Image Quiz

Abdominal Pain in a Nine-Year-Old Boy (continued)

Answer: Aneurysmal bone cyst of the left pedicle and lamina at L1, extending to the right lamina and vertebral body.

Radiographic Interpretation

The plain anteroposterior radiograph of the lower abdomen (Fig. 1) demonstrates an expansile, radiolucent eccentric lesion of the L1 vertebra. The left lumbar pedicle appears to be missing (the "winking owl" sign). The transverse T2-weighted (Fig. 2, left image) and coronal T2-weighted (Fig. 3) magnetic resonance images demonstrate a large expansile lesion involving the lamina, pedicle, and posterolateral aspect of the vertebral body of L1. The lesion is multiloculated and cystic, and septations and fluid/fluid levels are present. The space available for the cord at this level is noticeably reduced. The transverse computed tomographic image (Fig. 2, right image) demonstrates an expansile eccentric lesion with thin cortical margins and a narrow zone of transition. The sagittal computed tomographic reconstruction of L1 (Fig. 4) shows a cystic lesion of the posterior elements with involvement of the posterior aspect of the vertebral body. No soft-tissue mass was seen on any images.


Fig. 1
Fig. 2
Fig. 3
For larger view, click on image

Fig. 4
Fig. 5
Fig. 6

Histologic Interpretation

Both low-powered (Fig. 5) and high-powered (Fig. 6) photomicrographs of the lesion demonstrate blood-filled spaces (arrows) surrounded by several focal giant cells as well as spindle-shaped cells lining the walls of these spaces. Some of the spaces are slit-like, whereas others are dilated. Hemosiderin is also seen on high-power views.

Treatment

After an open incisional biopsy was performed along with intraoperative frozen-section analysis, which was consistent with a diagnosis of aneurysmal bone cyst, a wider exposure was carried out and intralesional curettage was performed. Loupes and headlamp were used to improve visualization. Lesional tissue was removed and the curettage was extended into the normal remaining vertebral body with use of a high-speed diamond burr, followed by bone-grafting with use of a combination of local autograft, demineralized bone matrix, and cancellous allograft cubes. Bovie cauterization was performed along with application of topical phenol, with care taken to avoid the dura mater and/or any other neurovascular structure. Following the resection, the stability of the spine was assessed. Because of the large size of the lesion, limited titanium instrumentation was performed (Fig. 7). A postoperative thoracolumbar orthosis was prescribed for comfort.


Fig. 7		 Fig. 7 Postoperative anteroposterior (left) and lateral (right) radiographs, showing the instrumentation between T12 and L2.

For larger view, click on image

The patient's postoperative course was uneventful; he started walking on the second postoperative day and was discharged on the fourth day. At the time of the one-year follow-up, he was pain-free and had resumed full activities with no signs of recurrence. An extended follow-up will be necessary to assess the possibility of recurrence.

Discussion

Aneurysmal bone cyst represents between 1% and 6% of all primary bone tumors1,2. This benign but locally aggressive lesion is usually characterized as a highly vascular space-consuming lesion consisting of widely dilated vascular channels that are not lined by an identifiable endothelium3.

The origin of this lesion is still unknown, although up to approximately 20% of aneurysmal bone cysts are reported to occur in the presence of other lesions, such as fibrous dysplasia, osteoblastoma, osteosarcoma, giant cell tumor, and chondroblastoma2-6. Although aneurysmal bone cysts can be observed in any age group, 75% of affected patients are in the second decade of life2,7.

Aneurysmal bone cysts can be encountered in almost any bone. For example, in Dahlin's series of 289 primary aneurysmal bone cysts, the spine was affected in 14% of patients7. An aneurysmal bone cyst almost always involves the posterior elements; however, it can expand anteriorly into the vertebral body. At times, an aneurysmal bone cyst will span two or three adjacent vertebrae, although it has not been observed to violate the intervertebral disc8,9. In another large study of aneurysmal bone cyst in 156 patients with an average of 9.4 years of age (range, 1.5 to sixteen years of age), the spine was involved in thirteen patients (8.5%). The thoracic spine was the most frequent location2, but in some other series, involvement of the lumbar spine was slightly more prevalent8,10,11.

According to the criteria of Enneking12, an aneurysmal bone cyst usually behaves as an active benign (stage-2) or aggressive (stage-3) lesion, which can be a fast-growing lesion with a great potential for morbidity.

A patient with an aneurysmal bone cyst may be asymptomatic and the lesion may be found only incidentally2,5. Often, the usual symptom of aneurysmal bone cyst of the thoracic or lumbar spine is persistent and progressive back pain. Pathologic fractures are infrequent (occurring in 10% of patients). Neurologic symptoms are rare but are often seen in patients with an associated microfracture8,9.

Our patient presented with abdominal pain, and the lesion was an incidental finding on the plain radiograph. The size of the lesion and the potential risk for progression and fracture, which would thus lead to deformity and neurologic compromise, were indications for surgical treatment.

Imaging studies are often the means of diagnosis of aneurysmal bone cyst. On plain radiographs, expansion of the affected bone is evident. The lesion is often eccentric and septated and there is aggressive lytic destruction of the bone. The cortex is usually expanded and thin, with a narrow zone of transition3. Magnetic resonance images show a multiloculated expansile lesion with multiple characteristic, double-density fluid-fluid levels, septations, low signal on T1-weighted images and high signal on T2-weighted images13, and no soft-tissue mass. The fluid-fluid levels represent the settling of red blood cells within the cavity of the lesion, and while this finding is suggestive of aneurysmal bone cyst, it is not diagnostic. The fluid-fluid levels are not seen if the computed tomographic or magnetic imaging scans are performed too quickly for the red blood cells to settle. Magnetic resonance images also help in differential diagnosis, especially in the differentiation of aneurysmal bone cyst from unicameral bone cyst.

Several methods have been described for the treatment of aneurysmal bone cyst of the spine. Most lesions can be adequately treated with extended intralesional curettage and bone grafting. Selective arterial embolization can be used as primary treatment for these lesions and is sometimes useful to reduce the possibility of intraoperative bleeding3.

After treatment, local persistence of aneurysmal bone cyst occurs in 10% to 50% of patients 8,9,14,15. The rate is even higher in children who are younger than ten years old16 and is likely due to inadequate tumor removal. Radiotherapy, which was used in the past, is no longer indicated because of the risk of long-term complications, such as growth plate arrest and secondary, radiation-induced neoplasm8.

The authors' preferred method of treatment for aneurysmal bone cyst of the spine is a four-step surgical technique, as previously described14,17. Instrumentation and fusion are sometimes needed to prevent or correct coexisting instability. If instrumentation is needed, we advise the use of titanium hardware because it allows postoperative magnetic resonance imaging and thus early detection of recurrence15.

*The authors did not receive grants or outside funding in support of their research for or preparation of this manuscript. They did not receive payments or other benefits or a commitment or agreement to provide such benefits from a commercial entity. No commercial entity paid or directed, or agreed to pay or direct, any benefits to any research fund, foundation, educational institution, or other charitable or nonprofit organization with which the authors are affiliated or associated.

References

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