a 35-year - old man presented with a 7.0 cm anterior mediastinal mass on a chest radiograph during a routine medical evaluation at an outside hospital . the patient was transferred to the department of thoracic and cardiovascular surgery for further evaluation and treatment . the patient did not present with any specific medical problems or symptoms and had no family history of cancer or prior surgeries except for an operation due to a pneumothorax that was performed 12 years prior to presentation . fluorodeoxyglucose ( fdg)-positron emission tomography / computed tomography ( ct ) revealed two heterogenic hypermetabolic foci in the anterior mediastinum , with standard uptake values of 3.3 and 3.4 . no significant fdg uptake was observed in the lymph nodes or anywhere else in the body to suggest distant metastasis , except for physiological uptake in both testes ( fig . contrast - enhanced chest ct images showed multiple tumors in the anterior mediastinum . specifically , a 5.0 cm mass compressing the superior vena cava , a 1.5 cm nodule , and a 1.0 cm nodule were observed ( fig . ultrasonography of the testes revealed a normal shape and echogenicity without focal mass or infiltrative lesions . the serum levels of  human chorionic gonadotropin ( 0.1 iu / l ) and -fetoprotein ( 12.8 ng / ml ) were in the normal range . a biopsy was performed in the operating room , and the diagnosis of thymoma was determined based on frozen tissue sections . the largest mass ( 5.05.0 cm in size ) showed a lobulated , homogeneous , firm , cut surface with a partial hemorrhage . in the normal thymic tissue surrounding the largest tumor , two smaller masses ( 1.51.2 cm and 1.00.5 cm in size ) were seen , which had a well - defined , round , homogeneous , soft appearance . the medium - sized mass was located 1.5 cm from the largest tumor , and the smallest mass was 0.5 cm away ( fig . , low - power magnification of the largest mass showed a well - circumscribed , thick - walled capsule with a partially incomplete area , lobular architecture , and perivascular spaces containing proteinaceous fluid and lymphocytes . the tumor was composed of a dual population of lymphocytes and epithelial cells in equal proportions . under high - power magnification , the epithelial cells had a bland nuclear appearance with small inconspicuous nucleoli . the lymphocytes that were intermixed with the epithelial cells were small and mature - appearing . the two small masses without a capsule revealed clusters or islands of large epithelioid cells with prominent nucleoli and pale cytoplasm that were intermingled with small , infiltrating lymphocytic cells . interestingly , these two small tumors revealed a prominent lymphoid follicular hyperplasia with the formation of germinal centers , some microscopic cysts , and hassall s corpuscles in the surrounding area of the seminomatous component ( fig . , the epithelial cells of the thymoma showed diffuse positive reactions for cytokeratin 19 and p63 and negative reactions for placental - like alkaline phosphatase , cd5 , cd117 , and d2 - 40 ( fig . in contrast , the epithelial cells of the seminoma - like tumors showed diffuse positive reactions for cd117 , partially strong positive reactions for placental alkaline phosphatase ( plap ) and d2 - 40 , and negative reactions for -fetoprotein , cytokeratin 19 , and p63 ( fig . epithelial membrane antigen staining was observed in hassall s epithelial cells of the thymoma and in microcystic epithelial cells of the seminomas . in summary , histological and immunohistochemical analyses of the anterior mediastinal masses revealed a type b2 invasive thymoma and two seminomas . the simultaneous occurrence of a thymoma and seminoma in the thymus has not yet been reported except for two cases described by weissferdt et al .   in 2014 . however , they reported a thymoma with a component of a thymic seminoma combined within a single tumor . the seminomatous component showed seminoma - like epithelioid cells with distinct hyalinization adjacent to the area of the conventional thymoma . our case included one large thymoma and two small seminomas in the anterior mediastinum , with distances of 1.5 cm and 0.5 cm between the thymoma and the seminomas . because multiple thymic masses containing a thymoma and small seminomas are very rare , the possibility of metastasis from another organ or a morphological transformation of the thymic epithelial tumor cells into thymic carcinoma should be considered . the two small masses showed negative reaction for cd5 and pan - cytokeratin and positive reaction for plap and d2 - 40 by immunohistochemical staining . although cd5 loss in thymic carcinoma is known , the other immunohistochemical findings favor seminoma rather than thymic carcinoma . we could not find a case of a thymic carcinoma with a positive reaction for plap or d2 - 40 in the literature . our case showed site - associated features such as the presence of multiple cysts , reactive remnant thymic epithelium , epithelioid granulomatous reaction , fibrosis , and follicular lymphoid hyperplasia . in the seminomatous tumor fortunately , site - associated morphologic features of mediastinal masses are rare in gonadal tumors . although a biopsy of the testes was not performed , metastatic seminoma in the mediastinum from an intratubular germ cell neoplasia is unusual in the absence of retroperitoneal lymph node metastasis . these clinical and histological findings support the conclusion that these seminomas are of mediastinal origin rather than due to metastasis . primary mediastinal seminomas were first described in 1955 by woolner et al .   and have been well recognized for over 50 years . however , it is believed that mediastinal seminomas originate from extragonadal germ cells in the thymic gland due to chromosomal abnormalities . chromosome 12p abnormalities , which are specific genetic alterations found in testicular seminomas , are also present in mediastinal seminomas . specifically , przygodzki et al .   reported a unique pattern of a kit exon 17 mutations in mediastinal seminomas , which are rare in testicular seminomas . this may imply that the mediastinal seminoma develops through different pathways than that of its gonadal counterpart . regardless of the tumor location , seminomas show kit protein expression , which is also seen in thymic carcinomas but not in thymomas . histopathological analyses show that thymomas have heterogeneous morphological features such as spindle cells , lymphocytic infiltrate , and rhabdomyomatous components . additionally , cytokeratin expression in seminoma tumor cells , which reflects the differential potential or the morphological transformation of the seminoma into another nonseminomatous gcts , has been reported . moreover , a thymic carcinoma or seminoma may occur in combination with a thymoma within a single tumor . therefore , we cautiously hypothesize that thymoma , thymic carcinoma , and seminoma occur from cells of the same origin , such as embryonic stem cells or primordial germ cells , which have the ability to differentiate into a diverse population of cells . they obtain chromosomal and molecular abnormalities such as chromosome 12p abnormalities and kit mutations and transform from thymic epithelial tumors into gcts ( seminoma ) or from seminomas into thymic epithelial tumors . the etiology and pathogenesis of thymomas and seminomas in the thymus are unknown , and further research is required in this area . follicular lymphoid hyperplasia of the thymus can be seen in reactive , autoimmune diseases like myasthenia gravis , follicular lymphoid hyperplasia may be associated with specific antigens produced by the microenvironment of the seminoma . weissferdt and moran   postulated that the specific distribution of antigen - presenting dendritic cells in mediastinal seminomas is related to the pathogenesis of follicular lymphoid hyperplasia . in short , we report a case of one thymoma and two mediastinal seminomas that developed simultaneously in the thymus of a young adult . the seminomas revealed florid follicular lymphoid hyperplasia and microcystic changes that do not appear in testicular seminomas . in addition , no clinical or radiological evidence of gcts was found elsewhere . therefore , even though multiple masses were present , we considered these tumors to be synchronous primary thymic tumors rather than metastases from an occult tumor of the testis . to our knowledge , this is the first description of a synchronous occurrence of a separate thymoma and mediastinal seminomas within the thymus .