoplasms, and rarely FUS-ATF1. Angiomatoid fibrous histiocytoma is now recognized at an increasing number of sites and is known to display a variety of unusual histologic features. Objective. To review the current status of AFH, discussing putative etiology, histopathology with variant morphology and differential diagnosis, and current genetics, including overlap with other tumors harboring EWSR1- CREB1 and EWSR1-ATF1 fusions. Data Sources. Review of published literature, including case series, case reports, and review articles, in online medical databases. Conclusions. The occurrence of AFH at several unusual anatomic sites and its spectrum of morphologic patterns can result in significant diagnostic difficulty, and correct diagnosis is particularly important because of its small risk of metastasis and death. This highlights the importance of diagnostic recognition, ancillary molecular genetic confirmation, and close clinical follow-up of patients with AFH. Further insight into the genetic and epigenetic changes arising secondary to the characteristic gene fusions of AFH will be integral to understanding its tumorigenic mechanisms. (Arch Pathol Lab Med. 2015;139: ; doi: / arpa ra) Angiomatoid fibrous histiocytoma (AFH) is a rare soft tissue neoplasm of intermediate (rarely metastasizing) biologic potential and uncertain differentiation, which predominantly arises superficially in the deep dermis and subcutis of the extremities of children and young adults. 1 3 It was first reported as angiomatoid malignant fibrous histiocytoma in 1979 by Enzinger, 1 who described 41 cases of an unusual fibrohistiocytic sarcoma. There is little documentation of similar lesions in the literature before this perhaps because these were subsumed into descriptions of other entities such as sclerosing hemangioma, 2 dermatofibroma, and other lesions classified as fibrous histiocytoma. 1 Enzinger 1 initially proposed AFH as a variant of malignant fibrous histiocytoma (now termed undifferentiated pleomorphic sarcoma), albeit one that tended to arise in a much younger population and at superficial sites, in contrast to usual-type malignant fibrous histiocytoma, which typically arises in the deep soft tissues of older adults. Subsequent Accepted for publication July 11, From the Sarcoma Unit, Royal Marsden Hospital, London, England. The authors have no relevant financial interest in the products or companies described in this article. Reprints: Khin Thway, MBBS, BSc, FRCPath, Sarcoma Unit, Royal Marsden Hospital, 203 Fulham Rd, London SW3 6JJ, England ( khin.thway@rmh.nhs.uk). large investigations demonstrated the much more favorable prognosis of AFH after wide excision, 3 which led to its recognition as a distinct entity. Further to the characterization of the recurrent chromosomal rearrangements resulting in the EWSR1-CREB1, EWSR1-ATF1, and FUS-ATF1 gene fusions, AFH is of course now established as a translocation-associated neoplasm. Angiomatoid fibrous histiocytoma has now been documented in a variety of extrasomatic sites, and an increasing spectrum of histologic features has been described. We review the current status of AFH, discussing putative etiology, clinical findings, histopathology with morphologic variants and differential diagnosis, and current genetics, including overlap with other tumors harboring EWSR1-CREB1 and EWSR1-ATF1 fusions. CLINICAL FEATURES Angiomatoid fibrous histiocytoma is rare, accounting for approximately only 0.3% of all soft tissue tumors, 4 although given its histologic similarity to a variety of other neoplasms it is likely that it has been previously underdiagnosed and subsumed under a variety of other neoplastic categories, including vascular, fibrohistiocytic, and myofibroblastic types. Most patients present in the first 3 decades of life, although the age distribution is wide, and AFH can occur in infants (including congenitally) 5 and adults to the eighth and ninth decades. 2,4 There is no significant sex predilection. Angiomatoid fibrous histiocytoma most frequently occurs 674 Arch Pathol Lab Med Vol 139, May 2015 Angiomatoid Fibrous Histiocytoma Thway & Fisher

