Primary Tumors of the Spine

Radiologic-Pathologic Correlation

Abstract Introduction

Primary Benign Lesions of the Spine Primary Malignant Lesions of the Spine Conclusions

Source Information References

Primary Benign Lesions of the Spine

Enostosis

Enostoses, also commonly referred to as bone islands, were originally described by Stieda in 1905 (8). Other terms used to describe these lesions include calcified medullary defect and endosteoma. These lesions have a propensity to occur in the axial skeleton, particularly the pelvis, spine, and ribs. In previous descriptions, Onitsuka (9) and Schmorl and Junghanns (10) noted bone islands in only 1% of spines. However, Resnick and coworkers (11) detected enostoses in 14% of cadavers, making bone islands one of the two most common lesions to involve the spine (along with hemangioma, which occurs in 11% of all cadavers) (11,12).

Enostoses of the spine occur most frequently in the thoracic (T-1 to T-7) and lumbar (particularly L-2 and L-3) vertebral bodies (11). Interestingly, thoracic lesions are more common to the right of the midline, whereas lumbar enostoses occur more frequently in the left half of the vertebral body. Spinal enostoses are asymptomatic lesions that are discovered incidentally.

Histologically, an enostosis represents lamellar compact bone with a haversian system embedded within the medullary canal (13,14). These lesions are generally developmental as opposed to being present at birth and could be considered hamartomatous lesions (ie, normal tissue in an abnormal location). Spinal enostoses often lie just beneath the cortex, and, as with bone islands in other locations, commonly have radiating spicules at their periphery.

Radiographic or computed tomographic (CT) findings of enostoses are often characteristic, consisting of a circular or oblong, osteoblastic lesion with an irregular, spiculated margin (Figure 1). The appearance of the lesion margin has been described as "thorny radiations" or a "brush border." The surrounding trabecular bone is normal, with an abrupt transition to the sclerotic bone island. In the study by Resnick et al (11), spinal enostoses varied in size from 2 X 2 mm to 6 X 10 mm. This small size most likely accounts for why only 29% of enostoses were detected on radiographs before patient death (11). However, giant bone islands (>2 cm) can also occur in the spine, and their radiographic and pathologic appearances, other than size, are identical to those of smaller lesions (Figure 2).

The bone scintigraphic appearance of most enostoses is normal, without evidence of increased accumulation of radionuclide; however, uptake has been observed in some cases, most frequently in cases of giant bone islands. Hall and coworkers (15) reported positive bone scans in 33% (four of 12) of patients with large bone islands. The increased uptake of the radionuclide is likely related to increased osteoblastic activity.

Limited reports of magnetic resonance (MR) imaging of enostosis and our experience suggest that these lesions have low signal intensity, regardless of the pulse sequence used. Spiculated lesion margins may also be apparent. The signal intensity of the surrounding marrow is normal.

The natural history of enostosis is variable, and although most lesions remain stable, some may slowly increase or decrease in size, as reported by Onitsuka (9) in 31.9% of cases. The most important aspect of diagnosing spinal enostosis, for radiologists, is recognition of this relatively common benign and asymptomatic lesion. The condition with which enostosis is most easily confused is usually osteoblastic metastatic disease, and the distinction between these two entities can be particularly troublesome if lesion enlargement has occurred. In the vast majority of cases, however, the lack of increased activity on bone scans, a solitary abnormality, normal adjacent trabecular bone, and a thorny margin allow confident radiologic diagnosis. Vertebral biopsy, although rarely necessary, should be considered if the lesion increases in diameter by more than 25% within 6 months or 50% within 1 year (14).

Osteoid Osteoma

Osteoid osteoma involves the axial skeleton in 10% of cases (16). Clinical symptoms include painful scoliosis, focal or radicular pain, gait disturbance, and muscle atrophy. A patient history of painful scoliosis is important and should alert both radiologist and clinician to the possible diagnosis of axial osteoid osteoma, particularly because idiopathic scoliosis is usually not associated with pain. The pain associated with osteoid osteoma is often worse at night and is relieved by salicylates or nonsteroidal anti-inflammatory medication. Patients are usually affected between the ages of 10 and 20 years, and there is a male predominance (2:1 to 3:1) (16,17,18).

