Osteoid Osteoma

Osteoid osteomas are benign tumors of bone characterized by a small nidus of calcified osteoid tissue with a vascular connective tissue matrix.^18–24 The osteoid tissue and trabeculae stimulate bone formation, which results in the surrounding dense sclerotic margin. Because the lesion is typically more vascular than the adjacent normal bone, one can often see an increased reddish blue color on gross examination. Over time, the lesions undergo calcification. The upper limit of size is 1.5 cm, although most lesions are generally smaller than 1.0 cm.

Osteoid osteoma causes pain that is localized to the general anatomic area and is worse at night, with the child often awakening. Pain-related spinal deformity may be manifested in children with longer standing symptoms as torticollis with cervical lesions or as scoliosis with thoracolumbar tumors.

Osteoid osteoma and osteoblastoma together account for approximately 7% to 12% of all benign tumors of bone.^25–30 There is a gender difference that varies from a male-to-female ratio of 2 : 1 to 6 : 1. The most common age at initial evaluation is in the second decade of life. These lesions are commonly found in the posterior elements of the spine with a predilection for the lumbar spine, followed by the cervical, thoracic, and sacral segments. The imaging findings reveal a lytic defect with a sclerotic margin. Computed tomography (CT) demonstrates the lesion better than plain films do, especially with smaller tumors. Magnetic resonance imaging (MRI) can make the lesion appear larger with increased surrounding edema. The lesion frequently has a hypointense central area surrounded by isointense to hyperintense edema on T2-weighted imaging (Fig. 222-1). Other imaging modalities such as bone scans may help confirm the diagnosis but are not necessary with adequate CT and MRI evaluation. More likely, bone scans may be useful to help localize the area of interest if the history and examination do not provide adequate clues to guide imaging.

FIGURE 222-1 Osteoid osteoma at C5. This child suffered neck pain that had been especially severe at night for the preceding 8 months. He was intolerant of nonsteroidal anti-inflammatory drug and aspirin therapy. The lesion (arrow) is shown to be a sclerotic lesion surrounded by an area of lucency on CT (A). MRI shows bone edema (arrow) larger than the lesion on axial T2-weighted imaging (B). Surgical resection was localized to the lesion, with demineralized bone matrix and bone chips (arrow) placed into the defect, which eventually formed solid bone (C and D).

Osteoid osteoma is thought to be clinically relevant for a few years and causes pain as it matures and calcifies. As a result, conservative, nonoperative treatment consists of salicylate or nonsteroidal anti-inflammatory drugs for several months to a few years. In children who are intolerant of or unresponsive to aspirin, surgical resection can offer quick resolution of symptoms. Aggressive, wide resection can carry the risk of iatrogenic deformity and instability; resection of the lesion is all that is required. Accurate localization is critical to achieve cure while limiting destabilization of the spine. Neuronavigation, intraoperative fluoroscopy, and intraoperative CT or MRI have the potential to improve accuracy, although a solid understanding of anatomy may be all that is required.

Osteoblastoma

Osteoblastomas are histologically similar to osteoid osteomas except that these lesions may have a thin or absent sclerotic margin and are usually larger than 1.5 cm.^{12,21,23,27,30–41} These lesions consist of osteoblasts, osteoid tissue, and trabecular bone surrounded by a highly vascular matrix. Instead of a thick sclerotic margin, these lesions are more expansile with growth into the epidural space. Forty percent of osteoblastomas have been found in the spine, primarily in the posterior elements and in the first 2 decades of life. Again, there is a male predominance with a male-to-female ratio of 2 : 1.^10,12,34,39

The initial symptom is pain, but it is often less severe and less localized and has less diurnal variation than seen with osteoid osteoma. Because of the indolent course and larger size, osteoblastomas can be accompanied by neurological signs of radiculopathy or myelopathy. Plain films are more likely to demonstrate this tumor as a lucent lesion with a thin sclerotic rim. These lesions are best visualized with CT or MRI. Depending on the degree of vascularity, there can be brisk enhancement (Fig. 222-2).

FIGURE 222-2 Osteoblastoma at C6. This 14-year-old boy had considerable neck and right arm pain for several months. MRI shows a lesion that is isointense to hypointense to bone on T1-weighted (A) and T2-weighted (B) imaging. There is a lack of enhancement with the administration of gadolinium (C). The expansile lesion (arrow) is enlarging the lamina with effacement of the thecal sac (D).

Although osteoblastomas are benign lesions, they can be locally aggressive with a higher recurrence rate. As a result, careful attention to surgical margins is important. The expansile lesion destroys adjacent bone and can create spinal instability either through destruction of normal elements or as a result of complete resection (Fig. 222-3). Malignant transformation of a benign osteoblastoma is rare, although cases have been reported. Treatment with radiation has been reported for lesions that are either surgically inaccessible or incompletely resected.³⁴ There are reports of radiation promoting malignant transformation.⁴² Accordingly, radiation therapy should be given only when surgical resection is not possible for an osteoblastoma that is clinically aggressive.

