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Aneurysmal Bone Cyst

Summary

Aneurysmal bone cyst (ABC) is a solitary, expansile and erosive lesion of bone. The cause of this non-neoplastic lesion is unknown.

Most patients are under 20, but the tumor can occur at any age.
The tumor presents with pain for several weeks with local swelling. Any bone may be involved, but the most common sites include the long bones and the posterior parts of the spinal vertebra.
On xrays, the lesion has a destructive, lytic, and expansile appearance.
Most ABC's are sucessfully treated with curettage and packing with bone chips or bone cement.
Complete Information on this Tumor
Introduction and Definition: 

This lesion is not a true neoplasm, but rather is thought to be a reactive lesion that may be caused by a local arteriovenous malformation or vascular injury. The true cause is unknown. There is a definite relationship to local trauma in some cases, and other cases are associated with another tumor such as osteoblastoma, chondroblastoma, fibrous dysplasia, or other. A large proportion of these lesion appear to arise de novo without any definite traumatic or neoplastic cause.

Incidence and Demographics: 
ABC is found most commonly during the second decade and the ratio of female to male is 2:1. ABC's can be found in any bone in the body. The most common location is the metaphysis of the lower extremity long bones, more so than the upper extremity. The vertebral bodies or arches of the spine also may be involved. Approximately one-half of lesions in flat bones occur in the pelvis.
Symptoms and Presentation: 

Gradually increasing pain, a mass, or with a pathological fracture through the lesion. Rapid increase is lesion size has been reported in a few cases.

X-Ray Appearance and Advanced Imaging Findings: 
On plain film, an ABC is normally placed eccentrically in the metaphysis and appears osteolytic. The periosteum is elevated and the cortex is eroded to a thin margin.The expansile nature of the lesion is often reflected by a "blow-out" or "soap bubble" appearance. CT scan can also help delineate lesions in the pelvis or spine where plain film imaging may be inadequate. CT scan and MRI can narrow the differential diagnosis of ABC by demonstrating multiple fluid-fluid levels within the cystic spaces. MRI is useful to confirm the multiple fluid-fluid levels and the non-homogeneity of the lesion. ABC appears on both T1 and T2 MRI with a low signal rim encircling the cystic lesion. A careful search for radiological signs of the precursor lesion, if any, is recommended. Some of these precursor lesions may have a flocculent chondroid matrix that may be a clue to their pathogenesis.
Laboratory Findings: 
No relevant findings
Differential Diagnosis: 
Giant cell tumor, UBC, Telangectatic osteosarcoma
Preferred Biopsy Technique for this Tumor: 
Incisional / combined with curettage if certainty of diagnosis is high
Histopathology findings: 
On gross examination, an ABC is like a blood filled sponge with a thin periosteal membrane. Soft, fibrous walls separate spaces filled with friable blood clot. Microscopically, the ABC has cystic spaces filled with blood. The fibrous septa have immature woven bone trabeculae as well as I macrophages filled with hemosiderin, fibroblasts, capillaries and giant cells. The treatment approach will vary depending of the location and aggressiveness of the lesion. A slow growing, indolent ABC has been observed to regress spontaneously. Selective embolectomy of nutrient vessels and percutaneous injection of a fibrosing agent are newer treatment modalities. Percutaneous injection of methylmethacrylate was used successfully by Herve Deramond for an aggressive ABC lesion in the second cervical vertebra.
Treatment Options for this Tumor: 
Curettage is normally used. Large lesions may require other treatments, such as embolization. The cyst can be packed with bone chips or polymethylmethacrylate cement. Bone fragility must be addressed with plates, screws, or rods as indicated. Percutaneous transvascular treatments have be used with good results, and are especially useful in difficult to access lesions of the spine and skull base.
Preferred Margin for this Tumor: 
Intralesional
Outcomes of Treatment and Prognosis: 
Recurrence is seen in approximately 20% of cases and is more common in younger children.
Special and Unusual Features: 
One theory of the etiology of primary ABCs is that these lesions are secondary to increased venous pressure that leads to hemorrhage which causes osteolysis. This osteolysis can in turn promote more hemorrhage causing amplification of the cyst. Another theory is that these lesions do not arise de novo by rather develop secondarily within another primary tumor such as osteoblastoma, and subsequently enlarge and destroy all or most of the primary tumor.
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