Osteomas have been reported in patients from 10 to 79 years of age, with most in the fourth and fifth decades (13,46). Males and females are equally affected. The signs and symptoms associated with osteoma vary
depending on the size of the lesion and its location (37). Small lesions are usually asymptomatic and incidental findings. Large paranasal sinus tumors in particular may produce a variety of manifestations. Attached to the sinus wall and covered by the sinus mucoperiosteum (38), they often block the nasal ducts, causing mucoceles, sinusitis, nasal discharge, headache, and pain. At times a patient may lose the sense of smell (38). Occasionally, paranasal osteomas exhibit more dramatic features such as orbital or intracranial invasion (41). Tumors near the orbit can produce exophthalmos, double vision, and pressure on the optic nerve that may result in loss of vision. At other sites, they can erode the wall of the cranial fossa, perforate the dura, and compress the frontal lobe. Paranasal osteomas are usually discovered in the third and fourth decades, and the finding is most often incidental. One retrospective study involving 16,000 patients found a frequency of 0.4% for these tumors (11). They occur twice as frequently in males as in females and are believed to have a growth peak at the time of maximum skeletal development; they are only occasionally found before puberty (47).

Osteomas seldom develop in the long and short tubular bones (1,5,20,49) and are extremely rare in the flat bones (7,33,48). Tumors at these sites are known as parosteal osteomas. Like osteomas elsewhere, they are usually asymptomatic. Most parosteal lesions measure 1 to 4 cm in diameter, rarely reaching the dimensions reported by Baum et al. (16.5 × 4.2 cm) and Mirra et al. (6 × 5 × 5 cm) (1,33). Many long bone lesions are multiple.

It is worth noting the association of osteomas with the familial adenomatous polyposis syndrome (Gardner syndrome), an autosomal-dominant disorder with 25% new mutations, frequently seen in Mormons in Utah (10,18). It is caused by mutations in the APC gene, which maps to chromosome 5q21–q22 (3). In addition to bone lesions, the syndrome is marked by multiple cutaneous and subcutaneous lesions (e.g., sebaceous cysts, skin fibromas, and desmoid tumors) and intestinal, particularly colonic, polyposis (17). The bone tumors may precede the appearance of the intestinal polyps, which have a marked propensity to carcinomatous change (8,17). Osteomas have also been reported in association with tuberous sclerosis, an autosomal-dominant disorder affecting TSC1 (9q34) or TSC2 (16p13) gene, characterized by mental retardation, epilepsy, cutaneous hamartomas, and benign juvenile or inflammatory polyps of the colorectum (3,33). Finally, osteomas of the soft tissues (so-called osseous choristomas) are rare occurrences that have been described in the thigh, in the intraoral soft parts, and in the bronchi (12,29,42,45).

Conventional radiography is the basis for the diagnosis of osteoma. Radiographs typically show a dense, ivory-like sclerotic mass with sharply demarcated borders attached to the bone (Fig. 2-2). Conventional tomography (Fig. 2-3) and computed tomography (CT) (Fig. 2-4) are effective in demonstrating lack of

cortical invasion (50), which not infrequently is a feature of parosteal osteosarcoma, or lack of cortical and medullar continuity with a host bone, a feature of osteochondroma (see “Differential Diagnosis,” later). Magnetic resonance imaging (MRI) shows low signal intensity on both T1 and T2 sequences, consistent with cortical bone (50).

Histologically, two types of osteoma may be encountered: cancellous and compact (51). Most investigators consider the former to be the forerunner of the latter. The cancellous (or trabecular, or spongy) osteoma reveals a cancellous, trabecular architecture. Trabeculae are thin with fatty marrow present in the intertrabecular spaces (25) (Fig. 2-5A). They usually contain more woven bone and show evidence of active bone formation with transformation to lamellar bone (39). Woven bone consists of a more or less mature matrix with a plait of collagen fibers; it contains roundish osteocyte lacunae in high numbers per space unit. Lamellar bone is composed of narrow parallel layers of mature matrix with densely packed collagen fibers; it contains spindle-shaped osteocyte lacunae in smaller numbers than woven bone. Compact osteoma, on the other hand, consists of dense, compact, mature lamellar bone (9) (Figs. 2-5B, C). No Haversian systems are present and only occasionally marrow spaces are visible. A compact osteoma usually exhibits empty osteocyte lacunae in varying numbers, probably as a result of increased distance between the small marrow spaces and the cells that interferes with cell nourishment by diffusion.

