meningeal hemangiopericytoma ( mhpc ) was first reported by stout and murray in 19422832 ) . mhpc tumors arise from zimmermann 's pericytes around capillaries and post - capillary venules . begg and garret have reported several similarities between mhpc and angioblastic meningioma , and have additionally used immunohistochemistry and genetic analyses to demonstrate the ways in which mhpc differs from meningioma516 ) . although mhpc was initially considered to be a transformation of meningioma , it was ultimately recognized as a distinct pathological entity with clinical behaviors , immune cell features , and ultrastructural features that differ from meningiomas7182226 ) . in 1993 , the world health organization recognized mhpc as a distinct clinicopathological entity , based on mhpc 's tendencies toward recurrence and extraneural metastasis , its distinct clinical behavior , and its immunohistochemical , ultrastructural , and genetic characteristics28 ) . they tend to recur even after macroscopic total resection , with local recurrence rates as high as 91% . a significant number of patients with mhpc live as long as 15 years and beyond after the initial surgery , developing second and third local recurrences as well as distant metastasis , thus mandating vigilant long - term follow - up26 ) . in this study , we explored treatment outcomes , recurrence , and clinicopathological characteristics in patients with surgically treated mhpc . this study included 15 patients who underwent surgical resections performed by one senior neurosurgeon between 1997 and 2013 . clinicopathological characteristics , such as age , sex , tumor location , tumor size , extent of surgical resection , adjuvant radiotherapy , recurrence , and survival were retrospectively collected from medical records . prior to surgery , the researchers explained to the patients that their medical records would be used for research . complete resection ( cr ) refers to cases in which the tumors were not visible to the naked eye and could not be seen in postoperative imaging . we analyzed characteristics such as age , sex , tumor size and location , extent of resection , status of adjuvant radiotherapy , and the histological grade of the recurrence . all statistical analyses were performed using spss ( version 16.0 , ibm spss inc . , chicago , il , usa ) . the relationships between various factors and recurrence times were assessed using the log - rank test . this study included 15 patients who underwent surgical resections performed by one senior neurosurgeon between 1997 and 2013 . clinicopathological characteristics , such as age , sex , tumor location , tumor size , extent of surgical resection , adjuvant radiotherapy , recurrence , and survival were retrospectively collected from medical records . prior to surgery , the researchers explained to the patients that their medical records would be used for research . complete resection ( cr ) refers to cases in which the tumors were not visible to the naked eye and could not be seen in postoperative imaging . we analyzed characteristics such as age , sex , tumor size and location , extent of resection , status of adjuvant radiotherapy , and the histological grade of the recurrence . all statistical analyses were performed using spss ( version 16.0 , ibm spss inc . , chicago , il , usa ) . the relationships between various factors and recurrence times were assessed using the log - rank test . the patient 's ages ranged from 33 to 76 years ( mean 47.2 years ) . twelve ( 80% ) patients had tumors in the supratentorial region and 3 ( 20% ) patients had tumors in the infratentorial region . one patient had received transcatheter arterial chemoembolization and adjuvant radiotherapy for hemangiopericytoma of the liver prior to the diagnosis of intracranial mhpc . in this case , the tumor had already spread to the lungs and spine , as identified using a preoperative positron emission tomography scan . two ( 13% ) patients with tumors that were at least 100 mm in size had received tumor embolization before surgery . patient characteristics , such as the extent of tumor resection , adjuvant radiotherapy , recurrence , time to recurrence , and status at last follow - up , are summarized in table 1 . recurrence occurred in 3 ( 20% ) patients : 1 patient ( case no . 15 ) developed recurrence at the original site ( fig . 1 ) and 2 patients ( case nos . recurrence occurred in 1 patient who had received radiosurgery and 2 patients who had not received adjuvant radiotherapy . there was no significant relationship between recurrence and adjuvant radiotherapy ( p=0.3 ) ( fig . 