2 superficially in the deep dermis and subcutis of the extremities and usually presents as a slowly growing, superficial nodular mass that seldom causes tenderness or pain and is often mistaken clinically for a hematoma or a hemangioma. 1 Lesions can also present with symptoms related to the anatomic site or may be detected incidentally. 6 Other more frequent sites for AFHs are the trunk and head and neck; occurrence in nonsomatic soft tissue sites is rare but increasingly documented, and unusual primary sites include the brain, 7,8 lungs, 7 9 mediastinum, 6,9,10 retroperitoneum, 6 omentum, 11 ovary, 6 vulva, 6 and bone. 9,10 Extrasomatic AFHs also show a higher mean age at presentation (35 years compared with years for somatic cases) 6 and tend to be larger neoplasms. 6 Some patients experience systemic symptoms such as pyrexia, anemia, and malaise, 11 suggesting tumoral cytokine production, and this is also thought to be more frequent in extrasomatic AFH. 6 Angiomatoid fibrous histiocytoma does not have a proven relationship with any other specific conditions, although one case has been reported on the knee of a child with human immunodeficiency virus. 12 Reports of its occurrence as a second neoplasm in other malignancies are rare and include a supraclavicular AFH occurring in a 27-year-old man 16 months after chemotherapy for disseminated testicular cancer, 13 an inguinal mass in a child with stage III retroperitoneal neuroblastoma, 14 and in an 18-year-old woman with a 4-year history of treatment for Hodgkin lymphoma. 15 It is unclear whether these might have occurred secondarily to treatment, although it appears more likely that they were incidental. Other examples of AFH associated with other neoplasms include a vulval lesion detected at the same time as endometrial and ovarian carcinoma 6 and a mediastinal lesion detected during surveillance computed tomography scan for nasopharyngeal carcinoma treated with radiation therapy 13 years previously. 6 Most AFHs behave relatively indolently, with local recurrence in up to 15% of cases. 2,4 Less than 5% have been reported to metastasize, 1,3,5 sometimes after multiple recurrences, 1 and this is predominantly to regional lymph nodes but exceptionally to the lungs, liver, or brain. 3,16 Deaths from distant metastases are very rare 1 and may occur 2 decades after initial presentation, 1,3 although long-term survival and cure are still possible in the presence of metastatic disease. 1,3 Genetically confirmed primary AFH with metastatic behavior has only rarely been documented, 17 and metastatic AFH has been genetically confirmed in only one case. 18 Local recurrence is related to infiltrative margins and head and neck location, 3 probably reflecting the difficulty in obtaining adequate surgical clearance at this site. Extrasomatic AFHs appear to show higher recurrence rates compared with those of somatic soft tissue. 6 Invasion into deep fascia or muscle correlates with both local and distant metastatic behavior. 3 Appropriate management is considered to be wide local excision with follow-up, although adjuvant radiation therapy or chemotherapy may be indicated for metastatic or unresectable disease. 19 ETIOLOGY The etiology and differentiation of AFH remain unclear, and its constituent ovoid and spindle cells do not wholly phenotypically resemble any mature cell type. Ultrastructural investigations have been conflicting and have failed to show specific findings, which may be at least partly because of sampling error and poor tissue preservation. While AFH was originally thought to be of fibrohistiocytic derivation, akin to malignant fibrous histiocytoma, 15,20,21 no conclusive ultrastructural or immunohistochemical evidence of this was found. Early studies also described vessels in varying stages of formation with endothelium, basal lamina, and sometimes pericytes, 22 and a modified endothelial cell was proposed as an originator, 20 but this was subsequently disproved by the consistent lack of immunohistochemical expression of endothelial markers. One study 23 appeared to show smooth and striated muscle cells in addition to histiocyte-like and fibroblast-like cells, and short interdigitating processes connected by desmosome-like junctions have been described in several tumors. 