FIGURE 222-3 Osteoblastoma at C6 (same patient as in Figure 222-2). The expansile, hypodense lesion (arrow) is seen on sagittal (A) and axial (C) CT scans. The patient underwent complete resection of the lesion without disruption of the facet joint laterally (B and D); resolution of the patient’s symptoms was achieved.

Osteochondroma

Osteochondroma, also known as chondroma or osteocartilaginous exostosis, is a benign bone hamartoma. These cartilage-capped bony lesions contain normal-appearing bone. The pathology is derived from normal bone in an abnormal location, most commonly in the cervical and thoracic spine.^43–49 There is a male preponderance of these lesions, and they occur in the second to fourth decades of life. Osteochondromas are thought to develop at secondary sites of ossification and are found in the spinous process, lamina, and transverse process. In the rare instances in which they are located in the pedicle or vertebral body, it is important to watch for neural compression and spinal stenosis.⁵⁰ Plain film imaging reveals a pedunculated lesion coming out of normal cortex. CT shows stippled calcification. The cartilage portion is hyperintense, whereas the bony portion is isointense on MRI.

Multiple lesions are seen in patients with hereditary multiple exostoses.^{44,46,47,49,51} This genetic disorder has an even greater male preponderance. Although solitary lesions rarely degenerate to the malignant lesions osteosarcoma or chondrosarcoma, these tumors can develop in 5% to 10% of patients with multiple cartilaginous exostoses. When malignant transformation has taken place, imaging changes from a well-defined lesion with normal adjacent soft tissue to an indistinct, rapidly growing lesion with associated calcifying soft tissue. The cartilaginous cap is typically less than 1 cm for chondroma; chondrosarcoma develops a thicker cap, often greater than 2 to 3 cm.

Treatment is conservative observation in most cases. Those in whom significant pain, neural compromise, or malignant transformation develops will need resection.

Hemangioma

Hemangiomas are found incidentally in 10% to 12% of the population. These benign, osteolytic lesions contain multiple thin-walled vessels. They are found primarily in the vertebral body.^21,52–54 Histologically, there are two types of hemangiomas based on the size of the vascular channels: capillary and cavernous. Cavernous hemangiomas are more common and have larger venous channels. They are most often found in the thoracic spine.

Imaging often includes plain films, CT, MRI, and angiography. CT shows a “honeycomb” appearance because of the resorption of horizontal trabeculae with thickened vertical trabeculae. MRI is useful to determine the extent of neurological compression. Angiography demonstrates an extensive vascular blush; this radiographic study is useful for identification of the vascular feeder and the venous drainage, as well as for embolization with particles or ethanol.^54,55

The vast majority of hemangiomas are asymptomatic and found incidentally. Because they are more common in adults, the infrequent pediatric patient may benefit from short-term follow-up. Once it is determined that this is a static lesion, no further follow-up is recommended. In the rare patient with significant pain, neurological compression, or spinal instability, treatment is indicated.^56–61 Because these lesions are highly vascular, preoperative embolization is important. With the advent of less invasive techniques, percutaneous vertebroplasty may provide definitive treatment. Other less involved approaches include posterior decompression with or without fusion. If the lesion is large or involves multiple areas of the spinal column, 360-degree resection and reconstruction will be necessary for definitive treatment. There are reports of stereotactic radiation therapy and ethanol embolization as primary treatment, but this is less described in the pediatric population and the immature spine.

Giant Cell Tumor

Giant cell tumors are lytic, expansile lesions containing multinucleated giant cells.^36,62–67 The presence of increased osteoclastic giant cells explains why these tumors are also known as osteoclastomas. Giant cell tumors tend to be very vascular and often exhibit hemorrhages of varying age. The most common location for these lesions is in the long bones, but they can rarely be found in the spine. Those in the spine can be located in both the vertebral body and the posterior elements. The prognosis is thought to be better for spinal lesions than for those found in the long bones.

Pain is common; radiculopathy and weakness occur as the tumor enlarges and begins to compress the neural elements. Imaging reveals a large destructive and expansile lesion with a thin rim of bone.²⁸ MRI and CT are important to delineate the extent of the tumor and guide surgical planning. Pathologically, homogeneous mononuclear stromal cells are admixed with giant cells.

Treatment is determined by accessibility and size, and resection and subsequent spinal stabilization are often required.^{36,64–66,68–72} Because these lesions are vascular and may be similar to aneurysmal bone cysts, preoperative embolization may be useful to limit bleeding. Giant cell tumors can be locally aggressive, which has prompted the use of adjuvant radiation therapy to control the disease. Malignant transformation with metastases to the lung is possible.⁷³

Aneurysmal Bone Cyst

Aneurysmal bone cysts are benign, lytic, vascular lesions found in any bone but are typically seen in the long bones, spine, and ribs.^74–78 The majority of aneurysmal bone cysts are found in the spine, with up to 30% to 50% being associated with other lesions such as eosinophilic granuloma, angioma, osteoblastoma, and bone cysts. Between 60% and 70% are found in the posterior elements, with the remaining being localized to the vertebral body. These lesions arise during the first 2 decades of life, with a slight female predilection.