2 ) . among the patients who developed recurrence during follow - up , the average time to the first recurrence was 66.7 months . the 10-year recurrence - free survival ( rfs ) rate was 80% , whereas the median rfs was 146 months . the mean ki-67 index differed significantly between patients who did and did not develop recurrence ( 43% vs. 14% ; p=0.001 ) ( fig . 3 ) . comparison of 2 groups in terms of age , sex , tumor size , location , adjuvant radiotherapy status , histological grade , the ki-67 index are summarized in table 2 . the relationship between recurrence and the resection rate could not be assessed because cr was performed in all patients . one of them underwent radiosurgery ( 18 gy at the 90% isodose line)for the single lesion . the others underwent conventional radiotherapy ( 50 gy in 25 fractions ) because of tumors at the multiple brain sites . although the patient who received radiosurgery experienced temporal recurrence within 3 years , neither of the patient who received conventional radiotherapy experienced recurrence . we investigated recurrence status according to histological grade and found that 2 ( 67% ) of the patients with recurrence had grade ii disease , and 1 ( 33% ) of the patients with recurrence had grade iii disease . among patients without recurrence , 8 ( 67% ) had grade ii disease and 4 ( 33% ) had grade iii disease . accordingly , recurrence rates did not differ significantly according to histological grade ( p=0.931)33 ) . the 5- and 10-year overall survival rate was 88.3% , while the 5- and 10-year recurrence - free survival rates were 83% and 52% , respectively ( fig . two patients died of respiratory problems during the follow - up period : 1 patient with metastatic lung tumors died of acute respiratory distress syndrome in 12 months after surgery , and the other patient died of pneumonia 1 month after surgery . because mhpc carries high risks of recurrence and spread to other sites , precise identification of the tumor 's features is necessary to provide appropriate treatment and accurate assessments of prognosis45891214152225 ) . long - term studies have shown that mhpc tends to recur in brain and spinal meninges , but not at the original site4578151622 ) . galanis et al.12 ) and guthrie et al.15 ) have emphasized the importance of achieving cr during the first surgery to reduce the risk of recurrence and prolong survival . they have also suggested that aggressive surgical therapy plays an important role in the success or failure of treatment . in a study of mhpc , schiaritis et al.28 ) reported rfs rates of 77% after cr and 3% after incomplete resection . they stated that the time to recurrence after cr was 63 months longer than the time to recurrence after incomplete resection . in the current study , this value is substantially lower than has been observed in other studies that included cases of incomplete resection . accordingly , the results of the current study also tend to suggest that cr poses a lower risk of recurrence than incomplete resection . a literature review of recent studies on meningeal hemangiopericytomas showed an average cr rate of 57.7% ( table 3 ) . adjuvant radiotherapy has been reported to reduce recurrence rates and extend survival24102330 ) . in 1976 guthrie et al.15 ) reported a 52% recurrence rate in patients treated with adjuvantradiotherapy , as compared with an 86% rate in those who did not receive adjuvantradiotherapy . further , the average times to recurrence were 75 and 34 months for patients who did and did not receive adjuvantradiotherapy15 ) . in guthrie et al.'15)s study , adjuvant radiotherapy was performed in 4 patients with surgical findings of an unclear boundary between the tumor and the normal brain tissue ; biopsy results showed where the normal brain tissue had been invaded by tumor cells in the boundary area . in 1 of the 3 patients who developed recurrence , recurrence occurred in the boundary of the area where the tumor had originated . it is logical to suggest that adjuvant radiotherapy is helpful when incomplete resection is performed ; however , even in cases of cr , adjuvant radiotherapy may be helpful when the boundary area of the tumor is not clear . radiosurgery is an effective treatment for recurrent mhpc with well - separated , small tumors . bastin and mehta4 ) reported that the tumor was effectively reduced by a single fraction of stereotactic radiosurgery for recurrent mhpc . galanis et al.12 ) performed radiosurgery on 17 tumors in 7 patients who had recurrent tumors or persistent tumors , even after they had received conventional fractionated radiotherapy with 50 to 60 gy . guthrie et al.15 ) demonstrated that recurrence rates reduced with increasing levels of radiation , and found no recurrence in patients who had received more