15,22,24 Immunohistochemical studies, including the findings of desmin-positive cells within the lymphoid proliferation, postulated a myoid or myofibroblastic line of differentiation, 4,15 such that an alternate name of angiomatoid myosarcoma was suggested. 11 Another theory has been of origin from fibroblastic reticulum cells that provide a structural supporting function in the stroma of lymph nodes, a subset of cells that have been shown to express desmin. 25 Pluripotent mesenchymal stem cells have been postulated as potential originating cells for several sarcoma types, and the same suggestion was made for AFH by some authors in the 1980s. 5 Further to the characterization of the recurrent chromosomal rearrangements that result in the EWSR1- CREB1, t(12;22)(q13;q12) EWSR1-ATF1, and t(12;16)(q13;p11) FUS-ATF1 gene fusions, AFH is of course now established as a translocation-associated neoplasm. Translocations are thought to be either initial or early steps in tumor formation 32 and result in gene fusions that frequently lead to the formation of novel, tumor-specific chimeric transcription factors 33 that can cause dysregulation of gene expression. 34 The propensity of certain genes to partner with others is not clearly understood but might be partly because of their proximity within the 3-dimensional chromosomal arrangement within the nucleus. 35 The fusion products generated presumably instigate novel differentiation programs corresponding to the properties of the specific, as yet unknown, target cell of AFH, with secondary genetic and epigenetic events contributing to neoplastic transformation. HISTOPATHOLOGY Grossly, AFHs are firm, multinodular and multicystic, hemorrhagic masses, with cut surfaces varying from grayish-yellow to white. They are usually small, ranging from 2 to 4 cm, although they can reach 10 cm (median, 2.5 cm). 1 Histologically, tumors are circumscribed and lobulated or multinodular, often with a thick, often incomplete fibrous pseudocapsule (Figure 1, A). Up to 80% are surrounded by a dense lymphoplasmacytic infiltrate or cuff, which may include germinal center formation 1,3,4 (Figures 1, B). Angiomatoid fibrous histiocytoma shows a wide morphologic spectrum, and the only constant finding is of sheets and short fascicles of ovoid, epithelioid, or spindle cells with bland, vesicular nuclei (Figure 1). The cells often have a fibroblastic or histiocytoid appearance with moderate amounts of eosinophilic cytoplasm (Figure 2). Mitotic figures are usually infrequent, although atypical forms may be present (Figure 3, A). 36 Smaller numbers of lesions show pleomorphism, which may be diffuse or marked, 37 but cellular atypia and increased numbers of mitotic figures are not associated with a worse clinical outcome. 3 Intralesional hemorrhage is seen at least focally in most cases, leading to Arch Pathol Lab Med Vol 139, May 2015 Angiomatoid Fibrous Histiocytoma Thway & Fisher 675

3 Figure 1. A, Angiomatoid fibrous histiocytoma. Tumors are circumscribed and lobulated or multinodular, often with a thick, frequently incomplete fibrous pseudocapsule. B, Up to 80% of angiomatoid fibrous histiocytomas are surrounded by a dense lymphoplasmacytic infiltrate or cuff, which often includes germinal center formation.1,3,4 C through F, Angiomatoid fibrous histiocytoma shows a wide morphologic spectrum, and the only constant finding is of sheets and short fascicles of ovoid, epithelioid, or spindle cells with bland, vesicular nuclei. Intralesional hemorrhage is seen at least focally in most cases, leading to the formation of variably sized blood-filled pseudoangiomatous spaces that sometimes occupy the major portion of the tumor. These lack endothelial linings and are instead lined by flattened neoplastic cells (C and F). There may be prominent hemosiderin deposition (D), shown here as prominent hemosiderin-laden macrophages (right of field). The bland islands of cells can mimic granulomas (E) (hematoxylin-eosin, original magnifications 340 [A], 3100 [B and D through F], and 3200 [C]). 676 Arch Pathol Lab Med Vol 139, May 2015 Angiomatoid Fibrous Histiocytoma Thway & Fisher

