Introduction

Aneurysmal bone cysts are expansile, lytic, bone lesions consisting of multiple thin walled, cystic cavities containing blood. Aneurysmal bone cyst is a rare and benign lesion of the skull with obscure pathogenesis.1 Jafe and Lichtenstein coined the term in 1942, to describe a peculiar bone lesion with a vascular lining and characteristic “soap bubble” radiological picture of expanded bone.6 Aneurysmal bone cyst is a rare lesion of the skull with obscure pathogenesis.8 Approximately, 36 to 50% of all aneurysmal bone cysts are found near one end of long bone and 25% involve the vertebrae, hyoid, mandible and odontoid.18,23  Sixty-three cases of aneurysmal bone cysts of skull have been reported in the literature, of which 11 were of temporal bone.15 An unusual case of aneurysmal bone cyst in the right temporal region is reported here.

Case report

An 18 year old boy was admitted with headache, right earache and gradually increasing painless swelling in the right temporal region for 3 months. There was no history of trauma. On examination there was a non-tender ill-defined mass, in the right temporal region; which was firm to hard and fixed to bone with smooth surface. Skin over the swelling was normal. There was no neurological deficit. All routine investigations were within normal limits.

Figure 1

X-ray skull revealed a radiolucent area in the right temporal bone.

Figure 2

CT scan head (plain and enhanced) showed extra axial heterogeneous mixed density mass in the right temporal region, taking irregular variegated enhancement. The mass showed medial extension compressing left temporal lobe. There was erosion of petrous part of temporal bone.

Figure 3

MRI showed large extra axial T1 intermediate signal intensity with small hyperintense areas and T2- FLAIR hyperintense mass (about 06cmx07cm) in the right temporal region. No significant perifocal oedema. The mass shows multiple fluid-fluid levels. Overlying extracranial soft tissues show swelling and altered signal intensity. Involvement of overlying calvari.

On post gado T1W1, the mass shows heterogenous contrast enhancement. The overlying bone and extra calvarial soft tissues also show moderate contrast enhancement.
A right temporal question mark skin flap was raised with temporalis muscle. The underlined temporal bone was thinned out with erosion of petrous and greater wing of sphenoid. The tumor was highly vascular had well defined capsule and was soft in consistency. Dark coloured blood was found from the cyst. The squamous part of temporal bone was thinned out and temporal dura was adherent to it, which remained intact. Inferiorly, tumor mass was extending upto the floor of middle cranial fossa and posteriorly to the petrous bone. Gross total excision of the tumor was done. Postoperative recovery was uneventful.
Histopathological examinations from different histopathologists revealed:
1. Specimen showed bone and soft tissue. These contain many spaces filled up with blood. Some of the spaces are empty. These spaces are lined by plump cells. The stroma contains osteoid. In some areas, calcification seen. In other areas, haemosiderin laden macrophages are seen. A good number of osteoclastic giant cells are seen. The stromal cells are spindle shaped. Features are compatible with aneurismal bone cyst.
2. Sections made from submitted specimen showed meningothelial cells arranged in whirled pattern. Focal areas of haemorrhage and bony tissue are also seen. Findings are consistent with meningioma.

Discussion

This case report has special emphasis on histopathology reports which brought into a diagnostic dilemma. This may be related with the sample from the specimen   containing bony component as well as meningeal component. But the patient has been treated and folloed up as a case of aneurysmal bone cyst.
Aneurysmal bone cyst is a benign, non-neoplastic lesion that presents most frequently under the age of 20 years.18, 5, 21 The involvement of the skull is rare. The incidence of aneurysmal bone cyst in the skull varies from 3 to 6% of all aneurysm bone cysts.3, 13, 17 It usually presents as scalp mass.9, 4, 14
Occasionally, they may present as intracranial space occupying lesion7 or cerebral hemorrhage.14 Age of presentation varies, but they usually present in the second or third decade, with equal sex distribution.23, 15 In 1942, Jaffe and Lichtenstein6 introduced the concept of aneurysmal bone cyst as a lesion with characteristic radiological appearance of ballooned out distension of the periosteum, usually outlined by a paper thin subperiosteal bone shell which is overlined by a region of disintegrated cortex. Trauma has been put forward as an important etiological factor.22 Edling8 regarded aneurysmal bone cyst as one of the manifestation of solitary dysfibroplasia of bone, suggesting a defect in development of the epiphyseal plate but it does not explain its occurrence in the mature bone. Lichtenstein18 suggested that it could result from local circulatory disturbance, because of sudden vascular occlusion of venous drainage of that segment of bone or development of an arterio-venous shunt. This results in progressive blood spaces in the medulla, which lead to gradual distension of the bone with atrophy. Jaffe11 reported that a pre-existing lesion of bone may initiate an osseous A-V fistula.
Aneurysmal bone cyst in coexistence with other lesions of bone have been reported18, 2, 20  and include unicameral bone cyst, non-ossifying fibroma, gaint cell tumor, chondroblastoma, fibrous dysplasia, osteofibrous dysplasia of campanacci, fibrous histiocytoma osteoblastoma and cartilaginous hematoma of chest wall of infants.
  CT scan is superior to plain radiology in defining extent and soft tissue extension of an aneurysmal bone cyst, particularly in the skull. Multiple small fluid levels are important characteristics of aneurysmal bone cyst on CT scan, which represents sedimentation of red blood cells within blood filled cavities.10 MRI also shows fluid levels, particularly in TlWI.12 Other findings include prolonged relaxation time, complete delineation of the margin of the lesion by a rim of low intensity signal and internal septation creating cystic cavities where wall contains diverticulum like projections.
Pathologically, these cysts contain multiple fluid filled cavities separated by multiple septa lined by multinucleated giant cells. These large spaces filled with blood do not have an endothelial lining, but are rather delimited by cells with the morphology, ultrastructural and immunohistochemical features of fibroblasts, myofibroblasts and histiocytes. Aneurysmal additional feature of a peculiar degenerated calcifying fibromyxoid tissue reported by Rosai.20 The risk of recurrence is increased with an increase of mitotic figures.
  The treatment option of choice for these lesions is total excision when possible, which is curative7, 14  but difficult to accomplish in cases where the skull base is involved.16 In these cases, partial resection and curettage of the inner contents may suffice to halt progression of the disease or even cause regression, but there may be recurrence. As these lesions are non-neoplastic and benign, the use of radiation therapy is not recommended, although it is reported in the literature.19 There is no role for chemotherapy.

Conclusion

The histopathology of this temporal region mass – aneurysmal bone cyst vs meningioma remains a diagnostic dilemma in the final diagnosis. But the patient has been managed as aneurysmal bone cyst of temporal bone. His postoperative recovery is uneventful without any recurrence in subsequent follow up.