Aneurysmal bone cyst (ABC) is an expansile, benign but locally aggressive bone lesion composed of blood-filled multilocular cystic cavities. It typically occurs in young patients aged 10-20 years and commonly involves the metaphysis of long bones, posterior spinal elements, and pelvic bones. The lesion can show rapid expansion, is surrounded by an eggshell-thin cortex, and contains pathognomonic fluid-fluid levels. In approximately 30% of cases, secondary ABC develops on pre-existing primary lesions such as giant cell tumor, osteoblastoma, chondroblastoma, or telangiectatic osteosarcoma. Treatment includes curettage with grafting, embolization, or medical therapy (denosumab). Malignant transformation does not occur but local recurrence rate is 20-30%.
Age Range
10-30
Peak Age
15
Gender
Female predominant
Prevalence
Uncommon
Aneurysmal bone cyst is considered a reactive vascular lesion resulting from local hemodynamic disturbance. In primary ABC, translocation in the USP6 (ubiquitin-specific protease 6) gene has been identified (most commonly t(16;17) — CDH11-USP6 fusion); this genetic alteration leads to matrix metalloproteinase (MMP) overexpression and vascular remodeling. Increased MMP activity accelerates bone destruction, while abnormal vascular channels create blood-filled cystic spaces. The fluid-fluid levels on MRI reflect gravity-dependent sedimentation of blood products within the cyst — serum above (T1/T2 low-intermediate signal), cellular components and hemoglobin degradation products below (variable signal on T1). Expansile growth thins the bone cortex, but the periosteum's capacity for new bone formation maintains a thin shell of bone.
Fluid-fluid levels resulting from gravitational sedimentation of blood products in multilocular cystic spaces are pathognomonic for aneurysmal bone cyst. The upper layer consists of serous plasma (high T2 signal), the lower layer of cellular components and hemoglobin degradation products (variable signal). Should be seen in at least two different compartments. Telangiectatic osteosarcoma can also show fluid-fluid levels but is distinguished by thick, irregular septations and solid enhancing component.
Expansile, multilocular lytic lesion surrounded by an eggshell-thin cortex. Thin septations divide the cyst cavity into multiple compartments. Internal density is consistent with hemorrhagic fluid, ranging from 20-70 HU — fresh blood shows higher density, old blood lower density. Fluid-fluid levels may be seen on CT but are more conspicuous on MRI. Matrix calcification is generally absent. Periosteal reaction may be of aggressive type (Codman triangle). In some cases, cortical destruction and soft tissue component may accompany.
Report Sentence
An expansile, multilocular lytic lesion is seen, surrounded by a thin bone shell; internal densities are consistent with hemorrhagic fluid with fluid-fluid levels identified.
On T1-weighted sequences, a multilocular cystic lesion is seen; signal in each compartment varies depending on the stage of blood products. Fresh blood (deoxyhemoglobin) shows low signal, subacute blood (methemoglobin) shows high signal. Septations appear as thin low-signal bands. Fluid-fluid levels are prominent, with the upper layer (serous plasma) showing low-intermediate signal and the lower layer (blood products) showing variable signal. Solid component is absent in primary ABC; if a prominent solid enhancing component is present, secondary ABC or telangiectatic osteosarcoma should be considered.
Report Sentence
On T1-weighted sequences, a multilocular cystic lesion is seen with variable signal intensities in compartments depending on the stage of blood products and prominent fluid-fluid levels.
On T2-weighted sequences, pathognomonic fluid-fluid levels are seen. In each cystic compartment, the upper layer shows high signal (serous fluid/plasma), and the lower layer shows variable signal (cellular components, hemoglobin degradation products). Fluid-fluid levels should be present in at least two different compartments. Septations appear as low-signal, thin, regular bands. Expansile growth is prominent, with the bone shell appearing as a thin low-signal line. Surrounding bone marrow and soft tissue edema may be seen. The thickness and regularity of septations and solid components are critically important in benign-malignant differentiation.
Report Sentence
On T2-weighted sequences, pathognomonic fluid-fluid levels are seen in multiple compartments, consistent with aneurysmal bone cyst.
On STIR, multilocular cystic spaces demonstrate markedly high signal, with increased lesion contrast due to suppression of surrounding bone marrow fat signal. Perilesional bone marrow edema may be seen as high signal. Fluid-fluid levels are also visible on STIR but may be less conspicuous than on T2 as some blood product signals may also be suppressed. Bone shell integrity and soft tissue extension are well evaluated on STIR. The true size of the entire lesion and surrounding tissue reaction are best seen on STIR.
Report Sentence
On STIR, the expansile multilocular cystic lesion demonstrates markedly high signal with accompanying perilesional bone marrow edema.
On post-gadolinium sequences, enhancement is seen in thin septations and the peripheral bone shell/periosteum. Cyst cavity contents do not enhance — they are avascular hemorrhagic fluid. Septal enhancement should be regular and thin. Thick, irregular, or nodular septal enhancement raises suspicion for secondary ABC or telangiectatic osteosarcoma. Solid enhancing component is not expected in primary ABC. Surrounding soft tissue reactive enhancement may be seen.
