Teaching Case

A 24-year-old female patient presented to the clinic with the complaint of mild pain on the left jaw. An extraoral examination revealed a swelling of size 1.7x1.3cm around the body left mandible posterior-inferior aspect which is firm, non-tender, and bony hard.

A non-enhanced Computed Tomography (CT) and Magnetic Resonance (MRI) of facial bones were performed.

MRI Turbo Spin Echo images shows a heterogenous multiloculated cystic lesion involving the body of left mandible extending from the left second premolar to molar. The lesion is multilocular with fluid-filled cavities appearing iso to hypointense on the T1 weighted image (Figure 1A) and heterointense on T2 weighted imaging (Figure 1B) with characteristic fluid-fluid levels (blue arrow) indicating the various stages of blood within the cystic lesion.  No evidence of intralesional or extra lesional soft tissue component noted ruling out the possibility of secondary aneurysmal bone cyst (ABC).

CT shows sharply defined oval-shaped expansile multiloculated lytic lesion involving the left body of the mandible with thinned out inner cortex which is discontinuous at places and thin internal septations (Figures 2A, 2B, 2C), which on soft tissue window shows multiple fluid levels (Figures 3A, 3B, 3C) with no evidence of internal solid component or matrix calcification noted within.

An aneurysmal bone cyst is a rare benign osteolytic bone neoplasm of unknown origin, characterized by several sponge-like blood- or serum-filled, generally non-endothelial spaces of various diameters that may contain osteoid tissue and osteoclast-like giant cells [1]. ABC can be primary and secondary, with the later showing the imaging characteristics of the accompanying. ABCs are uncommon in the craniofacial skeleton unlike the long bones and spine; only 2% involvement of mandible with the body (90%) and mandibular (10%) ramus being the main locations [2].

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 [3].

Imaging plays a crucial role in diagnosing and further characterizing of ABC. Radiographs demonstrate sharply defined, expansile osteolytic lesions, with thin sclerotic margins.  CT is better at assessing cortical breach, matrix calcification and extension into soft tissues.  MRI can demonstrate the characteristic fluid-fluid levels exquisitely as well as identify the presence of a solid component suggesting that the aneurysmal bone cyst is secondary.  The cysts are of a variable signal, with a surrounding rim of low T1 and T2 signal.

Focal areas of high T1 and T2 signal [4] are also seen presumably representing areas of the blood of variable age. Malignant transformation of primary ABC is rare although the diagnostic possibility of a missed primary lesion with secondary ABC should be considered. The presence of extra lesional soft tissue components, marked bone erosion, matrix calcification, enhancing septa on post-contrast imaging should raise the suspicion of secondary ABC (6).

Primary ABCs usually show a favourable outcome with an overall cure rate of 90%-95% (5).  Treatment of Primary ABC is usually aimed at the complete excision of the lesion.  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% (7).

Teaching Points

Primary ABC of the mandible is a rare entity; however, it is a component of the varied diagnostic spectrum of cystic lesions of the mandible. Presence of extra lesional soft tissue component marked bone erosion with extension to adjacent soft tissue, presence of osteoid/chondroid matrix, enhancing solid components among fluid-fluid levels with a wide zone of transition & enhancing septa on post-contrast imaging should raise the suspicion of secondary ABC rather than the primary ABC.