4 Figure 2. A, Angiomatoid fibrous histiocytoma. The cells often have a fibroblastic or histiocytoid appearance with moderate amounts of eosinophilic cytoplasm. B, The cells are often arranged in syncytial-like sheets. C and D, The cells are usually relatively uniform, although smaller numbers of angiomatoid fibrous histiocytomas show cellular pleomorphism, which may be diffuse or marked but is not associated with a worse clinical outcome. Other architectural patterns include storiform distributions (C) or whorls (D). E, Occasionally, there are interspersed tumoral giant cells. F, The stroma may be markedly sclerotic (hematoxylin-eosin, original magnifications 3200 [A and D] and 3400 [B, C, E, and F]). the formation of variably sized blood-filled pseudoangiomatous spaces sometimes occupying the major portion of the tumor, which lack endothelial linings and are instead lined by flattened neoplastic cells (Figure 1, C and F). Hemosiderin deposition can be a prominent feature (Figure 1, D). Approximately one-third show completely solid histology without pseudoangiomatoid spaces. 38 The stroma may be myxoid (Figure 3, B) (and can rarely account for the Arch Pathol Lab Med Vol 139, May 2015 Angiomatoid Fibrous Histiocytoma Thway & Fisher 677

5 Figure 3. A, Angiomatoid fibrous histiocytoma. Mitotic figures are usually infrequent, although atypical forms may be present. B, The stroma may be myxoid and can rarely account for the predominant portion of the neoplasm. C, Approximately half of angiomatoid fibrous histiocytomas express desmin, which may be diffuse or focal, while angiomatoid fibrous histiocytomas can occasionally express other markers of myoid differentiation such as smooth muscle actin, calponin, and rarely h-caldesmon; however, they are consistently negative for skeletal muscle markers such as myogenin or MyoD1. D, There can be variable expression of epithelial membrane antigen, which is also expressed in the abundant plasma cell population within the lymphoid cuff surrounding the tumor (right of field) (hematoxylin-eosin, original magnifications 3400 [A] and 3100 [B]; and immunohistochemistry, original magnification 3200 [C and D]). predominant portion of the neoplasm) 39 or occasionally sclerotic or desmoplastic-like (Figure 2, F), giving an appearance resembling poorly differentiated carcinoma. 6 Occasionally, tumoral giant cells are seen (Figure 2, E) and rarely reactive osteoclast-like giant cells within pseudoangiomatoid spaces. 38 Unusual morphologic features, reported in smaller numbers of AFHs, include nuclear grooving, clear cells, rhabdomyoblast-like cells, groups of small cells with scanty cytoplasm reminiscent of Ewing sarcoma, and pulmonary edema like and reticular patterns of cells in myxoid stroma. 6,40 In addition, some cases show schwannoma-like features, including nuclear palisading and stroma containing prominently hyalinized vessels. 38 There are no reliable histopathological parameters that predict behavior, 3 and both primary AFH and metastatic AFH are frequently morphologically typical, including the genetically documented case of metastatic AFH. 18 Angiomatoid fibrous histiocytoma lacks a specific immunoprofile, so that immunohistochemistry is supportive rather than diagnostic. Approximately half of these neoplasms express desmin (Figure 3, C), which may be diffuse or focal, and occasionally there is expression of other markers of myoid differentiation such as smooth muscle actin, calponin, and rarely h-caldesmon. However, they are consistently negative for skeletal muscle markers such as myogenin or MyoD1. Expression of epithelial membrane antigen (Figure 3, D), CD99, and CD68 has been variably reported, ranging from approximately 40% to 50% of lesions 4,22,41 to all lesions in one study. 6 There may be occasional focal CD21 expression, although the majority are negative for this. Vascular endothelial markers such as CD31, CD34, and factor VIII related antigen are also negative, as are CD35, S100 protein, cytokeratins, and lysozyme. 41 The surrounding lymphoid infiltrate is composed of a mixture of B lymphocytes and T lymphocytes, and within this lymphoid population there are often scattered desmin-positive cells. 42 The Ki-67 proliferative index is usually low, at 2% to 4% Arch Pathol Lab Med Vol 139, May 2015 Angiomatoid Fibrous Histiocytoma Thway & Fisher

