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Received Oct 5, 2017; Accepted Dec 10, 2017
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1. Introduction
Myoepithelial tumors (METs) of soft tissue and bone are rare tumors of uncertain histogenesis. The first deep tumor was described in the retroperitoneum [1], followed by a large series in 1997 including cases in soft tissue [2]. Rare cases in bone are more recently reported, and these lesions tend to occur in the acral region of the lower limbs usually in middle age male patients [3–7]. Axial localization of METs has to be distinguished from chordoma.
Histologically, METs are made of a homogeneous population of myoepithelial cells. They could be considered as a part of a continuum with mixed tumors when ductal differentiation is present. They may harbor chondroid and bone differentiation as observed in classical mixed tumors. The diagnosis of MET requires the coexpression of both epithelial markers and S100 protein [8, 9]. They share morphological and immunohistochemical features with their counterparts described in skin and salivary gland.
A different genetic pattern distinguishes METs arising in the skin from those in deep soft tissue and bone. EWSR1(22q12) gene fusions have been detected in half of METs arising in deep soft tissues and in up to 70% cases of intraosseous MET [7, 10]. Several partners of EWSR1 are described: POU5F1(6p21.33) (16%), PBX1 (16%), PBX3, ZNF444, ATF1, KLF17, and NFATC2 [10–16]. The EWSR1-POU5F1 more often occurs in children and young adults while the EWSR1-PBX1 occurs in middle-aged adult patients [11, 14]. The tumors of the first subgroup show a solid or nested growth arrangement of tumor cells showing at least partially a clear appearance of the cytoplasm. The subgroup with EWSR1-PBX1 rearrangement presents a bland sclerotic appearance or clear cell morphology with a diffuse EMA staining. However, none of the EWSR1-rearranged tumors show the presence of ductal or glandular differentiation or cartilage/bone matrix formation [10]. Rearrangement of the FUS(16p11) gene has also been reported in rare cases of MET arising in deep soft tissue as well as in bone [15, 17]. Two gene partners have been characterized, KLF17