ANEURYSMAL BONE CYST

DR. MAHESH CHAUDHARYRESIDENT(PHASE-A, MARCH 2014)DEPARTMENT OF RADIOLOGY & IMAGINGB.S.M. MEDICAL UNIVERSITY, DHAKA

Definition

• An aneurysmal bone cyst is a benign, expansile, osteolytic lesion with a thin wall, containing blood-filled cystic cavities.

• The term aneurysmal is derived from its macroscopic appearance – sponge like tumour containing numerous giant cells.

Etiology

The true etiology of ABCs is unknown.

It’s believe that ABCs are the result of a vascular malformation within the bone. Three commonly proposed theories are as follows:

i. ABCs may arise without evidence of another lesion are classified as primary ABCs (65-99%)

ii. ABCs may be caused by a reaction secondary to another bony lesion(1-35%).

iii. ABCs may arise in an area of previous trauma.

Incidence and Demographics

ABC is found at any age Around 75%: before 20 years and rare after 30 years Female: Male = 1.5:1.

Site - can be found in any bone in the body. The most common location is the metaphysis of long bones of lower extremity, more so than upper extremity.

The vertebral bodies or arches of the spine also may be involved, half of the cases involving more than one vertebra.

Approximately one-half of lesions in flat bones occur in the pelvis.

Presentation

Patients usually present with pain, a mass, swelling, a pathologic fracture, or combination of these symptoms in the affected area.

Neurologic symptoms may develop when involving the nerve, typically In the spine.

Other findings may include the following:• Deformity• Decreased movement, weakness, or stiffness• Occasionally, bruit over the affected area• Warmth over the affected area

Histology

Aneurysmal bone cyst consists of blood-filled spaces of variable size that are separated by connective tissue containing trabeculae of bone or osteoid tissue and osteoclast giant cells.

They are not lined by endothelium.

Development of ABC

It follows 3 stages-

Stage Description

I Initial phase Osteolysis without peculiar findings.

II Growth phase •Rapid increase in size of osseous erosion.•Enlargement of involved bone.•Formation of shell around central part of lesion.

III Stabilization phase Fully developed radiological pattern.

Imaging Modalities

1. X - ray

2. CT scan

3. MRI

4. Nuclear Imaging

5. Angiography

X-Ray Appearance

• ABC is normally placed in the metaphysis and appears as a osteolytic lesion. 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.

• The lesion rarely penetrates the articular surface or growth plate.

• The accuracy of x-ray is high, especially when lesions in the appendicular skeleton.

ABC: Antero posterior and lateral x-ray of left knee demonstrates an expansile, lucency in metaphysis. No fracture. The lesion does not transgress the growth plate.

X-ray of tibia and fibula demonstrates an expansile lucent lesion in the proximal tibia with evidence of a pathological fracture(arrows).

A B

Computed Tomography• Cross-sectional CT is the most useful imaging examination,

because it can demonstrate the intraosseous and extraosseous extents of the lesion.

• CT can be used to determine the nature of the matrix of the tumor, especially when tumors are in complex locations, such as the facial skeleton, spine, thoracic cage, and pelvis.

• Spinal CT can demonstrate stenosis of the spinal canal due to involvement of the posterior elements.

CT of aneurysmal bone cyst. Lateral (A) and oblique (B) x-ray of the right ankle of a 24-year-old woman show a radiolucent, trabeculated lesion in the talus. (C) Coronal anterior and (D) coronal posterior CT sections demonstrate the internal ridges of the lesion.

CT scan of aneurysmal bone cyst arising from lamina and internal mass of C6 resulting in a unilateral dislocation of the facet joints in a 10 year old girl.

Magnetic Resonance Imaging• T1WI - show predominantly low to intermediate signal

intensity with or without fluid levels. Acute hemorrhage into the cyst may have high signal intensity.

• T2WI - show areas of low to intermediate signal intensity or some areas of heterogeneous high signal intensity, depending on the contents of the cyst. A rim of low signal intensity with internal septa may produce a multisystem appearance.

