a 37 year - old chinese man presented to our hospital for evaluation of a painless left cervical mass in september 2009 . at that time , he underwent a cervical ultrasound , which showed a thyroid mass with clear margin . his surgeons noted a single well circumscribed tumor with 3 cm in diameter in the left lobe of the thyroid , while no abnormality with the right lobe and without lymphadenectasis of neck were found . the pathologic specimen showed the tumor was an orbicular - ovate solid mass with hard texture and 4cm2.5cm2.5 cm in size . the cut surface was pale and firm , and had a whorled appearance ( fig.1 ) . microscopically , the tumor was composed of spindle cells with a mixture of hypocellular ( collagenous ) and hypercellular area . the spindle cells arranged in various patterns , including storiform , wavy , desmoidlike , palisading and hemangiopericytoma - like ( fig . the cells were spindle or orbicular - ovate with finely dispersed chromatin and inconspicuous and small nucleoli . no cytological atypia , necrosis , and hemorrhage were found . in the margin of the tumor , the mitotic figures were rare [ < 1 /10 high - power field ( hpf ) ] . immunohistochemically , tumor cells were strongly positive for cd99 , bcl-2 and cd34 , but negative for desmin , neurone - specific enolase ( nse ) , smooth muscle actin ( sma ) , s-100 , or cd68 . ki-67 index is lower than 5% ( fig . solitary fibrous tumor ( sft ) is a rare spindle - cell neoplasm , which was first reported by klemperer and rabin in 1931 as a pleural tumor 1 . subsequently , it has been recognized in other organs of the body 2 - 13 . furthermore , distinct immunohistochemical features and electron microscopy findings supported a fibroblastic differentiation of the tumor . therefore , it is now widely accepted that sft is a quite ubiquitous neoplasm of likely mesenchymal origin . sft arising in the thyroid is very rare . since taccagni et al . first reported in 1993 it often occurs in adults , with an average age of 49.5 years ( range from 28 to 76 years old ) . among 24 cases , including the present one , 15 cases were male and the other 9 cases were female [ table 1 ] . thyroid sft has no pathognomonic clinical feature and presents with a painless slowly increasing mass . the patient reported with the intrathoracic tumor had increasing respiratory symptoms that were thought to be caused by his well - known asthma . the etiopathogenisis of sft is not clear , but in some cases occurred in the thyroid presented with a background of long - standing goiter 1 , 12 , 14 , however , the herein reported case showed no goiter and found by phsycal examination . most thyroid sfts appeared as large well - circumscribed solid lesions , with whitish to grayish color on cut surface . microscopically , the lesions were characterized by a proliferation of spindle cells arranged in various patterns , including storiform , wavy , desmoid - like , palisading , and hemangiopericytic . immunohistochemically , tumor cells were strongly positive for vimentin , cd34 , cd99 , and bcl-21 . all these above features are similar to its counterparts occurred in the pleura and other sites . there were no distinct pathologic criteria that may definitely differentiate benign from malignant variants of sft . in the reported 23 cases of thyroid sft , only one case had recurrence and pulmonary metastasis . the features that suggest aggressive behavior include high cellularity , moderate to marked cytological atypia , higher frequency of mitoses ( > 4/10 hpfs ) , evidence of tumor necrosis , and infiltrating margins 14 . so , long - term follow - up should be recommended especially to the cases with aggressive features . the differential diagnosis of sft of the thyroid gland includes riedel thyroiditis , fibrosing hashimoto 's thyroiditis , leiomyoma , peripheral nerve origin tumors and undifferentiated carcinoma . riedel thyroiditis and fibrosing hashimoto 's thyroiditis are diffuse fibrous mass with marked infiltrating lymphocytes , wheras the sft of thyroid is well - circumscribed mass without notable lymphocytes . for leiomyoma and peripheral nerve origin tumors , immunohistochemical staining are specific positive for sma and s-100 , repectively , while nagetive for cd34 and bcl-2 . undifferentiated carcinoma is charecterized by highly pleomorphic tumor cells that lack any organoid growth pattern . a component of papillary carcinoma , follicular carcinoma or poor differentiated thyroid carcinoma often can be found and express the epithelial markers .
solitary fibrous tumor ( sft ) is a rare spindle - cell neoplasm , especially in the thyroid . 
 we report a case of primary solitary fibrous tumor of the thyroid gland in a 37 year - old chinese man . 
 the tumor was characterized by bland - looking spindle cells admixed with thin and thick collagen fibers . 
 on immunohistochemistry study indicated that tumor cells were diffusely positive for cd34 , bcl-2 and cd99 , and negative for desmin , nse , sma , s-100 , and cd68 . 
 the patient remains well 16 months after excision . 
 the morphologic and immunohistochemical features of the thyroid sfts are similar to their reported counterparts in other anatomic sites .