Report Sentence
On post-contrast sequences, enhancement is seen in thin septations and peripheral rim, while cyst cavity contents do not enhance; no solid nodular enhancement is identified.
On diffusion-weighted imaging, cystic components show no restricted diffusion; low signal on high b-value and high signal on ADC maps. Septations and peripheral rim do not show diffusion restriction. If a solid component shows diffusion restriction, telangiectatic osteosarcoma or the solid (primary lesion) component of secondary ABC should be considered. ADC values in cystic regions typically measure >1.5 × 10⁻³ mm²/s.
Report Sentence
On diffusion-weighted imaging, no restricted diffusion is identified in cystic components, and no solid component showing diffusion restriction is seen.
Criteria
ABC developing de novo without underlying primary bone lesion; USP6 gene translocation positive, constitutes ~70% of cases
Distinct Features
No solid enhancing component, all cystic spaces contain fluid-fluid levels. Thin regular septations, thin bone shell. Radiologically classic ABC appearance — multilocular, expansile, fluid-fluid levels. USP6 FISH test positivity confirms diagnosis.
Criteria
ABC developing on a pre-existing primary bone lesion (giant cell tumor, osteoblastoma, chondroblastoma, fibrous dysplasia, telangiectatic osteosarcoma); constitutes ~30% of cases
Distinct Features
Contains solid enhancing component (representing the primary lesion) in addition to cystic ABC component. The solid component may show typical radiologic features of the primary lesion. Careful evaluation on MRI is needed — solid component may show restricted diffusion on DWI. Biphasic appearance (cystic + solid) is the diagnostic clue of secondary ABC.
Criteria
Contains predominantly solid component with minimal or absent classic cystic spaces and fluid-fluid levels; constitutes <5% of all ABCs, shows ABC pathology on histology
Distinct Features
Radiologically may resemble an aggressive solid tumor — fluid-fluid levels may be minimal or absent. Shows enhancement, making differentiation from malignant tumors (especially osteosarcoma) difficult. Biopsy is usually necessary. USP6 FISH test is helpful in diagnosis. Clinical and radiological diagnosis is made only with histopathological correlation.
Criteria
ABC located in posterior vertebral elements (pedicle, lamina, spinous process); separately evaluated due to spinal cord compression risk and different surgical approach
Distinct Features
Expansile cystic lesion in posterior elements with fluid-fluid levels. Risk of spinal canal extension and neural compression — spinal cord and nerve root compression should be evaluated with MRI. May extend to vertebral body. Combined embolization + surgical approach is preferred. Requires urgent intervention in the presence of neurological deficit.
Distinguishing Feature
Simple bone cyst is unilocular with homogeneous fluid signal and typically does not contain fluid-fluid levels; ABC shows multilocular structure with multiple fluid-fluid levels and septations. Simple bone cyst is centrally located while ABC may be more eccentric.
Distinguishing Feature
Telangiectatic osteosarcoma can also show fluid-fluid levels but is distinguished by thick (>3mm), irregular septations, solid enhancing nodular component, and extraosseous soft tissue mass. Osteoid matrix production and aggressive periosteal reaction (Codman triangle, sunburst pattern) are more prominent in osteosarcoma.
Distinguishing Feature
Giant cell tumor (GCT) is located in the epiphysis (after growth plate closure), is an eccentric lytic lesion with prominent solid enhancing component; ABC is metaphyseal and predominantly cystic. Secondary ABC can develop in GCT — in this case a solid + cystic biphasic pattern is seen. Low signal areas on T2 (hemosiderin) and diffusion restriction (solid component) may be seen in GCT.
Distinguishing Feature
Osteoblastoma is a solid enhancing lesion showing matrix calcification or osteoid production; ABC is predominantly cystic without matrix calcification. Osteoblastoma can coexist with ABC in posterior spinal elements (secondary ABC). Osteoblastoma may show more sclerotic reaction and nidus-like central structure.
Urgency
moderateManagement
Curettage with bone grafting; preoperative embolization for large/vascular lesions; denosumab or doxycycline for recurrent/unresectable cases; serial MRI for follow-upBiopsy
NeededFollow-up
Post-treatment MRI every 3-6 months for first 2 years to assess recurrence; annual follow-up thereafterAneurysmal bone cyst is a benign but locally aggressive lesion requiring treatment. Standard treatment is curettage with bone grafting or cement application. Preoperative embolization reduces bleeding risk in large or vascular lesions. Recurrence rate is 20-30%, with incomplete curettage being the most important risk factor. Denosumab or sclerotherapy (doxycycline, polidocanol) may be applied for recurrent or surgically inaccessible lesions. Spinal ABC with neurological deficit constitutes an urgent surgical indication. Biopsy is generally recommended to exclude telangiectatic osteosarcoma and secondary ABC. Primary ABC does not undergo malignant transformation.
Aneurysmal bone cyst is a locally aggressive benign lesion. Recurrence rate is 15-20%. Treatment is curettage + adjuvant (PMMA/bone graft) or selective arterial embolization. In secondary ABC, the underlying lesion (GCT, osteoblastoma, chondroblastoma, telangiectatic OS) should be investigated.