The majority of axial osteoid osteomas (75%) are located in the posterior elements (Figure 3) of the vertebra (pedicles, articular facets, and laminae), and only 7% are in the vertebral body (17,19). The remainder of cases involve the transverse and spinous processes. The lumbar spine is most commonly affected (59% of cases), followed by the cervical (27%), thoracic (12%), and sacral (2%) segments (20,21).

Pathologically, the nidus of an osteoid osteoma is a small (<1.5-2.0 cm in diameter), round mass of pink to red tissue, which reflects the vascularity of the lesion (Figure 3e). Microscopically, the nidus is composed of well-organized, interconnected trabecular bone with a background of vascularized fibrous connective tissue (16) (Figure 3f). Frequently, the nidus is surrounded by a variable degree of reactive cortical bone.

The classic radiographic appearance of osteoid osteoma is a round to oval, discrete radiolucent area (representing the nidus), with variable surrounding sclerosis (Figure 3a). Central calcification may be present. However, the com- plex anatomy of the spine often obscures the nidus so that only sclerosis or a dense pedicle is apparent on radiographs. The differential diagnosis for a dense pedicle includes osteoblastic metastasis, enostosis, unusual infection, lymphoma, and reactive sclerosis caused by abnormalities of the facets (eg, contralateral absent pedicle or spondylolysis). If scoliosis is present, the osteoid osteoma nidus is localized to the concave portion at the apex of the curve.

Bone scintigraphy demonstrates marked increased uptake by the osteoid osteoma nidus (Figure 3b). Occasionally, a more distinctive double-intensity scintigraphic pattern is apparent, with a small, central, high degree of radionuclide uptake, which corresponds to the nidus, surrounded by a less intense zone of tracer accumulation, which corresponds to the osseous reaction.

CT is particularly useful for identifying the nidus and has largely supplanted conventional tomography for this purpose. Most investigators consider CT to be the optimal modality for detecting osteoid osteoma (21). The nidus is seen as a well-defined, low-attenuation lesion less than 1.5-2.0 cm in diameter, with or without central calcification, surrounded by a variable degree of sclerosis (Figure 3c).

The MR imaging findings of osteoid osteoma are variable. The nidus is generally low to intermediate signal intensity on T1-weighted images and intermediate to high signal intensity on T2-weighted images. However, areas of calcification have low signal intensity with all pulse sequences. The appearance of osteoid osteoma on MR images can be misleading when small lesions are located in the lamina or pedicle. The nidus may become obscured by signal changes due to associated surrounding sclerosis, marrow edema, and soft-tissue inflammation (22).

Treatment of spinal osteoid osteoma is complete resection of the nidus. Radiography of the specimen may be performed to ensure complete removal (Figure 3d). Because of the difficulty in localizing the nidus at surgery, intraoperative scintigraphy (in which a scintillation probe is used to detect the nidus) or tetracycline labeling (in which tetracycline is orally administered, absorbed by the nidus, and identified under ultraviolet light because of its fluorescence) may be used. Nidus localization by means of CT-guided needle placement before surgical resection has also been employed (16). New methods of treatment include percutaneous CT-guided removal and percutaneous ablation with a radio-frequency electrode, laser, or alcohol (16,23,24).

Osteoblastoma

Osteoblastoma and osteoid osteoma are similar but distinct lesions that can usually be differentiated on the basis of their clinical courses and radiologic appearances. Osteoblastomas occur in young adults, with 90% of cases diagnosed in the 2nd and 3rd decades of life; however, cases have been reported in patients aged 3-72 years (25,26,27). There is a male predominance (2:1). Clinical symptoms often differ from those of osteoid osteoma, with osteoblastoma producing dull localized pain, as opposed to the intense night pain caused by osteoid osteoma. Unlike osteoid osteoma, osteoblastoma is commonly manifested by neurologic symptoms, including paresthesias, paraparesis, and paraplegia. Scoliosis occurs less frequently with osteoblastoma than with osteoid osteoma and may be convex toward the side of the tumor.

