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% .