A 22-year-old female patient presented with a selling on the right side of the chin which is gradually increasing in size. She did not have any previous history of trauma. On examination, facial asymmetry was present with a firm, non-tender swelling involving the right side of the lower jaw. On aspiration, blood-tinged fluid was obtained.
CT was done which showed a well-defined, oval, expansile lytic lesion involving the left angle of mandible on the bone window. The cortex was thinned out with multifocal discontinuity and internal septations. The lesion showed narrow zone of transition without any periosteal reaction (Figure1). On soft tissue window, the lesion showed multiple internal septations and blood-fluid levels (Figure 2).
An aneurysmal bone cyst is characterized by blood-filled, non-endothelial spaces that may contain osteoid tissue and osteoclast-like giant cells . ABCs in the craniofacial skeleton are uncommon unlike the long bones and spine. Mandible is involved in only 2% of the cases, body (90%) and mandibular ramus(10%) being the main locations . ABC can be primary or secondary. All the cases of primary ABC exhibit a thinning of the cortex and an expansion of the lesion. Secondary ABCs can be morphologic mimics of a primary ABC and arise secondary to a pre-existing bone lesion, which can be of malignant nature.
Clinical presentation of the ABC varies from a small, indolent, asymptomatic lesion to rapidly growing, expansile, a destructive lesion causing pain, swelling, deformity, neurologic symptoms, pathologic fracture .
Radiographs demonstrate sharply defined, expansile osteolytic lesions, with or without thin sclerotic margins which is known as the soap-bubble appearance. CT helps in assessing cortical breach, matrix calcification. Internal content, septations and fluid-fluid level can also be seen within the lesion. The primary variant shows imaging features same as the previously mentioned features of ABC while the secondary variant may show features of the accompanying lesion. T2-weighted MR images show multiple cystic lesions that are divided by a thin septum, and fluid-filled cysts that indicated collection of blood. Malignant transformation of ABC is rare, but can raise a discussion about a missed primary lesion with secondary ABC. The red flags for secondary ABCs include presence of numerous blood-filled cavities divided by septa, cortical destruction and the presence of a soft tissue mass and thick septal enhancement, as observed using MRI. Thus, contrast-enhanced MRI is important in differentiating primary and secondary ABC.
Primary ABC shows an overall cure rate of 90%-95% . Complete excision of lesion is the main aim of treatment. The treatment modalities are percutaneous sclerotherapy, diagnostic and therapeutic embolization, curettage, block resection & reconstruction, radiotherapy and systemic calcitonin therapy. The recurrence rate of aneurysmal bone cysts is up to 20% . In our case curettage of the lesion was done and on histopatholgical examination, endothelium-lined vascular spaces with abundant pools of RBC were seen. Giant cells with hemosiderin pigments was also seen which is characteristic of ABC.
Take Home Message / Teaching Points
ABC is a rare entity among the lesions involving the mandible and because of its characteristic appearance can be diagnosed promptly on imaging. Presence of extra-lesional soft tissue component, presence of osteoid/chondroid matrix, enhancing solid components among fluid-fluid levels raise the suspicion of secondary ABC rather than the primary ABC.
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