The initial symptoms of aneurysmal bone cysts commonly include pain for several months, but the duration and character of the pain depend on the location. Posteriorly located lesions can be associated with pain, tenderness, swelling, and a subcutaneous mass. Anteriorly located lesions will cause pain and tenderness without swelling. Regardless of location, many patients with aneurysmal bone cysts have decreased range of motion and may also have neurological signs such as sensory disturbance, localized weakness, dysesthesias, and paraplegia or quadriplegia.

Imaging shows an irregular, osteolytic, expansile lesion with destruction of the normal anatomy.^74,75,77,78 On CT and MRI, there is an associated complex heterogeneous cyst with multiple fluid-fluid levels and blood products of varying age (Fig. 222-4). Angiography can show brisk flow with large feeding and draining vessels. Histologic analysis reveals multiple blood-filled cysts separated by fibrous bands and thin trabeculae contained by a thin subperiosteal shell. Blood products of different age are seen to be undergoing organization to varying degrees.

FIGURE 222-4 MRI of an aneurysmal bone cyst at T10. This 15-year-old girl had paraplegia, sensory loss, and incontinence. The cystic lesion (white arrow) was causing significant spinal cord compression (arrowhead). A complex, heterogeneous cystic lesion is seen on T2-weighted imaging (right upper and lower panels). Bright, heterogeneous enhancement of this lesion is seen on T1-weighted imaging with and without gadolinium enhancement (left lower and upper panels).

Treatment centers on resection of the lesion and stabilization of the spine.^{74,76,79–86} Aneurysmal bone cysts can be quite vascular; as a result, preoperative embolization is useful for limiting blood loss (Fig. 222-5). Because these lesions are often destructive of multiple columns of spinal support, stabilization with instrumentation and bone grafting is likely to be necessary (Fig. 222-6).^{36,74,87–91} The recurrence rate can be as high as 10% to 20%; clinical examination and imaging over long-term follow-up are required. It can be difficult to distinguish maturation of the bone graft and bone healing from recurrent tumor. Adjuvant therapy with radiation and liquid nitrogen have been used in an attempt to reduce the recurrence rate.

FIGURE 222-5 Angiogram of an aneurysmal bone cyst at T10. This patient underwent embolization as part of the preoperative evaluation. A significant reduction in blood flow is seen when pre-embolization (left) and postembolization (right) images are compared.

FIGURE 222-6 CT scan of an aneurysmal bone cyst at T10. The patient underwent surgical resection followed by bone grafting and stabilization. She was able to start walking again 4 days after surgery, and complete recovery of her motor, sensory, and bladder function was noted at last follow-up.

Langerhans Cell Histiocytosis

This lytic lesion is either known as or part of many named disease processes, including eosinophilic granuloma, histiocytosis X, Langerhans cell histiocytosis, Hand-Schüller-Christian disease, Letterer-Siwe disease, and Calvé’s disease.^92–95 Langerhans cell histiocytosis is commonly seen in the first 2 decades of life with a male-to-female ratio of 3 : 2. Symptoms from this lesion typically begin as localized pain but can include tenderness and fever.^{21,40,53,63,96–99} Vertebra plana is often used to describe this lesion because of the flattened vertebral body. Progression of symptoms results from the instability created by bone destruction or from epidural compression of nerve roots and the spinal cord (Fig. 222-7).^100–103

FIGURE 222-7 Langerhans cell histiocytosis at C4. Vertebral body collapse (vertebral plana) is seen on this sagittal CT scan (A). The lesion is isointense to hypointense on T1-weighted (B) and hyperintense on T2-weighted (C) imaging. Epidural and prevertebral enhancement is apparent after the administration of gadolinium (D).

Multiorgan involvement can occur and affect the liver, lung, lymph nodes, skin, mucosal membranes, bones, and pituitary gland. As a result, the work-up should include a skeletal survey, chest radiograph, and liver function tests; a clinical history and examination focusing on endocrine dysfunction, prominent lymph nodes, and skin manifestations are also important. The most common bones affected include the skull, long bones, ribs, spine, and facial bones. It is important to determine whether the disease is solitary or multifocal. Solitary lesions can be treated locally by either intralesional steroid injection or surgical resection. Multifocal disease should be treated systemically with chemotherapy because of the high rate of recurrent disease and central nervous system lesions. Biopsy, guided by either CT or fluoroscopy, can help differentiate Langerhans cell histiocytosis from other lesions such as infections, lymphoma, neoplasia, or metastatic disease.¹⁰⁴ In patients with high suspicion for eosinophilic granuloma, simultaneous injection of methylprednisolone may improve the pain and promote healing (Fig. 222-8).