Aneurysmal Bone Cyst
The Aneurysmal Bone Cyst (ABC) is a benign tumor of bone found most
commonly in female patients between 10
and 20 years of age. It has a predilection for the femur; though,
also commonly found in the tibia, spine and pelvis. Two-thirds of spinal lesions will present in the posterior elements; a distinction it
shares with osteoid osteoma, Giant Cell Tumor (GCT), and osteoblastoma.
Radiographically, it initially appears
as an aggressive lytic lesion with
permeative cortical destruction. It
will progress to form a large aneurysmal bulge with a reactive cortical rim
(Figure #3). Painful stress fractures
in the thinned cortical bone are possible and usually account for the
symptomatic presentation of these otherwise clinically silent lesions. Grossly they are filled with aged blood that
has been termed to have the appearance of “crank-case
oil.” Histologically, numerous blood lakes are the hallmark separated
by fibrous septae (Figure #4). Giant
cells are frequently encountered. The
etiology is unknown but most think trauma is the likely inciting event with
subsequent hemorrhage creating a hydraulic
dynamic effect which progresses into an expanding hemorrhagic lesion. The natural progression of this lesion is
spontaneous evolution, often preceded by trauma to the region (such as
fracture). An involuted ABC will leave
a thick rim of cortical bone easily identified radiographically.
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Figure #3
ABC of the proximal
tibia.
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Figure #4
Low-power micrograph
of an ABC. Note the multiple
“blood lakes” |
Therapeutic modalities are aimed at
hastening the involution process. This
most commonly includes intralesional
curettage with bone grafting.
Large, expanding lesions can often be reigned-in by embolization. Radiation therapy has been successfully used
in the past but is no longer considered an option due to long-term carcinogenic
risk. Prevention of pathologic fracture
is imperative as the defects often leave little structural bone available for
repair. This is particularly crucial in
the peritrochanteric region and a course of limited weight bearing may be required.
The differential diagnosis of ABC
includes: GCT, Solitary Bone Cyst
(SBC), Ewing’s Sarcoma, telangectatic
osteosarcoma, and chondromyxoid fibroma.
Differentiating factors with GCT include the fact that GCT rarely
present under the age of 20 and have twice the frequency of ABCs. In the spine, GCT often arise in the
vertebral body; whereas, ABCs are more often in the posterior elements. ABCs can be nearly indistinguishable from
other lesions, which necessitates timely
biopsy of multiple regions to ensure a more malignant lesion is not the
causative agent.