6 GENETICS Angiomatoid fibrous histiocytoma is associated with the following 3 characteristic translocations: t(2:22)(q33:q12) (forming the EWSR1-CREB1 fusion gene), 43,44 t(12:22)- (q13:q12) (forming the EWSR1-ATF1 fusion gene) 7,44 46 and t(12:16)(q13:p11) (resulting in the FUS-ATF1 fusion gene). 47,48 EWSR1 and FUS are multifunctional proteins belonging to the FET (previously TET) family of RNA-binding proteins, which are implicated in central cellular mechanisms such as theregulationofgeneexpression,maintenanceofgenomic integrity, and processing of messenger RNA/microRNA. 49 They are structurally similar, although they differ in exon/ intron structures in the 5 0 parts, and FUS is smaller than EWSR1 overall. 50 FUS and EWSR1 (along with TAF15, another member of the FET family) are rearranged with various transcription factor genes in sarcomas and more rarely in hematopoietic and epithelial cancers. 51 FUS can serve as an alternative to EWSR1 in other sarcomas (eg, FUS-ERG in Ewing sarcoma 51 and acute myeloid leukemia 52 and FUS-DDIT3 in myxoid liposarcoma), 53,54 and EWSR1 and FUS appear to serve as alternative 5 0 partners with the ATF1 gene. 47 EWSR1-CREB1 is the most frequently described gene fusion to date, having been described in more than 90% of cases, 43,44 although EWSR1-ATF1 appears to be more common in AFH occurring in extrasomatic soft tissue sites. 6 FUS-ATF1 has been described least commonly. 40 Occasional cases with EWSR1 rearrangement have failed to show involvement of ATF1 or CREB1, suggesting that these may have as yet unidentified fusion partners. Tumors with different fusions can occur at specific anatomic sites; for example, both EWSR1-CREB1 and EWSR1-ATF1 fusions have been documented in pulmonary AFH. 8,28 The type of gene fusion has not yet been shown to correlate with clinical, histologic, or immunohistochemical differences. 27 Therefore, molecular and molecular cytogenetic investigations are crucial in confirming the diagnosis of AFH, and fluorescence in situ hybridization (FISH) for EWSR1 rearrangement and reverse transcription polymerase chain reaction (RT-PCR) to assess for specific fusion transcripts are valuable ancillary diagnostic aids, which should ideally be used in a complementary manner. It should be noted that in the largest series of AFH diagnoses by FISH to date, 24% did not harbor detectable EWSR1 (or FUS) rearrangement. 55 This series found EWSR1 rearrangements in 13 of 17 (76.5%) AFHs but no FUS rearrangements in any of the cases, indicating that EWSR1 is a common genetic event in AFH and that FISH for EWSR1 rearrangement is useful diagnostically. While FISH generally has a higher sensitivity for detecting tumors and a better success rate than RT- PCR, 56 EWSR1 may not always be detectable when a fusion transcript is detected by RT-PCR, which may be because of variant translocations with submicroscopic insertions or exchanges of genetic material undetectable by commercial break-apart probes. 18 While FUS-ATF1 fusions have so far not been detected in other neoplasms, EWSR1-CREB1 and EWSR1-ATF1 fusions are not unique to AFH and have been consistently described in a number of different tumor groups that are clinically and usually histopathologically distinct from AFH. 57 The significant diversity of soft tissue neoplasms associated with EWSR1 gene rearrangements is well known 58 ; EWSR1 is capable of fusing with several different partner genes, which can sometimes result in the formation of histologically identical neoplasms but can also fuse with the same genes to generate morphologically and behaviorally different tumors. It is not fully understood how identical gene fusions can generate such different tumor types, although a possible explanation might be that these neoplasms originate from specific progenitor cells in different anatomic sites, 59 with the differing phenotypes corresponding to the particular cell of origin. Overall, gene expression patterns are likely to vary considerably between tumor types, contributing to their significant clinicopathological differences. 