osteoblastoma ( ob ) was first described in the english literature by jaffe and mayer in 1932 . in the year 1956 , jaffe and lichtenstein independently proposed the term  benign ob  to identify an osteoblastic osteoid - forming lesion similar to osteoid osteoma ( oo ) , but having a greater growth potential . some of these seemingly innocuous tumors do , however , tend to have local destructive growth pattern and recurrence . dorfman labeled these variants as aggressive osteoblastoma ( ao ) and suggested that the presence of epithelioid osteoblasts was a distinctive histologic feature in these variants . ob is rarely known to involve the skull and when they do , mandible followed by the fronto - temporal calvarial regions tend to be the most frequently reported sites . although , there are reports of involvement of skull base like the sphenoid sinus , we are not aware of any reports of involvement of the craniovertebral junction by the aggressive variant of this tumor . herein , we report the clinico - radiological and pathological findings of ao affecting the craniovertebral junction in a 34-year - old man and present a review of the literature . the present case report is about a 34-year - old gentleman who presented with a painful swelling behind the angle of left mandible of 5 months duration . he also complained of change in character of voice , dysphagia and weakness of left upper limb for 3 months prior to admission . on neurologic examination , he had ix , x and xi cranial nerve palsy on the left side . there was a 3.5 cm  3.0 cm bony hard swelling underneath the angle of left mandible leading to obliteration of post auricular groove . on examination of the oral cavity , the swelling was bulging into the tonsillar fossa and compressing the palatal arches . since , the mass was palpable externally , fine - needle aspiration was attempted pre - operatively ( twice ) and on both occasions , but it was found to be inconclusive . the axial and reformatted sagittal computed tomography showed a large expansile and destructive bony lesion involving the foramen magnum on the left side from lower clivus up to the axis . thin sclerotic rim was visible around most of the periphery of the lesion [ figure 1a  d ] . the lesion had involved the lateral masses and part of anterior and posterior elements of upper 2 cervical vertebrae . ( a - d ) computed tomography scan of the craniovertebral junction shows expansile mass involving the clivus , occipital squama including the jugular foramen on the left side . vertebral foramen of c1 is obliterated on the left side and anterior arch is involved more extensively than the posterior one . internal calcifications can be seen and a thin peripheral bony rim can be seen around the tumor , which is absent at places magnetic resonance imaging showed a heterogenous mass ( size approximately 5.3 cm  6.2 cm  5.5 cm ) with strong post contrast enhancement . the lesion was displacing left internal carotid artery peripherally causing mild compression and had involved the left vertebral artery , the oblique part of v3 segment [ figure 2a  d ] . ( a - c ) the tumor is heterogeneously but avidly enhancing and extending inside the spinal canal but no significant neuraxial compression is seen . mass is anteriorly pushing the oral cavity and nearly obliterating it on the left side . ( d ) compression and thinning of ipsilateral vertebral artery ( the oblique segment of v3 ) and displacement of internal carotid artery . ( e and f ) post - operative images show surgical cavity with residual enhancement anterolaterally with pre - operative impression of a high grade bony lesion , the patient was planned for surgical decompression of the mass through left sided far lateral approach . at surgery , near total excision of the tumor was achieved , deliberately leaving a thin rim of tumor tissue attached to posterior pharyngeal wall to prevent opening the oral cavity [ figure 2e and f ] . histopathological evaluation of the resected tissue showed epithelioid osteoblasts lining the bony trabeculae separated from the thin walled vessels by the osteoclastic giant cells . hence , a pathological diagnosis of ao was made [ figure 3a and b ] . ( a ) irregularly laid down osteoid rimmed by epithelioid osteoblasts ( single arrow ) separated by thin walled vascular channels and multinucleate ostoclastic giant cells ( double arrow ) ( h and e , 200 ) . ( b ) part of bony trabaculae , osteoid , epithelioid ostoblasts along with tripolar mitotic figure ( single arrow ) ( h and e , 400 ) these were first described in the english literature by jaffe and mayer in 1932 . in the year 1956 , jaffe and lichtenstein independently proposed the term  benign ob  to identify an osteoblastic osteoid - forming lesion similar to oo , but having a greater growth potential . it most commonly affects males during the second decade and often arises within the vertebral column and long bones . approximately 10 - 12% of ob occur in the maxillofacial skeleton , especially the mandible . the pathological spectrum of osteoid forming primary bone tumors includes oo and ob at one end and osteosarcoma ( os ) on the other . oo and ob differ only in size , potential for progression and propensity to produce extreme pain . nevertheless both are benign and respond well to surgery . on the other side , os is known to be malignant with frequent recurrences and metastasis in spite of seemingly adequate surgery . there are , however , some tumors with features nearly similar to the conventional obs which tend to display aggressive behavior with recurrences and a locally destructive growth pattern . dorfman examined one such case of his and 23 others which were reported until then and along with weiss , named them ao . these include presence of large epithelioid