Typically asymptomatic, a bone island is often an incidental finding on radiography performed for another purpose. It is discovered more commonly in adults than in children and shows no sexual predilection. The
pelvis, the femur, and other long bones are preferential sites of involvement, although the lesion may be found anywhere in the skeleton, including the carpal and tarsal bones as well as the ribs (74). The spine is a rare site of involvement (57,77), accounting for only three (1.4%) of 209 bone islands reviewed by Onitsuka (75). These vertebral bone islands involved the thoracic and lumbar segments.

Regardless of its site or size, a bone island exhibits a consistent radiographic picture. The lesion appears as an ovoid, round, or oblong focus, homogeneously dense and sclerotic, in the cancellous bone (spongiosa). It is commonly oriented with the long axis of the bone parallel to the cortex. Highly distinctive for this lesion are radiating bony streaks, referred to as “thorny radiation” (70,71) or “pseudopodia” (67), aligned with the long axis of the host bone’s trabeculae, that merge with the trabeculae in a feathered or brush-like fashion (Figs. 2-19 and 2-20). In the great majority of cases, bone islands range in size from 1 mm to 2 cm (79). “Giant” bone islands, defined as lesions larger than 2 cm (65), have also been reported (Fig. 2-21). The largest lesion on record, described by Brien et al. (56) measured 10.5 × 4.8 × 4.0 cm and was located in the proximal femur. Another notable giant bone island reported by Park et al. (76) measured 10.0 × 1.7 × 1.0 cm, located in the tibia. An island reported by Gold et al. (65) measured 5 × 5 × 4.5 cm, also located in the tibia. Similarly, a bone island reported by Smith (79) measured 3.5 × 4 cm and was located in the ilium, and by Greenspan and Klein (66) measured 4 × 3 × 2.5 cm and was located in the distal femur. In addition to showing the same radiographic features as smaller lesions, giant bone islands may also give the “cumulus cloud” impression (73) (Fig. 2-22).

Bone islands usually do not show radiologic evidence of change in size over time. Nevertheless, several investigators have reported size changes in bone islands, some of which exhibited metabolic activity (55,64,71,72a,74,75). In the large series of patients who underwent multiple examinations described by Onitsuka (75), 44 of 138 bone islands (31.9%) showed changes in radiographic size over periods ranging from 3 to 23 years. After exclusion of 19 of these lesions because they had enlarged proportionally during adolescent growth, 21 of the remainder had increased in size and four had become smaller. Other evidence of growing bone islands is offered by Blank and Lieber (55), who reported six cases, by Ngan (74), who described

three cases and emphasized the possible confusion of such lesions with sclerotic metastases, and by Hoffman and Campbell (71), who reported the unstable appearance of a bone island. In the latter work the instability of a bone island was demonstrated by its disappearance and reappearance over a period of 6 years in a patient with hyperparathyroidism.

On CT a bone island appears as a low-attenuation focus (63), exhibiting (as on radiography) its characteristic brush borders (Fig. 2-23A). Occasionally the lesion’s “pseudopodia” may show more rounded contours (Fig. 2-23-B). On all MRI pulse sequences a bone island exhibits the low signal intensity characteristics of cortical bone (Figs. 2-24 and 2-25).

On skeletal scintigraphy a distinctive feature of bone islands is that they usually exhibit no activity. This is generally ascribed to the fact that their level of metabolic activity is about the same as that of the surrounding cancellous bone. Therefore, radionuclide imaging has been and continues to be the means of differentiating bone islands from more aggressive bone lesions. Nevertheless, reports in the literature of histologically confirmed bone islands that showed increased radiotracer uptake on bone scan (61,65,67,69,78) have raised a note of caution about the usefulness of scintigraphy in distinguishing a bone island from clinically more significant lesions (Fig. 2-26).