23 EWSR1-CREB1 fusion is also seen in 2 other neoplasms, clear cell sarcoma like tumor of the gastrointestinal tract (CCSLGT) and primary pulmonary myxoid sarcoma (PPMS). CCSLGT is a rare, aggressive neoplasm occurring predominantly within the wall of the small bowel, stomach, or large bowel, particularly in young adults It is composed of medium-sized, round, or ovoid cells with clear or eosinophilic cytoplasm, in sheets or less frequently papillary or alveolar distributions, with variable numbers of CD68-positive osteoclast-like giant cells. CCSLGT expresses S100 protein but not other melanocytic markers and often also variably expresses neuroendocrine markers. Electron microscopy has also shown that some CCSLGTs contain dense-core secretory granules and other features of neuroendocrine differentiation, 63 such that some authors have proposed that CCSLGT should be designated malignant gastrointestinal neuroectodermal tumor. 63 CCSLGTs are associated with EWSR1-CREB1 gene fusions and less frequently with EWSR1-ATF1 fusions. Primary pulmonary myxoid sarcoma is a rare pulmonary neoplasm that arises predominantly endobronchially and characteristically harbors EWSR1-CREB1 gene fusion. 64 While most have behaved relatively indolently, cases have been shown to metastasize to distant sites (kidney and brain), with brain metastasis leading to death. 64 Primary pulmonary myxoid sarcomas are lobulated tumors composed of clusters and cords of largely bland spindle or ovoid cells, often with a reticular pattern within prominent myxoid stroma, which is Alcian blue positive and sensitive to treatment with hyaluronidase. 64,65 Primary pulmonary myxoid sarcomas bear a striking histologic resemblance to extraskeletal myxoid chondrosarcoma but lack the characteristic translocations of extraskeletal myxoid chondrosarcoma that fuse NR4A3 on chromosome 9q22 to a variety of partner genes Primary pulmonary myxoid sarcomas generally express only vimentin but are occasionally weakly and focally immunoreactive for epithelial membrane antigen. The reticular pattern characteristic of PPMS is sometimes seen in AFH, which may cause diagnostic confusion in AFHs arising in the pulmonary region. Histologic overlap between different tumor types harboring identical gene fusions might conceivably be owing to the induction of specific sets of transcription factors that result in particular morphologic phenotypes. EWSR1-ATF1 fusion is also found in clear cell sarcoma of tendons and aponeuroses (CCS), which typically occurs in the lower extremity soft tissues of young to middle-aged adults, and is composed of solid nests of uniform, ovoid to spindled cells separated by thin fibrous septa. CCS shows diffuse expression of S100 protein and melanocytic markers, and more than 90% also express either HMB-45 or MelanA. 57 EWSR1 rearrangements in AFH have not been shown to cause the downstream activation of the MiTF pathway and melanogenesis seen in CCS, 23,36,37 and MiTF- M and SOX10 expressed in CCS with EWSR1-ATF1 are not Arch Pathol Lab Med Vol 139, May 2015 Angiomatoid Fibrous Histiocytoma Thway & Fisher 679

7 detectable in AFH with this fusion. 46 This supports the idea that the fusion products of EWSR1-ATF1 instigate different differentiation programs, which correspond to the properties of the presumably different originator cells of AFH and CCS, resulting in distinct secondary genetic and epigenetic events. EWSR1-ATF1 fusion is also a consistent finding in hyalinizing clear cell carcinoma of the salivary glands, 74 a rare, low-grade salivary gland neoplasm composed of nests and cords of clear cells within hyalinized stroma, and has also been described in a single case of a pelvic myoepithelioma 75 and in an angiosarcoma of the parotid gland. 