mucoepidermoid carcinomas of the thymus are extremely rare malignant mediastinal neoplasms and account for 2% of thymic carcinomas ( 1 , 2 ) . until now , radiologic reports of mucoepidermoid carcinoma of the thymus have been rare , but the described gross findings are multilocular cystic structures with focal areas of induration within the walls of the cyst or well - circumscribed homogeneous tumor masses ( 3 ) . we report a case of mucoepidermoid carcinoma of the thymus with radiologic and histologic findings . a 53-yr - old male was admitted to our hospital complaining of right chest pain . erect posteroanterior and lateral radiographs of the chest showed a well marginated anterior mediastinal mass in the entire right lung zone , compressing the lower trachea , right bronchus , and heart toward the left side ( fig . ct scan of the chest demonstrated a 201610 cm - sized , cystic mass with focal heterogeneously enhanced solid portion anteromedially in the anterior mediastinum . direct invasion of the anterior chest wall and pericardium by the mass was suggested , because pericardial fat plane was obliterated and the margin between the mass and the adjacent right anterior chest wall and pericardium was indistinct ( fig . minimal amount of right pleural effusion and pericardial effusion were associated . at surgery , after aspiration of fluid within the cystic mass , pericardial invasion was noted . histologically , squamoid tumor cells formed cords or solid sheets within the fibrous stroma admixed with mucin - secreting epithelium lining gland - like spaces ( fig . the thymic carcinomas are a heterogeneous group of aggressive epithelial malignancies that have a strong propensity for early local invasion and wide spread metastases . squamous cell carcinoma and lymphoepithelioma - like carcinoma are the most common cell types and usually occur in middle - aged men with a mean age of 46 yr ( 4 - 6 ) . radiologically , thymic carcinomas commonly manifest as large , poorly defined , infiltrative anterior mediastinal masses and associated with areas of necrosis , hemorrhage , calcification , or cyst formation ( 2 , 6 , 7 ) . mucoepidermoid carcinoma was first recognized as a distinct pathologic entity in salivary glands by stewart et al . in 1945 . since then , this tumor has been identified in several other organs , including lung , esophagus , anus , cervix , and skin . although extremely rare , mucoepidermoid carcinomas of the thymus have also been described ( 3 ) . so far , only thirteen cases have been reported in the english literature ( 3 , 8 , 9 ) . the patients ranged in age from 17 to 66 yr , with a mean age of 35 yr on reported cases ( 3 ) . clinically , most patients were asymptomatic or had symptoms of chest discomfort , retrosternal pain and dyspnea . but two distinct gross findings , as described by moran et al . , are multilocular cystic structures varying in size with focal areas of induration within the walls of the cyst and well - circumscribed , homogeneous tumor masses ( 3 ) . our case presented as a 20 cm - sized , cystic mass with focal heterogeneously enhanced solid portion anteromedially in the anterior mediastinum . the invasion of anterior chest wall and pericardium was suspected due to indistinct margin between the mass and adjacent right anterior chest wall and pericardium , and surgically proved . histologically , the lesions showed a spectrum of features that ranged from those of well - differentiated , to moderately well - differentiated , to poorly differentiated mucoepidermoid carcinoma , with sheets and solid islands of squamoid cells admixed with mucin - secreting epithelium lining gland - like spaces ( 3 ) . the histogenesis of these tumors is still unclear ; however , the demonstration of transitions between the tumor cells and the epithelium lining the cysts as well as residual non - neoplastic thymic remnants within the walls of the cysts in four of six cases supports the theory that these tumors arise from thymic epithelium ( 3 ) . differential diagnoses of masses of the anterior mediastinum associated with extensive cystic changes include thymic cysts , thymomas , teratomas , seminomas , hodgkin 's disease , and metastasis ( 10 - 14 ) . and also mucoepidermoid carcinoma of the thymus should be included in the differential diagnosis , although the carcinoma is an exceedingly rare anterior mediastinal malignant tumor ( 3 ) . it is very difficult to differentiate from each other by only radiologic findings ( 10 - 14 ) . however , mucoepidermoid carcinoma of the thymus should be considered when a large cystic mass with focal induration within the anterior mediastinum on ct scan is noted .