osteoblasts lining the bony trabeculae , presence of a moderate number of mitotic figures and invasion into the surrounding bone and soft - tissue . conventional obs are lined by osteoblasts and lack any mitotic figures and invasion into surrounding structures . in contrast , oss have abundant osteoid , have abundant mitotic figures and have areas of necrosis . due to the relative rarity , both ob and ao often present with local pain and swelling ; however , ao tends to affect an older age group than ob , usually arising in the third or fourth decade . the radiographic appearance of ao is similar to ob , consisting of a circumscribed lytic defect sometimes surrounded by a sclerotic rim , although a more aggressive appearance including significant cortical expansion and destruction can be seen like in our patient . obs at certain locations like spinal canal and skull base , irrespective of whether they contain epithelioid cells or not , would have aggressive clinical behavior . in these cases , the clinical aggressiveness appears more dependent on the precise location and size of the tumor than on its microscopic features . therefore , while epithelioid osteoblasts are critical to the diagnosis of ao , tumor size and location seem to be more important considerations as far as treatment and prognosis are concerned . it should be remembered that tumors greater than 4 cm in diameter or those located in anatomic sites that impact the surgeon 's ability to completely remove it are more likely to recur or cause local tissue destruction . radiologically , our patient presented with an expansile lytic mass with internal calcifications and thin peripheral rim that appeared broken at places . as these tumors are highly vascular , secondary aneurysmal bone cyst like changes may occur and cause diagnostic dilemma . however , unlike true abc , there would be areas where the tumor would have entirely solid areas . from the treatment point of view , gross total excision is desirable in obs in general . complete excision can be achieved either by curettage or by en block resection , e.g. , spondylectomy . however , in patients like ours , complete excision is often not possible by dint of tumor location . hence , adjuvant therapy like chemo / radiotherapy has a role to play in aos . adjuvant therapy is advocated in unresectable tumors , incomplete excisions , tumor with aggressive histologies and recurrent tumors . long - term relapse free survival of up to 25 years have been reported after radiotherapy . although , there is controversy as to what should be the optimal dose of radiotherapy ( 60 gy vs. 40 gy ) , a dose of 50 gy given in a standard fractionated fashion ( 2 gy , 5 days a week for 5 weeks ) is probably the most effective regimen . radiotherapy usually causes either stabilization of tumor growth or its partial reduction , rarely , if ever complete tumor resolution . tumor response to radiotherapy immediately results in pain relief and causes ossification of the affected area in the longer run . these include chances of radiation induced bone necrosis with frank malignant degeneration in the residual tumor and radiation myelitis in spinal ao 's like in our patient . the indications are same as radiotherapy and people who advocate chemo , do so to avoid the concerns with radiotherapy mentioned above . methotrexate appears to be an effective agent although polytherapy ( methotrexate , doxorubicin and cisplatin ) have also been used . progression free survival up to 33 months have been reported with combination chemotherapy . in spite of all kinds of treatment discussed above , ao remains a difficult proposition with recurrences , malignant degeneration and finally an inevitable death . although , recurrence rate in obs in general is around 10% after treatment , it is as high as 50% with aos . this underscores the need for an active follow - up schedule for these patients and further research for other treatment options . ao may affect craniovertebral junction and should be kept as differential diagnosis while dealing with extradural bony lesions in this location . we stress the need for meticulous histopathological examination in such cases for proper diagnosis and prognostication . maximal surgical debulking followed by adjuvant chemo / radiotherapy appears to be the optimal treatment strategy as of now . these patients should be stringently followed - up as they have high chances of tumor recurrence / progression .
osteoblastoma ( ob ) is a rare bony neoplasm constituting around 1% of all primary bone tumors . 
 although the vertebrae and long bones are the most common sites affected by ob , skull remains a relatively uncommon site of occurrence . 
 aggressive variant of ob is histologically intermediate between an indolent conventional ob and a malignant osteosarcoma . to the best of our knowledge , 
 aggressive osteoblastoma ( ao ) affecting the craniovertebral junction has not been previously described in the literature . in this report , we present a 34-year - old gentleman who presented with a mass involving the left side of the neck and oral cavity along with ipsilateral lower cranial nerve paresis . 
 computed tomography and magnetic resonance imaging scans of the craniovertebral junction revealed a heterogeneously enhancing expansile lesion with areas of destruction involving the clivus , left sided jugular foramen and left side of first two cervical vertebras . 
 angiography showed distortion of the v3 segment of the left vertebral artery and shift of the ipsilateral internal carotid artery . 
 the tumor was maximally excised through far lateral approach . 
 histopathologic examination revealed a diagnosis of ao . 
 the patient was referred for radiotherapy for the residual tumor and was doing well at 5 months follow - up .