Osteoblastoma of the spine accounts for 30%-40% of all osteoblastomas, and the lesions are equally distributed in the cervical, thoracic, and lumbar segments (25). Osteoblastoma most frequently involves the posterior vertebral elements (55% of cases), although extension into the vertebral body is also common (42%) (25,26). Osteoblastoma confined to only the vertebral body is rare.

Pathologically, the typical osteoblastoma is larger than 1.5-2.0 cm in diameter. Histologic examination may reveal features very similar to those of osteoid osteoma (interconnecting trabecular bone and fibrovascular stroma), but overall the microscopic pattern is not as well organized as that seen in osteoid osteomas (28). An ABC component can be seen in 10%-15% of osteoblastomas (28). Mayer (29) described a subgroup of osteoblastomas, which appeared similar to osteosarcoma and contained prominent epithelioid osteoblasts, and referred to these lesions as aggressive osteoblastomas.

Three radiographic patterns have been described with osteoblastoma (25,26,27,28,29). The first, which consists of a central radiolucent area (with or without calcification) and surrounding osseous sclerosis, is similar to the radiographic appearance of osteoid osteoma, but the lesion is larger than 1.5 cm in diameter. The second, an expansile lesion with multiple small calcifications and a peripheral sclerotic rim, is the most common appearance of spinal osteoblastomas (30) (Figure 4). The third pattern has a more aggressive appearance, consisting of osseous expansion, bone destruction, infiltration of surrounding soft tissue, and intermixed matrix calcification (Figure 5). Mineralization within an osteoblastoma may have the radiologic appearance (rings and arcs) of chondroid matrix.

At bone scintigraphy, osteoblastoma demonstrates marked radionuclide uptake. At CT, the lesion shows areas of mineralization, expansile bone remodeling, and sclerosis or a thin osseous shell about its margins (Figure 4b).

Although the MR imaging appearance of osteoblastoma has not been extensively reported, it is generally nonspecific, with low to intermediate signal intensity seen on T1-weighted images and intermediate to high signal intensity seen on T2-weighted images (25) (Figure 4c, Figure 4d, Figure 5b). MR imaging optimally depicts the effects of the tumor on the spinal canal and surrounding soft tissues, and extensive peritumoral edema has been reported (31).

Imaging studies of aggressive osteoblastoma, in our experience, reveal large, infiltrative, soft-tissue components that reflect its pathologic appearance and clinical behavior (Figure 5).

Clinical, imaging, and pathologic characteristics usually allow differentiation of osteoblastoma from osteoid osteoma in the spine. The radiologic features favoring osteoblastoma include lesion diameters larger than 1.5-2.0 cm, osseous expansion, soft-tissue component, and multifocal (as opposed to central) matrix mineralization. The clinical course of osteoblastoma is slow growth, compared with the stable lesion size usually encountered in cases of osteoid osteoma.

Treatment of spinal osteoblastoma is surgical resection, and the recurrence rate for conventional lesions is 10%-15% (25,26,27). The diagnosis of aggressive osteoblastomas is important because they have a far greater recurrence rate (approaching 50%), which is probably related to their larger size and the resultant inability to perform complete resection (27). Repeated local recurrence of aggressive osteoblastoma can ultimately lead to patient death, and, rarely, malignant transformation to osteosarcoma and metastatic disease has been reported.

Giant Cell Tumor

Involvement of the spine by GCT is not unusual, constituting 7% of all cases of GCT (32,33,34). In fact, the spine is the fourth most common location of GCT. The vast majority of these lesions occur in the sacrum, followed in order of decreasing frequency by the thoracic, cervical, and lumbar segments (32,33,34). Spinal lesions are more frequent in women and affect patients in the 2nd to 4th decades of life. Clinical symptoms are primarily pain (often with radicular distribution), weakness, and sensory deficits. Dramatic increase in lesion size can occasionally be associated with pregnancy and is presumably related to hormonal stimulation.

Pathologically, GCT is composed of abundant osteoclastic giant cells intermixed throughout a spindle cell stroma (35). Cystic areas (similar to those seen in ABC) and regions of previous hemorrhage with hemosiderin are also seen (36,37). In addition, prominent areas of fibrous tissue that are high in collagen content are a frequent finding, and in these regions, giant cells are uncommon. Although the vast majority of GCTs are benign lesions, malignant GCTs (seen histologically as sarcomatous stroma) have been estimated to occur in 5%-10% of cases (35). Many of these malignant GCTs are related to previous irradiation, which is not an unusual treatment for spinal GCT, and should be considered radiation-induced sarcomas.

