it is a relatively common tumor , comprising approximately 5% of all benign soft tissue tumors , and shows a predilection to affect the sensory nerves . intraosseous neurilemmoma , however , is very rare , accounting for less than 1% of benign bone tumors . the mandible is the most frequently affected site , followed by the sacrum ; this tumor rarely arises in the bones of the extremities . only 2 cases of intraosseous neurilemmoma around the elbow have been described in literature ; these cases arose in the distal humerus . a 21-year - old woman was referred to our orthopedic unit with a 1-month history of pain in the left elbow . the initiating factor , such as trauma , was not clear . on physical examination , the overlying skin was intact , and there was no evidence of warmth , erythema , or induration . no other masses were palpable . the elbows and forearms had a normal range of motion ( rom ) . plain radiographs revealed a well - defined , lytic , and expansile lesion , with thin marginal sclerosis and trabeculation in the proximal ulna ( fig . 1 ) . computed tomography ( ct ) showed considerable destruction of the cortex of the ulna , which connected with a mass of soft tissue . at the edge of 2a ) , which had invaded into the cortex , and no periosteal reaction was seen ( fig . 2b ) . magnetic resonance imaging ( mri ) showed the well - defined and lobulated lesion to be isointense to skeletal muscle on t1-weighted images , and heterogeneous and hyperintense on t2-weighted images . the lesion revealed uniform enhancement following gd - dtpa administration . because of uncertainty regarding the histological origin of the tumor , we performed an open biopsy . the pathologic specimen consisted of hypercellular regions with palisading nuclei ( antoni type a ) , and hypocellularity regions with myxoid background ( antoni type b ) . curettage was performed on the bone lesion , and the resulting deficit was grafted with beta - tricalcium phosphate ( fig . after 3.5 years of follow up , the patient is clinically asymptomatic , and the radiographs show complete graft incorporation ( fig . the patient was informed that the case would be submitted for publication and her consent was obtained . neurilemmomas may involve bone tissue via 3 mechanisms : ( 1 ) they may be intramedullary , producing rarefaction of the bone ; ( 2 ) they may be located within the nutrient canal , with the formation of a dumbbell - shaped tumor ; or ( 3 ) they may be extraosseous , eroding into the bone . with regard to the comparatively high incidence of intraosseous neurilemmoma in the mandible , it has been speculated that the long intraosseous path of the mandibular nerve may predispose to metaplasia of the schwann cells in its nerve sheath . however , some authors have refuted this theory , noting that the nerves innervating the long bones through the nutrient foramina are longer than the mandibular nerve ; nevertheless , the frequency of intraosseous neurilemmoma in the long bones is several times lower than that in the mandible . de la monte and colleagues have suggested that the death of sensory nerve fibers within bone tissue may account for the exceedingly rare occurrence of intraosseous neurilemmoma . we think that the high frequency of neurilemmoma in the mandible is not because of the long intraosseous course , but because the mandibular nerve consists of sensory nerves of the trigeminal nerve origin . the characteristic radiographic features of intraosseous neurilemmoma include the following : ( 1 ) a well - defined lytic lesion , ( 2 ) sclerotic margins , ( 3 ) lobulated or trabeculated contours , ( 4 ) cortical expansion , and ( 5 ) absence of central calcification . however , the radiographic findings are non - specific . it is difficult to differentiate intraosseous neurilemmoma from other benign neoplasms of such osseous lesions , including solitary bone cyst , aneurysmal bone cyst , giant cell tumor , non - ossifying fibroma , benign fibrous histiocytoma , desmoplastic fibroma , fibrous dysplasia , chondromyxoid fibroma , and enchondroma . on ct , we can confirm the origin of the tumor when there is soft tissue extension of the tumor . if the origin is intraosseous instead of periosteal , no periosteal reaction is seen , and the edge of the destructed cortex will be overhung by a soft tissue mass . young and colleagues pointed out that the aspect of the overhanging edges of cortical bone can assist in distinguishing between an intraosseous desmoplastic fibroma versus a periosteal desmoplastic fibroma . similarly , intraosseous neurilemmoma is considered to originate from intraosseous nerves rather than from periosteal nerves , if ct shows overhanging edges of cortical bone . in our case and in others this may be the characteristic finding of intraosseous neurilemmoma provided the tumor expands within the nutrient canal of the cortex . the final diagnosis of intraosseous neurilemmoma was not made until after histologic examination of tissue obtained during excision . the diffuse immunoreactivity for s-100 protein is indicative of a schwann cell origin , and helps distinguish this tumor from other benign spindle cell lesions of similar histology . the recommended treatment for intraosseous neurilemmoma is conservative resection and bone grafting , as malignant change is extremely rare . recurrence also is rare following complete local excision . in previous reports , after performing only curettage , curettage and bone grafting , or en block resection , local recurrence only occurred in 2 cases . these authors did not refer to the reason for local reference , but recurrence of the tumor is not evidence of malignant transformation . in our case owing to the infrequency of an intraosseous neurilemmoma , the diagnosis is often not even suspected until histologic evaluation after a biopsy . although very rare , intraosseous neurilemmoma should be taken under consideration in the differential diagnosis of painful , radiographically benign - appearing osseous tumor around the elbow . written informed consent was obtained from the patient for publication of this case report and accompanying images . a copy of the written consent is available for review by the editor - in - chief of this journal on request . case report writing , data collection , and discussion writing were done by munehisa kito and yasuo yoshimura . discussion writing was carried by kenichi isobe , kaoru aoki , takashige momose , and hiroyuki kato .