hemangiopericytoma is a mesenchymal tumor that occurs most commonly in the soft tissues of the upper and lower extremities , pelvis , and retroperitoneal space and seldom in visceral organs ( 1 ) . due to its vascular origin transarterial embolization is a procedure which can be performed prior to surgery of highly vascular tumors in order to avoid perioperative bleeding . although there are a few reports ( 3 , 4 ) of preoperative embolization of hemangiopericytomas , to our knowledge , there has been no report to apply a preoperative embolization of a primary hemangiopericytoma of the rib . a 62-yr - old man was admitted to our department with the complaints of progressive dyspnea , an aching chest pain on the right side , malaise and anorexia with weight loss on 4 december 2001 . his medical history was noteworthy for diabetes mellitus type ii , a 10-pack year history of smoking , and a moderate history of alcohol . on physical examination , a chest roentgenogram on admission demonstrated a large , indistinct opacity on the peripheral region of the right lower zone associated with blunting of the right lateral costophrenic sulcus . computed tomographic ( ct ) scans of thorax revealed a homogeneous mass expanding the right seventh rib and invading into the adjacent lung , and a right sided pleural effusion ( fig . histopathologic examination of the pleural biopsy specimens were reported as chronic pleuritis . to clearly identify the mass , a ct - guided , core needle biopsy was performed . the ultrasonography and ct of the abdominopelvic region , magnetic resonance imaging of both upper and lower extremities and ct of the cranium were reported as normal . preoperative angiography showed hypervascularization of the tumor from the right 7th and 8th ( fig . 2 ) , and also from 9th intercostal arteries . we performed embolization with polyvinyl alcohol ( pva ) particles . firstly , we embolized arterioles within the tumor for which we used pva , 250 m in size . then , embolization of the feeding arteries of the tumor was done with pva , 750 - 1,000 m in size . complete resection of the mass with 7th , 8th , and 9th rib with a surgical margin of 5 cm and a wedge resection of the adjacent lung was done . macroscopic evaluation of the resected tumor revealed an encapsulated and lobulated mass located posteriorly to the 6th to 9th ribs . microscopic evaluation of the tumor revealed a solid mesenchymal mass composed of indistinct fusiform to rounded cells arranged around slit - like and branching thin - walled vessels ( fig . immunohistochemically tumor cells stained positively for vimentin and focally for cd-34 , and negatively for cytokeratin , smooth muscle actin , s-100 and cd-31 . hemangiopericytomas , that are believed to be derived from the vascular pericyte , rarely originate from ribs , although they can be found in any region which contains pericytes ( 5 ) . but , it may be asymptomatic and may be detected as an abnormal shadow on a chest radiography film ( 3 ) . the radiographic appearance is that of a mass on the lateral chest wall ( 6 ) . rarely , it may reach a huge size occupying two thirds of one hemithorax ( 7 ) . in our case , thorax ct revealed that a large indistinct opacity on the peripheral region of the right lower zone on chest radiography was a homogeneous mass expanding the right seventh rib . microscopically the tumor consists of numerous vascular spaces of variable size and shape separated by aggegates of tightly packed oval to spindle - shaped cells with bland nuclei . mitotic figures and nuclear atypia are variable in number and degree ( 6 ) . in our case , microscopic evaluation of the tumor revealed a solid mesencymal mass composed of indistinct fusiform to rounded cells arranged around slit - like and branching thin - walled vessels . gomori silver impregnation method revealed a dense reticulin meshwork confirming the perivascular distribution of tumoral cells . the results of immunohistochemical studies were consistent with the diagnosis ( 8) . because hemangiopericytomas are highly vascular tumours , because the choice of treatment for primary ones is surgery , and because some authors ( 3 , 5 ) reported massive bleeding during the operation of hemangiopericytomas , we performed transarterial embolization preoperatively in order to prevent or decrease perioperative bleeding ( 3 ) . and then , the patient underwent a right thoracotomy during which there was no any significant bleeding . because the recurrence rate was reported as 50% ( 7 , 9 ) , long - term follow - up is necessary for the patient . in our case finally , our case report has two important features : 1 ) primary hemangiopericytoma of the rib is an extremely rare tumor ; 2 ) preoperative embolization of a primary hemangiopericytoma of the rib have not been reported heretofore .