• MRI images of aggressive lesions show tumor enhancement with gadolinium enhancement, especially when they are associated with other tumors.

• Spinal cord compression and signal-intensity alteration in the cord can be evaluated when neurologic symptoms are present.

• Fluid-fluid levels may be seen in the cysts. • To demonstrate this phenomenon, the patient must

remain motionless for at least 10 minutes before scanning.

MRI of aneurysmal bone cyst. Anteroposterior (A) and lateral (B) x-ray of the midshaft of right femur of a 15yr/F which show an expansive lesion arising eccentrically from the medial aspect of the bone. A thin shell of periosteal bone covering the lesion (arrows) and a buttress of periosteal reaction at its proximal and distal extent (open arrows), characteristic for ABC.

(C,D) Coronal T1WI demonstrate heterogeneity of the lesion and internal septations. Axial T1-weighted (E) and T2-weighted (F) images show fluid-fluid levels (arrowheads).

Axial T2(fat suppressed) MR image show multiple fluid-fluid levels within the expansile bone lesion in the proximal tibia.

Fluid - fluid level (non specific)FFL results from separation of 2 fluids of different densities within a cavernous space

Mnemonic• G: giant cell tumour• O: osteoblastoma• A: aneurysmal bone cyst• T: telangiectatic osteosarcoma• S: sarcomas

• C: chondroblastoma• S: solitary bone cyst• F: fibroxanthoma

(A) Coronal T1WI shows the lesion having low T1 Signal with the septations (only just visible).

(B) Post contrast T1WI shows enhancement of the septa which can be seen in ABCs

A B

T1

T1 C+

Nuclear Imaging

• Demonstration of a solitary lesion on bone scintigraphy helps distinguish an aneurysmal bone cyst from other bone tumors, a hemophilic pseudo tumor, etc.

• Radioisotope uptake is increased.

• The common pattern is the accumulation of the tracer in the periphery of the lesion, with little intensity in the center; this finding is present in about 65% of cases.

Increased radioisotope uptake peripherally with a photopenic center."doughnut sign"

Angiography

• On angiograms, ABCs are hypervascular lesions .

• This feature is contrary to that of other malignant lesions, such as osteosarcoma and chondrosarcoma, which have gross hyper vascularity.

• Hyper vascular regions in aneurysmal bone cysts may affect the prognosis, because the number and size of the lesions are positively correlated with the likelihood of lesional recurrence after treatment.

Angiography examination of ABC of a 13-year-old male showed an expansile lesion involving the left inferior pubic ramus and ischium.

Differential Diagnosis• Simple bone cyst - central location, before epiphyseal fusion - absence of expansion - lack of cortical discontinuity • Giant cell tumor of bone - occurs in patients over age 20 -40 year - expansile, eccentric, wide zone of transition - begin in epiphysis with extension into metaphysis - involves joints or adjacent bone or soft tissue• Osteoblastoma - may have a “soap bubble” expansile appearance - no fluid level on CT/MR

• Fibrous dysplasia - ground-glass opacities: 56% - homogeneously sclerotic: 23% - well circumscribed lesions - no periosteal reaction• Chondroblastoma - arising eccentrically in the epiphysis of long bone - internal calcifications can be seen in 40-60% cases - Size ranges from 1-10 cm, most are 3-4 cm at diagnosis• Chondromyxoid fibroma• - knee joint (2/3) - well defined radiolucent, eccentric in metaphysis - marginal sclerosis

Treatment

• Preoperative embolization

• Curettage & bone grafting

• Complete resection with bone graft

• Radiotherapy

Complication

• Pathological fracture

• Neurologic symptoms

• Mechanical disruption

Prognosis

Recurrence rate 20-30%

References:

David Sutton: Text book of Radiology & Imaging (7th Ed)

J. Maheshwori, Text book of Orthopedics (3rd Ed) Haaga: CT & MRI of whole body (5th Ed)

Radiopedia.org LearningRadiology.com

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