Similar to other translocation associated tumors, the t (1;3)(p36;q23-25) or t(11;X)(q13;p11) translocations are seemingly early causative events in EHE oncogenesis [5, 11] which initiate a novel transcription program in cells with endothelial properties .
The corresponding fusion proteins, WWTR1-CAMTA1 and YAP1-TFE3, may serve as chimeric transcription factors, which manifest their oncogenic function via a promoter switch [5, 12]. Another possible oncogenic effect could be due to the loss of regulatory domains of the C-terminus of WWTR1 or YAP1 and the N-terminus of CAMTA1 or TFE3
WWTR1 on 3q23-24, encodes a transcriptional co-activator involved in mesenchymal stem cell differentiation and is highly expressed in endothelial cells [5, 12], while CAMTA1, a calmodulin-binding transcription activator, located on 1p36.23, has been proposed as an oncogene under the control of the WWTR1 promotor. The latter is supported by the occurrence of an in-frame fusion of the C terminus of CAMTA1 to WWTR1, arguing against loss of function .
Alternatively, the YAP1-TFE3 in-frame fusion was recently found in a small and distinct subset of EHE .
YAP1, located on 11q13, is a member of the FAT-family genes similar to WWTR1, and encodes for another WW-domain containing transcriptional co-activator . The protein of TFE3, located on Xp11.22, is a member of the microphtalmia transcription factor family with oncogenic properties in several tumor types [10, 13].
Although these gene fusions have not been identified in other neoplasms, in particular other epithelioid vascular tumors, it is not clear yet whether they are unique to this entity [5, 9].
Using both, FISH and RT-PCR, we detected a WWTR1-CAMTA1 fusion in 23 cases with discrepancies in 5 cases whereby the results were positive with one method only (and negative for the other). This could be due to the primers used missing potentially new breakpoints (RT-PCR) and possibly a low number of tumor cells showing a fusion signal by FISH.
The known WWTR1 breakpoints are in intron 3 and 4, and the CAMTA1 breakpoints are located in intron 7 and 8, and at different sites within exon 9 [5, 9]. The resulting fusion-transcript variants which we found in our study were exon 4-exon 8 (n = 13), exon 4-exon 9 (n = 7) and exon 3-exon 9 (n = 3) (listed in decreasing frequency).
Two cases (6%) revealed the recently detected fusion YAP1-TFE3 by RT-PCR. One of them (Case 8) was included in the study by Antonescu et al. , in which the gene fusion was demonstrated by FISH. This case was not only remarkable because of the histomorphological characteristics as reported in the mentioned paper  but also because it occured in a lymph node as the primary site. A more frequently occurring metastasis should be excluded in such cases [2, 14]. In contrast, the other case with a YAP1-TFE3 fusion showed a classical morphology of a multifocal lung EHE (Case 31). Interestingly, these 2 cases involved adolescents/young adults in concert with the finding of young age reported by Antonescu et al. .
TFE3 rearrangements are also present in alveolar soft part sarcoma, certain pediatric renal cancers and a subset of PEComas [15–18]. The use of an antibody against the C-terminal portion of TFE3 seems to be a useful diagnostic tool in all rearranged tumors [10, 19], but one has to be aware of an unspecific staining pattern  as we found in a proportion of cases with WWTR1-CAMTA1 fusion.
ERG and FLI1 are transcription factors of the ETS-family which are expressed in endothelial cells. In addition to CD31, these 2 markers are helpful in highlighting the vascular nature of EHE [10, 20, 21] and showed expression in all cases examined. These markers are of course expressed in all other vascular lesions [8, 20, 21], therefore, detection of the aforementioned fusion genes may be a valuable discriminatory tool in diagnosing EHE, especially in difficult cases.
In 2008, a proposal for risk stratification was made based on clinicopathological features of 49 soft tissue EHEs. It seems that higher mitotic activity (>3/50 HPF) and tumor size exceeding 3 cm are associated with higher mortality, irrespective of anatomic site, presence of cytological atypia, tumor cell spindling, or necrosis . Due to the small number of our cases with available clinical data, we were able to show only tendencies for the low-risk and high-risk category. Furthermore, in skin lesions, a favorable outcome is well known similar to all other sarcomas at this site . Whether it is useful to include lung-, liver- and bone lesions in this risk stratification scheme remains uncertain so far, especially with respect to multifocal occurrence (or early metastases) .
Differential diagnoses of EHE depend on anatomic site and age. Carcinomas, myoepithelial tumors, epithelioid sarcoma, mesothelioma, extraskeletal myxoid chondrosarcoma, myxoid liposarcoma and especially other vascular tumors with epithelioid morphology, such as epithelioid hemangioma, cutaneous epithelioid angiomatous nodule, epithelioid angiosarcoma and pseudomyogenic hemangioendothelioma (epithelioid sarcoma-like hemangioendothelioma, PM-H) can enter the differential diagnoses [1, 2, 8, 22–24]. In bone, chondrosarcoma can also be a potential pitfall.