Radiologic studies of spinal GCT usually demonstrate an expansile lesion, with bone lysis being seen on radiographs (30,38). As with appendicular GCTs, spinal lesions show no evidence of mineralized matrix. In the sacrum, lesions are frequently large with destruction of the sacral foraminal lines (Figure 6), but this nonspecific finding is also seen with other large sacral lytic lesions. Sacral GCT commonly involves both sides of the midline, and extension across the sacroiliac joint is not infrequent (33,38). When GCT occurs in the spine above the sacrum, it usually affects the vertebral body, as opposed to the posterior elements as is seen with many other neoplasms (ABC, osteoid osteoma, osteoblastoma) (Figure 7). Extension into the posterior elements and paraspinal soft tissues and associated vertebral collapse are often apparent (30,34). Involvement of the adjacent intervertebral disks and vertebrae is not uncommon (34). The propensity of GCT of the spine to extend across the sacroiliac joint and disk space is unusual for many other spinal lesions, and this extension can simulate an infectious process. Paradoxically, GCTs of long bones do not share this invasive feature of spinal lesions and rarely extend across or through articular cartilage.

At bone scintigraphy, GCT may demonstrate diffusely increased uptake of radionuclide. However, sacral GCTs commonly demonstrate the "donut" sign of central photopenia and increased peripheral activity. Angiography of GCT usually reveals a hypervascular lesion. CT and MR imaging performed after intravenous injection of contrast material also show enhancement of the lesion, which reflects its increased vascular supply.

CT and MR imaging are important methods for delineating the extent of spinal GCT (Figure 6, Figure 7). On CT scans, the tumor has soft-tissue attenuation with well-defined margins that may show a thin rim of sclerosis (38) (Figure 6a). Areas of hemorrhage or necrosis may create heterogeneity, with foci of low attenuation. On MR images, the lesion often shows heterogeneous signal intensity, regardless of the pulse sequence used (35,37). Generally, the tumor has low to intermediate signal intensity on T1-weighted MR images. Interestingly, GCTs predominantly have low to intermediate signal intensity on T2-weighted MR images in 63%-96% of all cases (35,37). This appearance seems to be caused by the relative collagen content of fibrous components and hemosiderin within GCTs (Figure 6c, Figure 7c). Although this feature is not unique to GCT of the spine, it is very helpful in differential diagnosis, because most other spinal neoplasms (metastases, myeloma, lymphoma, and chordoma) show high signal intensity on long repetition time MR images. This appearance may be the initial imaging characteristic that is suggestive of the correct diagnosis. Evidence of hemorrhage may also be apparent with high signal intensity on T1- and T2-weighted images or fluid-fluid levels on CT or MR images (Figure 6b, Figure 6c). Focal cystic areas with low signal intensity on T1-weighted images and high signal intensity on T2-weighted MR images are frequent. A low-signal-intensity pseudocapsule about the margins of the tumor is also common.

Unlike other benign tumors of the spine, the prognosis of GCT is not as favorable (2,6,7). These lesions are often locally aggressive, and their size or location may not allow complete excision, which is used in treating long bone GCT. In many cases, complete resection is possible only in some sacral lesions, and imaging has a vital role to play in presurgical evaluation. In general, sacral lesions that spare the majority of the S-1 segment and the sacroiliac joint are amenable to complete excision. Although sacroiliac joint involvement can be determined by coronal oblique (imaging parallel to the long axis of the upper sacrum) CT or MR imaging, sagittal MR imaging is the optimal method for evaluating the upper extent of the lesion. GCTs that cannot be entirely excised are often treated with a combination of partial curettage and radiation therapy (39). Selective arterial embolization has also been used before attempted surgical excision or as the sole therapy in cases of unresectable GCT. Recurrence has been estimated at a rate of 40%-60% of cases and is seen radiographically as areas of new bone destruction.