76 DIFFERENTIAL DIAGNOSIS The differential diagnosis of AFH is wide, encompassing reactive lesions such as granulomas to both benign and malignant neoplasms. It is of importance that the only consistent finding in AFH is of its sheets of ovoid to spindled cells, and other characteristic features such as the fibrous capsule or the lymphoplasmacytic infiltrate may either be absent or may not have been sampled. 77 Angiomatoid fibrous histiocytomas of long duration can show extensive fibrosis, hemorrhage, and hemosiderin deposition, which can obscure the neoplastic cell population and give an appearance of organized hematoma. Granulomatous inflammation can also look similar, but vascular spaces are lacking in granulomas, and AFH has a more solid appearance. Aneurysmal benign fibrous histiocytoma also occurs superficially in young adults but is predominantly dermal and retains the typical architectural features of dermatofibroma such as epidermal hyperplasia and peripheral collagen bundles. This neoplasm also has a more heterogeneous cell population than AFH (often containing giant cells and siderophages), lacks a surrounding lymphoplasmacytic infiltrate, and is desmin negative. Spindle cell hemangioma often occurs in the dermis and subcutis of extremities and has cavernous vascular spaces but is poorly circumscribed, with true vascular spaces lined by attenuated endothelial cells. It can also contain epithelioid cells with intracytoplasmic vacuoles. Nodular Kaposi sarcoma is dermal and circumscribed with bland cytology and contains slitlike blood-filled spaces rather than the cystic ones seen in AFH. Kaposi sarcoma also expresses CD34 and human herpesvirus 8, in contrast to AFH. Palisaded myofibroblastoma of lymph node contains delicate palisading spindle cells and thick bands of collagen fibers ( amianthoid fibers ) and is desmin negative, while inflammatory pseudotumor of lymph node lacks the pseudovascular spaces. Follicular dendritic cell sarcoma usually occurs at nodal sites and is positive for CD21 (which is only occasionally and focally expressed in AFH) and CD35 but lacks both desmin and smooth muscle actin. Ewing sarcoma occurs in an age range similar to that of AFH, and while the classic form is generally morphologically distinct from AFH, atypical variants can show larger cells and pleomorphism. Membranous CD99 is seen frequently in AFH, including in those with round cells, 41 and both harbor rearrangements of EWSR1, but EWSR1-FLI1 and other gene fusions of Ewing family tumors have not been described in AFH. Rhabdomyosarcoma (RMS) can be morphologically similar and is desmin positive like many AFHs but usually occurs deeply and is infiltrative and lacks a peripheral lymphoid cuff. In addition, features typical of RMS such as rhabdomyoblasts and strap cells (embryonal RMS) or alveolar architecture (alveolar RMS) are absent in AFH, which is also negative for specific markers of skeletal muscle differentiation such as myogenin and MyoD1. Malignant extrarenal rhabdoid tumor most frequently arises in children younger than 3 years 78,79 and is composed of sheets of large polygonal cells. In contrast to AFH, malignant extrarenal rhabdoid tumor is usually poorly circumscribed and infiltrative, with cells frequently containing eccentric vesicular nuclei and intracytoplasmic eosinophilic inclusions of aggregates of intermediate filaments. Most malignant extrarenal rhabdoid tumors are positive for cytokeratin (often in a dotlike pattern) and show INI1 loss. When AFH is pleomorphic, it can mimic undifferentiated pleomorphic sarcoma, although the latter most frequently occurs in the deep soft tissues of older adults and lacks the characteristic gene fusions of AFH. The prominent lymphoid cuff present in many AFHs can lead to diagnostic confusion as metastatic tumor deposits within lymph nodes. However, metastatic nodal deposits show surrounding true nodal architecture, including subcapsular and medullary sinuses, in contrast to the randomly distributed germinal centers seen in AFH. Metastatic neoplasms usually demonstrate cytological atypia, and there may be a history of primary cancer or the finding of a primary site after