Aneurysmal bone cyst
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Aneurysmal bone cysts (ABC) are benign expansile osteoclastic giant cell-rich bony neoplasms, composed of numerous blood-filled channels and cystic spaces 1.
ABC accounts for the 'A' in the popular mnemonic for lucent bone lesions FEGNOMASHIC.
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The terms 'giant cell reparative granuloma of small bone' 2,3 or 'giant cell lesion of small bone' have been discouraged 1.
Aneurysmal bone cysts are rare. They are mostly seen in children and adolescents, with ~80% under the age of 20 years 2,3 but can occur at any age 1. Both genders are equally affected 1.
The definitive diagnosis of aneurysmal bone cysts is based on a combination of typical radiological and pathological features.
Diagnostic criteria according to the WHO classification of soft tissue and bone tumors (5th edition) 1:
a multicystic bone lesion with fluid-fluid levels on imaging
histological evidence that cyst walls are composed of fibroblasts, osteoclastic giant cells, and hemosiderin pigment as well as proof of new bone formation
The following molecular criterion is desirable:
USP6 gene (at 17p13.2 locus) rearrangement; occurs in 63% of cases
Aneurysmal bone cysts commonly present with pain and swelling. On rare occasions, this is the result of a pathologic fracture. If the spine is affected, they may present with symptoms related to nerve root compression 1,3.
subarticular zone stenosis with nerve root compression
Aneurysmal bone cysts consist of multiloculated blood-filled spaces of variable size separated by fibrous septa, surrounded by a thin reactive bone formation rich in multinucleated osteoclast-like giant cells 1.
They are typically eccentrically located in the metaphysis of long bones 1, adjacent to an unfused growth plate. Although they have been described in most bones, the most common locations are 3-5:
long bones (~50-65%):
typically eccentrically located in the metaphysis
especially femur, proximal tibia and fibula, and humerus
spine and pelvis (~20-30%):
especially posterior elements of the spine with extension into the vertebral body in 40% of cases 5
obturator foramen in pelvic location
short bones of hands and feet: more often with a central location
craniofacial: jaw, basisphenoid, and paranasal sinuses
epiphysis, epiphyseal equivalent, or apophysis: rare but important
Grossly aneurysmal bone cysts are well-defined multiloculated blood-filled cystic lesions with sponge-like septae and a peripheral component that is surrounded by a reactive thin bony shell 1.
Histologically aneurysmal bone cysts are characterized by the following 1,6:
blood-filled cystic spaces separated by septa containing woven bone, bland fibroblasts, and multinucleated osteoclastic giant cells
the woven bone follows the border of the fibrous septa, bordered by osteoblasts
The previously termed 'giant lesion of small bones' features the same morphological features as the solid subtype of aneurysmal bone cyst 1.
Aneurysmal bone cysts do not express H3.3pGly34Trp, a feature that can be used to differentiate them from giant cell tumors of bone with aneurysmal bone cyst-like changes 1.
Aneurysmal bone cysts display cytogenetic rearrangements of the USP6 gene. These rearrangements also occur in the aneurysmal bone cysts of the hand and feet but not in lesions of the jawbones 1.
Radiographs demonstrate a sharply defined, expansile solitary lucent bone lesion, with thin-walled cavities 3.
On CT aneurysmal bone cysts are characterized as lucent bone lesions with a mean density higher than fat 7. It might show concerning features such as cortical breach or soft tissue extension 7,8.
Additionally, CT can demonstrate fluid-fluid levels, which are harder to appreciate than on MRI and require viewing with a narrow window width 8.
MRI can demonstrate the characteristic fluid-fluid levels exquisitely, as well as identify the presence of a solid component and concerning features suggesting an aneurysmal bone cyst-like appearance of another tumor entity.
The cysts are of a variable signal, with a surrounding rim of low T1 and T2 signals. Focal areas of high T1 and T2 signal 4 are also seen, presumably representing areas of blood of variable age (see aging blood on MRI).
T1 (C+): septations may enhance 9
It is important to remember that the presence of fluid-fluid levels, although characteristic of aneurysmal bone cysts, is by no means pathognomonic, and is seen in other lesions as well, both benign and malignant (e.g. giant cell tumors (GCT), chondroblastoma, simple bone cysts and telangiectatic osteosarcomas).
Aneurysmal bone cysts are poorly vascular 10.
Doughnut sign: increased uptake peripherally with a photopenic center.
The radiological report should include a description of the following 7:
imaging characteristics e.g. solitary lucent bone lesion, high T1 or low T1 bone lesion
location within the bone (eccentric, central)
soft tissue extension
aggressive periosteal reaction
surrounding bone marrow edema
solid mass-like enhancement
The lesion can be categorized according to the bone reporting and data system as Bone-RADS 4 unless histology has been already obtained 7.
Treatment and prognosis
Although they are benign, aneurysmal bone cysts can display different clinical natural courses: quiescent, active or aggressive. Thus patients should be referred to an orthopedic oncologist 7. They have been traditionally treated operatively with intralesional curettage or excision or complete en bloc excision with bone grafting are options 3.
Depending on the type of surgery. The recurrence rate of 15-30% has been described 3. Percutaneous treatment with fibrosing agents has also been performed, either in isolation or as a precursor to surgical excision 3,11,12. Embolization is another option 3.
Spontaneous regression may occur rarely or also following partial removal 3,13.
Malignant transformation has been only observed after irradiation 3.
History and etymology
Aneurysmal bone cysts have been first described by the American bone pathologist Louis Lichtenstein in 1950 14.
The differential diagnosis depends on the modality.
On plain radiography (and to a lesser degree, CT), the differential diagnosis includes most of the lesions included in the mnemonic FEGNOMASHIC. Compared to the other lesions in this list, aneurysmal bone cysts are markedly expansile (hence, "aneurysmal") and have a thin cortical shell.
On MRI, the differential is much shorter, especially when age, location and plain film appearance are taken into account. The main differential includes both lesions with intrinsic fluid-fluid levels (see fluid-fluid level containing bone lesions) and those from which an aneurysmal bone cyst may arise:
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