a 59-year - old womon presented with an abnormal mass on chest x - ray that was discovered during a routine check - up .
we obtained a biopsy though endobronchial ultrasound and pathologically diagnosed a thymoma ( type a ) .
contrast - enhanced computed tomography showed a smooth , solitary homogeneously enhanced mass measuring 33.52 cm in the right paratracheal area , which had no calcification ( fig .
the resected specimen was a firm light - gray tumor with a smooth capsule that measured 4.532.5 cm .
postoperative follow - up proceeded without any problems , and the patient was discharged after 5 days .
microscopy revealed that the tumor was a thymoma ( world health organization type a+b2 , masaoka staging i ) ( fig .
the majority of thymomas are found in the anterior mediastinum . only a few thymic masses arising out of the anterior mediastinum have been described in the literature , and these have been found in an ectopic thymus location such as the neck , pulmonary hilus , or posterior mediastinum .
the thymus arises embryologically from the third pharyngeal pouch and branchail cleft on each side .
the thymic masses from each side then move toward each other and migrate from the midline to the anterior mediastinum and their final position .
the incidence of ectopic thymic tissue is 3% to 5% in the retroinnomiate vein ( i.e. , a paratracheal site ) .
surgical resection has been used for both a firm diagnosis and treatment in previously reported cases of a middle mediastinum thymoma .
sakurai et al .   reported that 18f - fluorodeoxyglucose positron emission tomography ( pet ) scan and 11c - acetate pet are useful for diagnosing a thymoma from the middle mediastinum as ectopic thymic tissue .
castleman disease is an atypical lymphoproliferative disorder , and masses are commonly located in the chest .
most patients are asymptomatic and lesions are accidentally found on chest x - ray as rounded mediastinal masses , often mistaken for a thymoma .
thymomas rarely arise in the middle mediastinum , and thymoma is not considered in the differential diagnosis of middle mediastinum masses . but thymomas appear to have malignant potential , and the overall rate of thymoma recurrence is approximately 20% .
 thymoma is a common anterior mediastinal mass , although thymomas have occasionally been found in the neck , pulmonary hillus , or posterior mediastinum . but 
 a thymoma within the middle mediastinum has rarely been reported . 
 we report a thymoma arising in the middle mediastinum with a review of the literature . 
CASE REPORT
DISCUSSION