Discussion
Aneurysmal bone cysts (ABC) are benign, non-neoplastic, osteolytic, and expansile lesions of the bone. The most common locations for ABCs are the metaphyses of long bones, vertebrae, and flat bones. They are usually detected in patients in the first two to three decades of their lives. ABC of the skull is an unusal entity, and the reported incidence is around 2.5 to 6% of the cases. ABC’s are very rarely found in the temporal bone, there are 17 cases with histological confirmation reported in the literature.
Most experiences with ABC’s have been obtained from the lesions located in the long bones and vertebrae. They may be primary or secondary. Although some theories have been proposed, the exact cause of primary ABC's is not yet clear. The development of focal dynamic alterations with secondary venous hypertension may cause a slow expansion of the cortex according to Lichtenstein. In the long bones nearly one third of the lesions are considered secondary to the pre-existing bone lesions, recently a similar association has been observed in ABC’s of the skull as well. Secondary ABC's arise from pre-existing bone lesions such as giant cell tumor, osteoblastoma, chondroblastoma, non-ossifying fibroma, angioma, fibrous dysplasia, unicameral bone cyst, and chondromyxoid fibroma.
Clinical findings of temporal ABC’s are related to the presence and location of the mass lesion. The most common mode of presentation for temporal ABC’s is the swelling around the temporal region that may or may not be associated with pain. Other presenting symptoms are decreased hearing, 5th and 7th cranial nerve paralyses, and headache.
The ideal treatment is total resection of the lesion, and where needed, repair of the bony defect in an appropriate manner. Preoperative angiography and embolization may make surgery easier in selected cases since ABC’s are highly vascular lesions and direct surgery can cause significant blood loss. Lesions involving the base of skull present some problems in which thorough curettage and partial excision is the treatment of choice. There is high recurrence rate even after thorough curettage.
Gross pathological studies reveal that ABC’s are composed of multiple sinusoidal spaces filled with unclotted blood and blood-tinged serous fluid. These spaces are separated by fibrous or bony septa, giving a honeycomb appearance. Since the lesion originates within the diploic space of the bone, enlargement causes expansion of the diploic space, and the lesion is then surrounded by a thin shell of outer and inner skull tables. Histologically ABC’s are composed of honey-comb-like spaces without endothelial linings, and are filled with hemorrhagic fluid. The cysts are separated by septa composed of proliferated spindle-shaped fibroblasts with scattered multinucleated giant cells, hemosiderin-laden macrophages, granulation tissue, and extravasated red blood cells. The septa also usually contain trabeculae of new bone formation.
MRI studies of ABC’s reveal the characteristic appearance of the lesions. They are well-delineated expansile lesions, surrounded by a fibrous capsule with hypointense signal intensity on all sequences. The lesions may have internal septations with hypointense signal intensity, separating multiple cystic components of the lesion. The cystic components are of heterogeneous signal intensities on all sequences, representing different stages of evolution of blood by-products. The fluid-fluid levels within the cystic components are secondary to the layering of the uncoagulated blood, although this appearance is characteristic for ABC’s, it is not specific, and may appear in other lytic bony lesions such as osteosarcoma, chondroblastoma, giant cell tumor, malignant fibrous histiocytoma. MRI studies may also visualize any occupation of the epidural space and brain compression in cases with an intracranial extension of the mass. Although they may expand intracranially in the orbital locations, they usually do not penetrate the dura. Following intravenous gadolinium injection the cystic lesions reveal intense contrast enhancement at the peripheral capsule and internal septations.
The diagnosis of aneurysmal bone cyst can be strongly suspected by correlating the radiographic and magnetic resonance imaging findings. However for definitive diagnosis, accurate histopathological evaluation is imperative to rule out many lesions simulating ABC's such as giant cell tumors, hemorrhagic cyst, enchondroma, metastases from renal cell and thyroid carcinoma, plasmacytoma, chondrosarcoma, fibrosarcoma, fibrous dysplasia, hemophilic pseudotumor, telangiectatic osteosarcoma.