Epithelioid hemangioma occurring in skin, soft tissue and bone is distinguished by a lobular architecture of well-formed vessels with a pericytic cuff, highlighted by ASMA/MSA (muscle specific actin) immunohistochemical reaction [1, 22, 25, 26]. In the center of the lesion, the plump epithelioid cells are sometimes rather tightly packed and arranged in solid sheets simulating a more aggressive tumor. The distinctive zonation pattern with peripheral maturation is a helpful finding [25, 26]. A myxohyaline stroma is not a feature of epithelioid hemangioma. As in EHE, vascular invasion can occur. Multifocality of epithelioid hemangioma with involvement of soft tissue and/or bone, skin and lymph nodes should not be confused with metastases of EHE [26, 27].
Cutaneous epithelioid angiomatous nodule is a circumscribed lesion composed of a sheet-like proliferation of epithelioid cells with eosinophilic cytoplasm. Intracytoplasmic vacuoles are numerous and mitotic figures can be present. This lesion lacks cords and strands of tumor cells in a myxohyaline or fibrous stroma which is seen in EHE .
Epithelioid cells with eosinophilic cytoplasm are present to a variable extent in approximately 70% of angiosarcomas (AS) of soft tissue  and, of course, they can be prominent in AS at other sites as bone, skin and parenchymatous organs. There is considerable nuclear pleomorphism present and a high mitotic index, which are not typical features of EHE. Lumen formation ranges from irregular vascular channels to intracytoplasmic vacuoles or when absent there can be a diffuse, nest- or sheet-like growth pattern. The hemorrhagic background seen in angiosarcoma is not characteristic for EHE. Keratins can be expressed in both EHE and angiosarcomas (in ca. 30% of the cases, respectively) .
Pseudomyogenic hemangioendothelioma (epithelioid sarcoma-like hemangioendothelioma, PM-H) is composed of loose fascicles and sheets of plump spindle cells and/or epithelioid cells with vesicular nuclei and bright eosinophilic cytoplasm. A myxoid background rarely exists in PM-H and (intracytoplasmic) vascular formations are described as more elusive. Angiocentric growth may occur in both entities. Expression of keratins is an overlapping feature, although CD34 and pan-keratin MNF116 are lacking in PM-H [8, 24]. Recently, SERPINE1-FOSB, has been identified as a specific fusion gene in PM-H .
Carcinomas tend to manifest as more pleomorphic, mitotically active tumors  evoking a desmoplastic response and usually exhibit stronger keratin expression whereas endothelial markers are absent.
Myoepithelial tumors are composed of variable proportions of epithelioid, spindle, clear and/or plasmocytoid cells. However, cords and strands embedded in a (chondro) myxoid/hyaline matrix is a shared feature and cytoplasmic vacuolation can occur . Endothelial markers are absent in myoepithelial tumors. S-100 and keratins, usually expressed in myoepithelial tumors , can also be occasionally positive in EHE as demonstrated in two of our cases . EWSR1 and PLAG rearrangements are characteristic genetic changes in myoepithelial tumors [30–33].
Extraskeletal myxoid chondrosarcoma shows a multinodular architecture. The monomorphous small round or short spindle cells are loosely arranged and show delicate anastomosing strands distributed in a prominent myxoid stroma. Although some vacuolated cells can be observed, the cytoplasm is more eosinophilic [1, 34] and vascular markers are not expressed. NR4A3 rearrangement is a pathognomonic genetic aberration .
Conventional epithelioid sarcoma (ES) tends to grow in confluent nodules with central necrosis or hyalinization. The epithelioid to spindled cells show deeply eosinophilic cytoplasm. Vacuolated cells can possibly be confused with intracytoplasmic lumina of EHE . When the matrix is myxoid, there are often still prominent collagen bundles , a feature which is not seen in the myxohyaline stroma of EHE. Immunohistochemically, this tumor type typically shows keratin- and EMA-expression, loss of INI1  and occasionally a membranous CD31 staining . CD34 and D2-40 can be either positive or negative in both tumor types [40–42]. Interestingly, ERG and FLI1 (antibody to the N-terminus) expression was recently shown in up to 70% and more than 90% of ES, respectively [40, 41]. Therefore, these markers can only be interpreted in the right context together with CD31 and INI1. The latter is retained in EHE  as shown in some of our cases.
Myxoid liposarcoma shows a prominent myxoid matrix with enhanced cellularity at the periphery of the tumor nodules. The primitive cells are uniformly round to oval-shaped. Small signet ring lipoblasts could possibly be confused with the vacuolated cells of EHE, but there is no abundant pale-eosinophilic cytoplasm in a myxoid liposarcoma. The delicate arborizing vasculature in myxoid liposarcoma is very characteristic. When genetics are taken into account, DDIT3 is the consistent fusion partner in this lesion .
As well as the aforementioned vascular lesions, chondrosarcoma is an important differential diagnosis in primary bone tumors and shows uniform atypical chondroblasts suspended in an extensive myxoid matrix. Cytoplasmic vacuolation can be misinterpreted but the nuclei in chondrosarcoma are hyperchromatic and the scant cytoplasm is eosinophilic . Vascular immunohistochemical markers are absent.
Epithelioid mesothelioma could be a relevant differential diagnosis at appropriate sites because of the abundant, sometimes vacuolated cytoplasm of mesothelial cells and the possible myxoid stroma. Although pankeratin and D2-40 can be positive in both malignancies, as also demonstrated in a number of our cases, calretinin, keratin 5/6, keratin 7, and WT1 are typically distinguishing mesotheliomas .