The phenomenon of scintigraphically active bone islands has prompted a number of explanations of the pathomechanism, which is still unclear. It has been linked to the large size of reported bone islands (78) and to their growth, based on the observation that uptake of the radiopharmaceutical agent is roughly proportional to the volume of a lesion. However, two large bone islands that were undetectable on bone scans, as reported by Hall et al. (70), indicate that a lesion’s size alone is not always a factor in this phenomenon. Others have pointed to a giant bone island’s histologic morphology and increased metabolic activity relative to the surrounding cancellous bone as the probable explanation for its striking radionuclide activity (65). Because tracer uptake implies greater metabolic activity and, usually, increased blood flow, it is logical to assume that a bone island will show increased activity if it becomes metabolically active. However, reports of radiographically documented growing bone islands (one with a positive and the other with a negative bone scan) that showed no histologic evidence of metabolic activity raise doubts about this pathomechanism (55,72).

A contribution by Greenspan et al. (67,69), correlating the radiologic and pathologic findings in six cases of bone island, suggested that the increased tracer uptake observed in two bone islands appeared to be directly related to the higher degree of bone remodeling and osteoblastic activity exhibited by these lesions in comparison with the scintigraphically “cold” lesions. On histologic examination, these “hot” bone islands were marked by a mixture of compact and trabecular bone,


with substantial amounts of woven bone. Vasculature was abundant, and osteoblastic activity with remodeling was significant. In contrast, the scintigraphically isoactive bone islands were composed solely of compact bone or mixed compact and trabecular bone, and showed negligible cellular activity and remodeling.

Microscopic examination of a bone island reveals a histologic picture that correlates with the radiographic findings. Enostosis is a focus of compact (cortical) bone in the spongiosa that shows thorn-like, thickened trabeculae radiating from the lesion and merging with the surrounding trabeculae of the host bone (58) (Fig. 2-27). Higher power magnification reveals mature lamellar configuration encircling Haversian systems of nutrient canals (Fig. 2-28). Bone islands occasionally contain foci of woven nonlamellar bone and, as reported in some scintigraphically active bone islands, they may consist of a mixture of lamellar and woven bone (65,67). Foci of osteoblastic and osteoclastic activity are rare (Fig. 2-29).

Patients with osteoid osteoma almost invariably present with a complaint of nocturnal pain that may be severe enough to cause awakening (96,103). This is apparently due to the presence of prostaglandins that are demonstrated in the nidus but not in the surrounding bone (106,127,153). Recently, cyclooxygenase 1 and 2 (Cox-1 and Cox-2), apparently the source of these prostaglandins, have also been detected in osteoid osteoma by immunohistochemical methods (116,130). Salicylates, such as aspirin, are remarkably effective in relieving the pain, usually in less than half an hour. This classical presentation is typical in greater than 75% of patients and provides an important clue to the diagnosis (148). Local swelling and point tenderness are present in some patients (81,85,137). There may be also neurologic signs and symptoms, which can include muscle atrophy, a decrease in deep tendon reflex responses, and variable degrees of sensory loss (149). In most cases, osteoid osteoma arises in the age range of 10 to 35 years (89,121). There is a marked predominance in males, with a number of studies reporting a male-to-female ratio of 2:1 to 4:1 (34,93,100,113,114,115,119,120,121) (Fig. 2-31).

Osteoid osteoma can arise in virtually any bone. The long bones are preferentially affected (approximately 65% of the lesions occur in the long bones), particularly the femur and tibia [over 53% of all osteoid osteomas are located in these bones with the proximal femur being most frequently affected (91)], in which the lesions usually occur near the end of the shaft. Less commonly affected sites are the phalanges of hands and feet (21%), the vertebrae (9%), the humerus (122), and the scapula (88).

Depending on its location in the particular part of the bone, the lesion can be classified as cortical, medullary (cancellous), or subperiosteal. Osteoid osteomas can be further subclassified as extra- or intracapsular (intraarticular) (98,121). Recently, Kayser et al. (117) hypothesized that many osteoid osteomas arising in the tubular bones may in fact originate in a subperiosteal site and later appear as the intracortical lesions.