Aneurysmal Bone Cyst

The term aneurysmal bone cyst (ABC) was originated by Jaffe and Lichtenstein (40) in 1942 to describe the radiographic appearance of a subset of unicameral bone cysts with marked expansile remodeling due to large blood-filled spaces. These lesions typically affect young patients, with 80% being less than 20 years of age, and there is a mild female predominance (41). The spine is involved in 12%-30% of cases (41,42). The thoracic spine is affected most commonly, followed in order of decreasing frequency by the lumbar and cervical segments (42). Sacral involvement is rare (43). Patients complain of back pain and neurologic symptoms resulting from encroachment on the spinal canal.

Pathologically, ABC often has a characteristic appearance consisting of multiloculated blood-filled spaces; this has led to the surgical description of the lesion as a "blood-filled sponge" (40,41). The blood-filled spaces are not lined by endothelium and therefore do not represent vascular channels. Many researchers believe ABCs result from trauma with a local circulatory disturbance (primary ABC) or are a secondary phenomenon produced by an underlying neoplasm (including GCT, osteoblastoma, chondroblastoma, and osteosarcoma) that causes venous obstruction or arteriovenous fistulae (41). The vast majority of ABCs (65%-99%) are considered primary lesions (ie, no underlying neoplasm is found) (41). Solid components are usually in septations interposed between the blood-filled spaces and are composed of fibrous tissue, reactive bone, and giant cells. In 5%-7.5% of cases, the solid component predominates histologically (44). These lesions have been called solid variant ABCs by Sanerkin and colleagues (45) and have a propensity to involve the spine.

Radiographs of spinal ABCs generally show marked expansile remodeling of bone centered in the posterior elements, although extension into the vertebral body is frequently seen (75%-90% of cases) (41,42). Expansion is usually most prominent with transverse and spinous process lesions and least extensive with lesions of the pedicle and lamina, a characteristic that may be related to the fact that these latter lesions are usually seen at an earlier stage because of clinical symptoms associated with compression of the spinal canal. A thin, outer periosteal rim and septations may be apparent. Spinal ABC, similar to GCT and chordoma, may extend into adjacent vertebral bodies, intervertebral disks, posterior ribs, and paravertebral soft tissues.

At bone scintigraphy, ABC frequently demonstrates peripheral increased uptake of radionuclide. This nonspecific pattern (donut sign) was reported in 64% of ABC cases by Hudson (46) but is also frequently seen with GCT. Angiography reveals a hypervascular lesion in 75% of cases, and, as seen with bone scintigraphy, the vascularity is predominantly peripheral (41,46).

CT and MR imaging are often rewarding in the evaluation of ABC of the spine because the appearance suggests the cystic nature of lesion (Figure 8). In addition, tumor extension and relationship to the spinal canal are particularly well seen on coronal and sagittal MR images. CT and MR imaging often show single or multiple fluid-fluid levels within ABCs, indicative of hemorrhage with sedimentation (47) (Figure 8c). Visualization of fluid-fluid levels requires that the patient be motionless for approximately 10 minutes (to allow layers to settle) and that the imaging plane be perpendicular to the fluid levels. In the series by Hudson (46), 35% of ABCs showed fluid-fluid levels at CT. In our experience, MR imaging is more sensitive in the detection of these areas. On T1- or T2-weighted images, they may have increased signal intensity due to methemoglobin in either the dependent or nondependent component. Fluid-fluid levels are not specific for ABC but are very suggestive of this diagnosis, particularly if the entire lesion is composed of this type of tissue (48). Other areas in ABCs usually show high signal intensity on T2-weighted MR images. Despite the aggressive expansile destruction seen radiographically, these lesions often have a soft-tissue-attenuation or low-signal-intensity rim on CT and MR images (all pulse sequences), respectively, that corresponds to an intact, thickened periosteal membrane. In our experience, gadolinium enhancement of these lesions on MR images is usually seen within the rim and septations, rather than the cystic spaces (Figure 8b).

Although ABCs are benign, treatment of spinal lesions is often more problematic than that of appendicular lesions because complete resection may not be possible without excessive morbidity. Overall recurrence rate is 20%-30%, although it increases with incomplete resection (41). Recurrence is seen radiologically as progressive bone destruction and lesion enlargement.

