an 18-year - old man presented with a soft tissue mass of five months ' duration in the right forearm . physical examination revealed a round , tender mass . on magnetic resonance scan of the right forearm , t1-weighted imaging revealed a circumscribed mass with intermediate signal intensity . the obtained slides were stained with hematoxylin and eosin and papanicolaou methods . on microscopic examination , the smear revealed clusters , sheets , and isolated cells in the background of myxoid materials ( fig . the nuclei were uniformly round to ovoid , with finely distributed chromatin and a small nucleolus . some cells had eccentrically - located nuclei and lesser amount of basophilic cytoplasm , resulting in a plasmacytoid appearance . neither mitosis nor necrosis was found . on immunohistochemical staining for cytologic slides , the tumor cells were occasionally positive for cytokeratin ( ae1/ae3 ) and s100 protein . the excised tumor measured 3.01.81.5 cm . the tumor was well - demarcated , yellow solid , soft , and myxoid ( fig . the tumor cells were present in solid sheets or reticular pattern without ductal component or necrosis . the tumor cells showed immunostaining for cytokeratin ( ae1/ae3 ) , epithelial membrane antigen ( ema ) , s100 protein , and glial fibrillary acidic protein ( gfap ) ( fig . soft tissue myoepithelioma is a rare tumor composed of neoplastic cells with features of myoepithelial differentiation . the tumor presents as a painless or painful mass in the dermis , subcutis , or deep soft tissue.3,4 while most myoepitheliomas are of salivary gland origin , they have been reported in the soft tissue , retroperitoneum,3 lung,5 and testis.6 the histogenesis of myoepithelial tumors arising in soft tissue is unknown . it likely reflects a different pattern of gene expression during oncogenesis rather than origin of a specific cell lineage.7 hallor et al.8 reported that a minimally deleted region of 3.55 mb at chromosome band 19p13 was identified in soft tissue myoepitheliomas . in the present case , the tumor consisted of epithelioid , spindle , and plasmacytoid cells forming solid sheets or reticular pattern in myxoid and fibrous stroma . the tumor cells revealed positive reaction for cytokeratin ( ae1/ae3 ) , ema , s100 protein , and gfap . the cytomorphologic and immunohistochemical features of the present case are similar to those of myoepitheliomas arising in the salivary gland.9 - 12 soft tissue myoepitheliomas , mixed tumors , and parachordomas are on the spectrum of tumor showing myoepithelial differentiation.1 characteristically , mixed tumor has more pronounced ductal differentiation , while parachordoma shows prominent cytoplasmic vacuolation . in the present case , ultrastructurally , myoepitheliomas show intermediate filaments , desmosomes , and basal lamina.13 intermediate filaments , desmosomes , and basal lamina were present in this case . soft tissue myoepitheliomas may be mistaken as other type of soft tissue tumors due to their cytomorphologically heterogeneous features . the differential diagnosis of soft tissue myoepithelioma includes mixed tumor , parachrodoma , schwannoma , smooth muscle tumor , ossifying fibromyxoid tumor ( ofmt ) , myxoid liposarcoma , extraskeletal myxoid chondrosarcoma ( emc ) , and metastatic carcinoma . immunohistochemically , parachordomas are positive for cytokeratin and s100 protein.14 schwannoma has wavy , point - ended nuclei in collagenous or myxoid background and nuclear palisading.15 smooth muscle tumor has cigar - shaped , blunt - ended nuclei and eosinophilic fibrillary cytoplasm.16 conversely , the cells of soft tissue myoepithlioma have more tapered nuclei . immunoreactivity for smooth muscle actin , desmin , and h - caldesmon supports a diagnosis of smooth muscle tumor . ofmt shows round and ovoid cells in myxoid matrix.17 ofmt is immunoreactive for s100 protein in approximately 70% of cases but negative for cytokeratin and gfap . myxoid liposarcoma shows lipoblasts and delicate , arborizing , thin - walled blood vessels in myxoid background.18 emc is characterized by spindle , stellate , or round cells in a myxoid or chondromyxoid matrix and shows variable immunoreactivity for s100 protein , neuron - specific enolase , and synaptophysin.19,20 generally , metastatic carcinomas show epithelial tumor cells with hyperchromatic nuclei , prominent nucleoli , and a high nuclear to cytoplasmic ratio . it can be excluded by the absence of immunoreactivity for s100 protein and myogenic markers . immunohistochemical expression for epithelial markers ( cytokeratin and/or ema ) , s100 protein or gfap is useful for confirmation of myoepithelial differentiation.3,7 although most cases of myoepithelial neoplasms of soft tissue are benign , approximate 20% have a risk for local recurrence.3 the histopathologic criteria for malignancy in soft tissue myoepithelial neoplasms are moderate to severe cytologic atypia , increase of nuclear to cytoplasmic ratio , nuclear pleomorphism , and readily identifiable nucleoli.2,3 no cytologic features of malignancy were present in this case of myoepithelioma . recognition of the cytomorphologic features of soft tissue myoepithelioma is necessary for the correct cytological diagnosis . soft tissue myoepithelioma should be included in the differential diagnosis of soft tissue epithelioid and spindle cell neoplasms .
soft tissue myoepithelioma is a rare neoplasm composed of myoepithelial cells . here , we describe the cytologic features of soft tissue myoepithelioma arising on the right forearm in an 18-year - old man . 
 the excised tumor ( 3.01.81.5 cm ) was well - demarcated , yellow - gray , soft , and myxoid . 
 the cytologic smears showed round to spindle , epithelioid , and plasmacytoid cells in the myxoid background . 
 the nuclei were uniform , round to ovoid , with finely distributed chromatin and eosinophilic or pale cytoplasm . 
 the tumor cells demonstrated immunoreactivity for cytokeratin ( ae1/ae3 ) , epithelial membrane antigen , s100 protein , and glial fibrillary acidic protein . 
 electron microscopy showed intermediate filaments , desmosomes , and basal lamina .