myoepithelioma is a benign tumor that can originate from any secretory system throughout the body . in the salivary gland , myoepithelioma is rare , accounting for 1% of all salivary gland tumors .
although myoepithelioma of the salivary gland originates most frequently in the parotid gland , myoepithelioma of the accessory parotid gland ( apg ) is extremely rare , and to date , only 5 cases have been reported in the english literature [ 2 , 3 , 4 , 5 ] .
because the clinicopathological and mri findings of myoepithelioma of the apg have not yet been fully described , and given the rarity of this condition , we hereby describe the clinicopathological and mri findings of an epithelioid myoepithelioma of the apg that was treated in our hospital .
a 31-year - old woman presented with a 5-year history of a slowly enlarging left mid - cheek mass in the pre - parotid region . upon examination , a mobile non - tender and smooth - surfaced mass measuring 20 mm in the greatest dimension was noted in the left pre - parotid region .
an unenhanced ct of the neck confirmed a well - circumscribed and homogenous mass measuring 18  10 mm and located anterior to the left main parotid gland and on the surface of the masseter muscle .
this lobulated mass demonstrated isointensity and hyperintensity relative to the muscle on t1- and t2-weighted mri images ( wis ) , respectively , and homogeneous enhancement on contrast - enhanced t1-wis ( fig .
c ) . a time - signal intensity curve ( tic ) generated from a dynamic mri study revealed a gradual enhancement pattern ( fig . 1d ) .
on t2-wis , the mass periphery was surrounded by hypointensity that was suggestive of a fibrous capsule .
the tumor was most likely considered a pleomorphic adenoma ( pa ) , given the clinical presentation and mri findings .
the mass and the surrounding salivary tissue were completely separated from the main parotid gland and connected to the parotid duct via a ductule .
all facial nerve branches and the parotid duct were successfully preserved , and postoperative complications such as facial paralysis and salivary fistula were not observed . grossly , the tumor was well encapsulated , solid , and lobular ( fig .
microscopically , the tumor primarily comprised cells with round nuclei and a few cells with oval and slightly spindle - shaped nuclei .
the neoplastic cells were arranged in an epithelial and mostly lattice - like structure with hyalinous and mucinous stroma .
2c ) , cam5.2 ( cytokeratin stain ) , s-100 protein , smooth muscle actin ( sma ) , cytokeratin14 , and ae1/ae3 ( pan - keratin stain ) , as well as negativity for carcinoembryonic antigen . based on these results , the final histopathological diagnosis of the tumor was epithelioid myoepithelioma .
pa is reportedly the most common subtype of benign apg tumors ; in contrast , myoepithelioma represents an extremely rare apg tumor pathology .
only 5 cases of myoepithelioma of the apg have been reported in the english literature .
kawashima et al .   reported a case in a 34-year - old woman , isogai et al .  
reported a case of plasmacytoid myoepithelioma of the apg in a 47-year - old woman , and sun et al .   reported a case in a 41-year - old man and one in a 22-year - old woman ;
recently , xie et al .   described the fifth case in a 65-year - old man who was treated via an endoscopic approach .
there have been only a few reports on the imaging findings of myoepithelioma of the parotid gland . according to wang et al .
, myoepithelioma of the parotid gland is characterized by a well - defined and smooth or lobulated mass with homogeneous or inhomogeneous enhancement on the ct . although mri is a widely used modality for evaluating parotid gland tumors because it provides a greater amount of morphological information than ct does , the mri features of myoepithelioma of the apg have not been previously described .
the typical mri findings of pa , in contrast , have been well described to include a lobulated or oval - shaped well - defined encapsulated mass , a mass with a hypointensity on t1-wis and hyperintensity surrounded by a hypointense capsule on t2-wis   as well as a gradual tic enhancement pattern in dynamic mri studies . in the present case ,
the myoepithelioma of the apg was isointense relative to the muscle on t1-wis , hyperintense relative to the muscle on t2-wis , and well enhanced on contrast - enhanced t1-wis .