than 50 gy of radiation . dufour et al.9 ) reported recurrences in 2 patients who had received less than 30 gy of radiation , but no recurrence in patients who had received doses of 50 and 64 gy . in our study , recurrence was observed in 1 patient who had received radiosurgery with 18 gy , but no recurrence was observed in either of the 2 patients who received conventional radiotherapy with more than 50 gy . even though we did not observe a statistically significant difference in recurrence rates , we consider a dose of at least 50 gy to be both helpful and within the range of tolerable doses . several clinicians have performed adjuvant radiotherapy before surgery for cases of mhpc that are associated with elevated surgical risk . carella et al.6 ) presented the hemangiopericytic variant of angioblastic meningioma , which had better responses to adjuvant radiotherapy than normal meningioma . in an analysis of 5 patients who had undergone adjuvant radiotherapy before surgery , uemura et al.35 ) reported that tumors were easily removed without substantial bleeding and , further , that many tumor cells were pathologically reduced after adjuvant radiotherapy . however , it is difficult to obtain histological findings before surgery , and it is necessary to fully consider adjuvant radiotherapy after surgery . preoperative adjuvant radiotherapy should be performed only when surgical treatment is predicted to be difficult . vuorinen et al.34 ) suggested that the ki-67 index was not correlated with clinical outcomes , even though survival times tended to be longer among patients with a ki-67 index < 5% . in our study , the ki-67 index was significantly higher in patients who developed recurrence , as compared with patients who did not develop recurrence ( 43% vs. 14% ; p<0.001 ) . galanis et al.12 ) reported recurrence in 32 of 34 cases , including 19 patients with recurrence at the original site , 11 with recurrence at both the original site and a distant site , and 2 with leptomeningeal seeding at the site of the original tumor , but no recurrence . in our study , 1 patient had a recurrent tumor at the original site , and 2 patients had recurrent tumors at different sites . however , leptomeningeal seeding was not observed . mhpc also tends to recur in at sites that are distant from the nervous system . extraneural metastasis simultaneously occurs in the bones , lungs , kidneys , pancreas , adrenal glands , and liver . galanis et al.12 ) showed that metastasis is most common in the bones and liver , whereas brunori et al.5 ) reported more frequent metastasis in the lungs , bones , soft tissues , and liver . the current study includes 1 case in which extraneural metastasis had already occurred in the lungs , bones , and liver at the time of diagnosis with intracranial mhpc . reported a 5-year recurrence rate of 33% and brunori et al.5 ) reported that the average time to the first recurrence was 84 months . guthrie et al.15 ) stated that the 10-year recurrence rate after surgery was 76% and goellner et al.14 ) reported that the 15-year recurrence rate was 76% . schiariti et al.28 ) reported that the time to recurrence after the first surgery averaged 80 months . in this study , the 5- and 10-year overall survival rate were 91.7% and 88.3% , respectively , while the 5- and 10-year recurrence - free survival rates were 83% and 52% , respectively . recurrence after surgery has been described as the last stage in the natural history of hpc45141517 ) . therefore , it may be necessary to continue follow - up observations for as long as is possible . the duration of observational follow - up has an important effect on the selection of treatment methods because a long duration of observational follow - up allows an effective method , such as radiosurgery , to be selected for small recurrent tumors ( in addition to surgical treatment ) , before the tumor has had the opportunity to grow large . moreover , it is important to identify metastatic status during follow - up because extraneural metastasis frequently occurs in mhpc . in retrospective analyses , stout et al.32 ) and adegbite et al.1 ) demonstrated rates of extraneural metastases of 12% and 57% , respectively . treatment methods , surgical outcomes , and many other factors may lead to differences in the rate of extra - neural metastasis . guthrie et al.15 ) presented 5- , 10- , and 15-year metastasis rates of 13% , 33% , and 64% , respectively . koyama et al.20 ) reported that metastases occurred an average of 8 years after initial treatment , and have tracked patients for 16 years . , cr must be followed by adjuvant radiotherapy and periodic follow - up that is commensurate with the degree of surgical resection .