synovial sarcoma is a relatively common and well defined soft tissue sarcoma , accounting for about 5% to 10% of all soft tissue sarcomas ( 1 ) . the mean age at the time of diagnosis is about 30 yr ( 1 ) . about 70% of cases occur in patients older than 20 yr old , but the remaining 30% of cases are diagnosed between ages 1 and 20 yr ( median , 13 yr ) ( 2 ) . approximately 85% of this tumor occur in para - articular regions of the extremities , with a predilection to the lower extremities . following the extremities , the tumors arise in the head , neck , and trunk ( 1 ) . synovial sarcoma has also been described at various anatomic sites including the skin , heart , lung , pleura , prostate , kidney , central nervous system , bone , esophagus , liver , ovary , vulva , peripheral nerve , and mediastinum ( 1 , 3 - 6 ) . to the best of our knowledge , only one case of primary synovial sarcoma that originated in the thyroid gland has been reported ( 7 ) . synovial sarcoma of the head and neck region account for about 10% of all cases ( 1 ) . when synovial sarcomas arise in these unusual sites , recognition and differential diagnosis becomes more difficult . here , we report a primary synovial sarcoma of the thyroid gland with clinical , cytologic , histopathological , immunohistochemical , ultrastructural , and molecular genetic features , and discuss the differential diagnosis between synovial sarcoma and other primary thyroid tumors . a 15-yr - old man had a chief complaint of a palpable neck mass for 4 months . the patient first recognized the neck mass 4 months before , and it rapidly increased in size during the latter 2 months . the neck computed tomography revealed a relatively well - demarcated solid mass at the superior and lateral portion of the left thyroid gland ( fig . 1 ) , suggesting that this lesion may be either a primary thyroid mass with exophytic growth or a soft tissue tumor pushing the thyroid , in particular , a neurogenic tumor . the fine needle aspiration cytology disclosed moderate cellularity with a predominance of bipolar spindle - shaped cells having oval- to spindle - shaped nuclei and finely granular chromatin without nucleoli . the plump spindle cells in the clusters were arranged in a fascicular or streaming pattern with a mild degree of pleomorphism . these cytologic findings were initially regarded as a benign follicular lesion and could not lead to a definitive diagnosis ( fig . the mass was encapsulated and covered by the same capsule as the thyroid , measuring 655 cm in dimensions . the cut surface showed a well - demarcated , lobulated and grayish tan solid mass with rubbery consistency ( fig . microscopically , the tumor was separated from the thyroid parenchyme by a thick fibrous capsule and showed a biphasic growth pattern , which was an admixture of spindle and epithelial cell components in almost equal proportions ( fig . the spindle cell component consisted of fascicles of atypical fibroblast - like spindle cells , and the epithelial component was composed of solid nests of plump epithelioid cells with well - formed glandular structures ( fig . the periphery of the tumor showed focal calcification and more densely packed spindle cells without an epithelial component ( fig . the glandular structures were lined by round or cuboidal cells with round to oval nuclei and abundant eosinophilic cytoplasm . some glandular lumens are filled with pale basophilic materials , which were periodic acid schiff and alcian blue positive . the intervening spindle cells were slightly plump and had elongated , vesicular nuclei with a scant amount of eosinophilic cytoplasm . on average , 15 mitotic figures were seen per 10 high - power fields both in epithelial and spindle cells . on immunohistochemical staining , the glandular elements were diffusely and strongly stained with cytokeratin , whereas the spindle cells were diffusely stained with vimentin and focally with epithelial membrane antigen ( fig . 5 ) . stains for muscle specific actin , smooth muscle actin , s-100 , carcinoembryonic antigen , synaptophysin , chromogranin , thyroglobulin , and thyroid transcription factor-1 ( ttf-1 ) were all negative in the tumor cells . on ultrastructural examination , the tumor was composed of solid nests of epithelial cells and fascicles of spindle cells , which were partially covered by basal lamina . the epithelial cells had desmosome - like cell junctions , whereas the spindle cells had no cell junctions . no tonofilaments , granules , microvilli , or other specific organelles were found ( fig . the syt - ssx fusion gene transcript was identified in the paraffin - embedded tumor tissue using the reverse transcriptase - polymerase chain reaction method ( fig . a synovial sarcoma is a distinctive soft tissue sarcoma and their immunohistochemical and cytogenetic features are well described ( 1 ) . while most tumors are located in the vicinity of large joints and intimately related to tendons , tendon sheaths and bursal structures , they rarely occur