Additional modes of treatment for spinal ABC include embolotherapy and radiation therapy (41,42,49). Embolotherapy, performed with multiple agents, has been employed with substantial success as both a preliminary procedure to lessen blood loss at surgery and also as the sole treatment (42,49). Cory and coworkers (49) reported lesion regression in four ABCs treated only with embolotherapy, and two of these were in the spine. These lesions are also very sensitive to radiation therapy, although subsequent complications, including radiation-induced sarcoma, are a serious consideration in this young patient population.

Osteochondroma

Spinal osteochondromas are uncommon, representing only 1%-4% of solitary exostoses and constituting 4% of all solitary spinal tumors (50,51). In patients with hereditary multiple exostoses, only 7%-9% of patients have a spinal lesion and usually there is only one spinal osteochondroma despite the multiplicity of lesions throughout the remainder of the skeleton (52,53). Spinal osteochondromas are usually discovered at the age of 20-30 years in patients with a solitary osteochondroma and 1 decade earlier in patients with hereditary multiple exostoses (50,51,52,53). There is a male predominance, which is more striking in solitary osteochondroma than in hereditary multiple exostoses. Myelopathy is present in 34% of patients with solitary lesions and 77% of patients with multiple osteochondromas, but the symptoms may not become apparent until after trauma (50). A palpable mass may be appreciated in those cases in which the lesion extends posteriorly, whereas dysphagia, hoarseness, and vascular complications are more likely in osteochondromas that protrude anteriorly.

Osteochondromas have been encountered at all levels of the spine, but they have a predilection for the cervical spine, particularly C-2. Lesions most commonly arise from the posterior elements, although some originate from the vertebral body as well.

Osteochondromas arise during development when physeal cartilage is trapped outside the physeal plate. The greater mobility of the cervical spine, which allows increased stress and microtrauma with displacement of epiphyseal cartilage, may account for the predisposition for osteochondroma (50). Radiation is an unusual cause of spinal osteochondroma, and lesions occur at the periphery of the radiation treatment field, with a direct dose response relationship (>2,500 rad [25 Gy] usually required) (54). Radiation-induced lesions are more likely in patients who were younger than 2 years of age when they underwent radiation therapy, and the latent period is from 17 months to 9 years (54).

Osteochondromas are composed of normal bone (cortex and marrow space) with a cartilage cap from which growth occurs. The pathologic and radiologic hallmark of osteochondroma is continuity of the lesion with the marrow and cortex of the underlying bone. Spinal osteochondromas may be sessile or pedunculated.

The diagnosis of spinal osteochondroma can be definitively made from radiographic findings in only a minority of cases (21%), usually in large lesions protruding posteriorly from the spinous process where cortical and marrow continuity is easily assessed (50,55,56) (Figure 9). Smaller lesions related to the lamina or pedicle, particularly those extending into the spinal canal (which are more likely to be symptomatic), are difficult to detect on radiographs (Figure 9). Indeed, in 15% of cases of spinal osteochondroma, the radiographs are considered normal (50,55,56).

Thin-section CT is the radiologic examination of choice for detecting the osseous characteristics of the exostosis and the pathognomonic marrow and cortical continuity of the lesion to the underlying bone (Figure 9b, Figure 9c). The relationship of the lesion to the surrounding soft tissues and spinal canal can be well evaluated with CT or MR imaging.

MR imaging often reveals yellow marrow centrally (which has high signal intensity on T1-weighted images and intermediate signal intensity on T2-weighted images) with a low-signal-intensity cortex (Figure 9d). The hyaline cartilage cap is often small and thin, with low to intermediate signal intensity on T1-weighted MR images and high signal intensity on T2-weighted images. In adults, spinal osteochondromas with marked thickening (>1-2 cm) of the cartilage cap should be viewed with suspicion of malignant transformation to chondrosarcoma (Figure 10); this characteristic is similar to that for appendicular lesions.

Surgical excision of osteochondroma is usually curative. Improvement of symptoms is seen in 89% of patients after surgery, even those with long-standing lesions (50,56). Incomplete resection can lead to recurrence, although this is unusual.

Abstract Introduction

Primary Benign Lesions of the Spine Primary Malignant Lesions of the Spine Conclusions

Source Information References