additionally , a tic generated from a gadolinium - enhanced dynamic mri study displayed a gradual enhancement pattern .
we preoperatively suspected the mass to be pa because the mri findings were completely consistent with those of pa .
as the name indicates , myoepithelioma comprises neoplastic cells exhibiting myoepithelial differentiation ; these cells display various morphologic patterns such as spindle , plasmacytoid , epithelioid , and clear types . because of this morphologic diversity
, it is usually difficult to definitively diagnose myoepithelioma by fnac before surgery , although das et al .  
reported a case of a successful preoperative parotid myoepithelioma diagnosis via a cytological examination and immunohistochemistry .
the main differential diagnoses of myoepithelioma include pa , adenoid cystic carcinoma , other salivary tumors without obvious ductal differentiation , and mesenchymal tumors .
in particular , we should always consider various low - grade malignant tumors when interpreting the fnac results from salivary tumors .
a histopathological examination of the resected specimen can yield a definitive diagnosis of myoepithelioma , and immunohistochemical staining plays an important role in the detection of neoplastic myoepithelial cells .
such cells are typically positive for cytokeratin , vimentin , s-100 protein , calponin , sma , and glial fibrillary acidic protein , but negative for carcinoembryonic antigen . in our case ,
the histopathological findings of an epithelial and lattice - like arrangement of neoplastic cells with hyalinous and mucinous stroma were also characteristic components of pa .
however , neither cartilaginous myxoid stroma nor a glandular structure was found in our case , and the tumor was entirely encapsulated by connective tissue .
additionally , positive immunohistochemical staining of the neoplastic cells for cam5.2 , cytokeratin14 , ae1/ae3 , sma , and calponin indicated that these neoplastic cells possessed both epithelial and myoepithelial characters .
complete resection along with the surrounding salivary tissue is recommended for myoepithelioma management in order to avoid tumor recurrence . in conclusion , a mid - cheek mass could easily be suspected as an apg tumor according to the clinical presentation and imaging findings .
however , because myoepithelioma is an extremely rare apg tumor subtype and the imaging findings of myoepithelioma resemble those of pa , which is the most common benign apg tumor subtype , we believe that it is nearly impossible to preoperatively distinguish myoepithelioma of the apg from pa of the apg or low - grade malignant tumors , even by using fnac , given that kawashima et al .   also misdiagnosed a myoepithelioma of the apg as a suspected adenoid cystic carcinoma ( after fnac ) .
 tumors of the accessory parotid gland ( apg ) are rare , and pleomorphic adenoma ( pa ) is the most common benign apg tumor subtype . 
 myoepithelioma of the apg is much rarer than pa , and to date , only 5 cases have been sporadically reported in the english literature . 
 we describe the clinicopathological and mri findings of an epithelioid myoepithelioma of the apg that was treated in our hospital . 
 the patient 's only clinical symptom was a slow - growing and painless mid - cheek mass . 
 the tumor was suspected to be pa before surgery based on the following mri findings : ( 1 ) a well - circumscribed and lobulated contour , ( 2 ) isointensity and hyperintensity relative to the muscle on t1- and t2-weighted images ( wis ) , respectively , ( 3 ) good enhancement on contrast - enhanced t1-wis , ( 4 ) peripheral hypointensity on t2-wis , and ( 5 ) a gradual time - signal intensity curve enhancement pattern on gadolinium - enhanced dynamic mri . 
 the tumor was completely resected via a standard parotidectomy approach , and the postoperative pathological examination of the tumor , including immunohistochemistry , confirmed the diagnosis of epithelioid myoepithelioma . as it is hardly possible to distinguish myoepithelioma from pa and low - grade malignant tumors preoperatively , 
 a pathological examination using frozen sections is helpful for surgical strategy - related decisions . 