within the joint cavity itself ( 1 ) . furthermore , true synovial lining cells are not stained with epithelial cell markers , such as cytokeratin and epithelial membrane antigen , but this tumor is stained with these markers ( 8) . also , a synovial sarcoma occurs in various locations that are unrelated to joint structures ( 1 , 3 - 6 ) . these findings suggest that synovial sarcoma is not related to normal synovium and may be derived from pleuripotential mesenchymal cells , capable of partial or aberrant epithelial differentiation ( 9 ) . from this point of view , it is possible that a primary synovial sarcoma can occur in the thyroid gland and may originate from the mesenchymal cells of the capsule or stromal tissues of the thyroid gland . kikuchi et al . ( 7 ) previously reported a 60-yr - old male patient with primary synovial sarcoma of the thyroid gland . the patient had an ill - defined solid mass , which occupied almost the entire thyroid parenchyma and extended to the thyroid cartilage and surrounding muscles . the tumor revealed monophasic synovial sarcoma that was composed of spindle cells . in our case , a 15-yr - old boy with an encapsulated , biphasic synovial sarcoma without invasion into the adjacent structures underwent a complete resection . the fine needle aspiration cytology showed a large quantity of spindle cells , either isolated cells or dense clusters around blood vessels in a radial fashion . in our case , the cytologic smear showed moderate cellularity with scattered benign - appearing follicular epithelial cells and several clusters of spindle - shaped cells or epithelioid cells with mild nuclear pleomorphism . the main histologic differential diagnosis includes a spindle cell variant of medullary carcinoma , undifferentiated ( anaplastic ) carcinoma , and spindle epithelial tumor with thymus - like differentiation ( settle ) . the spindle cell variant of medullary carcinoma affects an older age group and is positive for synaptophysin , chromogranin , and calcitonin . the undifferentiated carcinoma is uniformly positive for cytokeratin and shows an admixture of spindle cells , pleomorphic giant cells , and epithelioid cells . the poorly differentiated carcinoma shows solid cellular nests with a trabecular arrangement and is thyroglobulin and ttf-1 positive . settle shows very similar clinical , histologic , and immunohistochemical features to synovial sarcomas , especially in the case of a monophasic pattern . settle consists of bland , uniform - appearing spindle cells that merge with more epithelial - appearing structures , whereas a synovial sarcoma consists of more atypical - appearing and mitotically active tumor cells . the immunohistochemical profile of synovial sarcoma and settle is very similar , but over 80% of synovial sarcomas ultrastructural examination shows that spindle cells have a cell junction without basal lamina in settle , but spindle cells of synovial sarcoma have no cell junction . . a cytogenetic study may provide conclusive evidence to confirm the diagnosis of synovial sarcoma by detecting the syt - ssx fusion gene transcripts using the reverse transcriptase - polymerase chain reaction method ( 12 ) . unusual locations make the diagnosis of synovial sarcoma difficult , especially when the synovial sarcoma occurs in the thyroid . differential diagnosis from settle is very difficult due to their similar clinical , histological , and immunohistochemical features . ultrastructural and cytogenetic studies with recognition of synovial sarcoma are necessary to make a definitive diagnosis .
synovial sarcoma is a rare but distinct soft tissue neoplasm , most commonly occurring in para - articular regions of the extremities of young adults and also occurring in the head and neck region . to the best of our knowledge , 
 only one case of primary synovial sarcoma of the thyroid has been previously reported . here 
 , we report a 15-yr - old man who had a chief complaint of a palpable neck mass . 
 the neck computed tomography revealed a relatively well - demarcated solid mass in the left thyroid gland . 
 after fine needle aspiration cytology , total thyroidectomy and lymph node dissection were performed . grossly , the mass was covered by the same capsule as the thyroid gland , measuring 655 cm in dimensions and weighing 78 gm . 
 the cut surface showed a well demarcated , lobulated , grayish tan , and rubbery solid tumor . 
 histologically , this tumor was a biphasic synovial sarcoma . 
 immunohistochemical , ultrastructural , genetic studies , and cytologic findings were all consistent with synovial sarcoma . when synovial sarcomas arise in this unusual site , recognition and differential diagnosis become more difficult . 
 the differential diagnosis of a spindle epithelial tumor with thymus - like differentiation is very difficult due to their similar clinical , histological , and immunohistochemical features . 
 ultrastructural and cytogenetic studies for synovial sarcoma are necessary to establish a definitive diagnosis .