clinicoradiological correlation. Primary AFH in organs such as the lungs or brain can be mistaken for metastatic disease, and the only method of confirming these as primary lesions is by careful clinicoradiological correlation. A reticular pattern is now well documented in AFH, 40 including those occurring endobronchially, 80 which can lead to diagnostic confusion with PPMS, especially because the latter also shows patchy, predominantly lymphoplasmacytic chronic inflammation. However, the reticular architecture predominates in PPMS, which also lacks a lymphoplasmacytic cuff and is consistently negative for desmin. Myxoid stroma is described in some lung AFH but as a focal finding without the prominence seen in PPMS. 58 Myoepithelial tumors of soft tissue often also have a reticular pattern, demonstrate considerable immunophenotypical heterogeneity, and occur in age groups (between the second and fourth decades, with about one-fifth occurring in children) and sites (most frequently in extremities, limb girdles, and then head and neck and trunk) similar to those of AFH. Histologically, myoepithelial tumors show a wide morphologic spectrum but may demonstrate ductular or tubular differentiation or chondromyxoid-type stroma, which is never seen in AFH. While myoepithelial neoplasms often express epithelial membrane antigen, they typically coexpress S100 protein and cytokeratins to varying extents, and keratin expression is not seen in AFH. Myoepithelial tumors also variably express glial fibrillary acidic protein, smooth muscle actin, calponin, and CD10. EWSR1 rearrangements have been described in about half of soft tissue myoepithelial neoplasms, resulting in fusions with a small group of partner genes, including the POU5F1 gene, PBX1 gene, 81,84,85 and ZNF444 gene. 81,86 The association of myoepithelial tumors with EWSR1-CREB1, EWSR1-ATF1, or FUS-ATF1 fusions has not been documented, except in one case of a pelvic myoepithelioma, which was described as having the EWSR1-ATF1 fusion. 75 A proportion of myoepithelial tumors of the skin and soft tissue with tubuloductal differentiation show recurrent PLAG1 rearrangements similar to those in mixed tumors of the salivary glands (pleomorphic adenomas), 87,88 which have not been described in AFH. 680 Arch Pathol Lab Med Vol 139, May 2015 Angiomatoid Fibrous Histiocytoma Thway & Fisher

8 CONCLUSIONS Angiomatoid fibrous histiocytoma is a rare neoplasm of intermediate biologic potential most frequently occurring in the superficial extremities of children and young adults. The vast majority have an excellent prognosis, but correct diagnosis is important because of the risk of local recurrence and small risk of metastasis and death. Its presence at several unusual anatomic sites is being increasingly recognized, as is its widening spectrum of morphologic patterns. Coupled with its bland morphology, this can result in significant diagnostic difficulty. This highlights the importance of diagnostic recognition, ancillary molecular genetic confirmation (preferably utilizing complementary modalities of RT-PCR and FISH), and close clinical follow-up of all patients with AFH. Elucidation of the secondary genetic and epigenetic changes that result from the characteristic gene fusions of AFH will be crucial in our understanding of the mechanisms by which these neoplasms are generated. This work was supported by the National Institute for Health Research Royal Marsden Hospital/Institute of Cancer Research Biomedical Research Centre. References 1. Enzinger FM. Angiomatoid malignant fibrous histiocytoma: a distinct fibrohistiocytic tumor of children and young adults simulating a vascular neoplasm. Cancer. 1979;44(6): Hairston MA Jr, Reed RJ. Aneurysmal sclerosing hemanigoma of skin. Arch Dermatol. 1966;93(4): Costa MJ, Weiss SW. Angiomatoid malignant fibrous histiocytoma: a follow-up study of 108 cases with evaluation of possible histologic predictors of outcome. Am J Surg Pathol. 1990;14(12): Fanburg-Smith JC, Dal Cin P. Angiomatoid fibrous histiocytoma. In: Fletcher CDM, Unii KK, Mertens F eds. 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