Intraarticular osteoid osteomas preferentially affect the hip (149). However, lesions in the elbow, foot, wrist, knee, and vertebral facet joints have also been reported (101,105). These tumors are usually characterized by nonspecific symptoms that are indicative of an inflammatory synovitis (82,144). Sclerosis may be lacking and the periosteal reaction is absent. On physical examination, a joint effusion and synovitis may be identified (82,92). Intracapsular lesions may also cause a precocious appearance of arthritis. This can provide an important diagnostic clue in cases when the history is typical of osteoid osteoma but the nidus has not been confirmed by radiography (132). If a lesion is located near the growth plate, particularly in a young child, it may cause accelerated bone growth.

Osteoid osteoma involves the axial skeleton in 10% of cases (118,119,123,131,135). Osteoid osteoma of the spine most commonly involves the lumbar segment (59%). Less frequently affected are the cervical (27%), thoracic (12%), and sacral (2%) vertebrae (34,120,121,142). In patients with lesions of the neural arch painful
scoliosis often occurs (frequently as a result of a spasm), the concavity of the curvature being toward the side of the lesion (118).

Cytogenetic analysis of osteoid osteoma showed clonal chromosomal changes in one out of six studied cases (94). Two other investigated tumors revealed structural alterations of 22q13, a region containing genes that are involved in cell cycle regulation (86).

The probability of identifying osteoid osteoma by conventional radiography is dependent on the location of the lesion. Cortical lesions are usually characterized by a radiolucent nidus (representing the tumor itself) with a surrounding area of reactive sclerosis. A periosteal reaction may or may not be identified (Fig. 2-32A). An intramedullary nidus is less easily identified on radiography because very little or no reactive sclerosis is present (Fig. 2-32B). Occasionally, the nidus may exhibit a focus of central calcification (sclerotic center). The nidus of a subperiosteal lesion may be visualized either as a central radiolucent or sclerotic focus, with or without reactive sclerosis (Fig. 2-32C), or as a shaggy, crescent-like focus of periosteal reaction (120). The radiographic presentation of intracapsular lesions is frequently characterized by periarticular osteoporosis and in some instances by premature osteoarthritis (Fig. 2-32D) (132).

In some instances conventional tomography is used to evaluate osteoid osteoma, with better visualization of the nidus than can be obtained with conventional radiography (Fig. 2-33). It can also assist in confirming or excluding the presence of a sinus tract and is therefore useful when the differential diagnosis includes a possible bone abscess.

Skeletal scintigraphy is a highly sensitive method for detecting the nidus of osteoid osteoma (125,139,152). This modality can be particularly helpful in cases for which the symptoms are atypical and the initial radiographs appear normal (146). A three-phase radionuclide bone scan, using technetium-99 as a scanning agent, has been suggested (111), which can be especially valuable when intraarticular or intramedullary lesions are not clearly visualized by conventional radiography. Radionuclide tracer activity can be observed on both immediate and delayed images (Fig. 2-34). Osteoid osteoma is occasionally characterized by an interesting scintigraphic feature, the so-called double-density sign. This phenomenon is believed to be related to increased vascularity of the nidus (110,111). In agreement with the appearance of the lesion on radiography, the double-density sign is seen as a small focus of increased activity associated with the nidus; this focus is surrounded by a larger area of less intense activity, related to the reactive sclerosis that surrounds the nidus (Fig. 2-35). Identification of the double-density sign can help to differentiate an osteoid osteoma from a bone abscess.

Ultrasound can be helpful in diagnosis of intraarticular lesions (97), and Doppler duplex color technique can be used to localize the nidus (102).

As the definite study for diagnosis of osteoid osteoma, CT is recommended. Not only can CT identify the lesion (151) but it can also accurately determine its



extent, thus enabling exact measurement of the size and location of the nidus (103,105,112,140,141) (Fig. 2-36). The nidus usually appears as a well-defined area of low attenuation, which is surrounded by a variably sized region of high attenuation reactive sclerosis (Fig. 2-37). To delineate the lesion, thin (preferably 2-mm) contiguous sections are most appropriate (Fig. 2-38). CT is particularly useful to define lesions that arise in the axial skeleton, whose complex anatomy is less clear on routine radiography and conventional tomography (145) (Fig. 2-39).

The suitability of MRI for detection of osteoid osteoma remains unclear and published reports have had mixed results (99,109,150,155,157). Goldman et al. (105) reported on four cases of intracapsular osteoid osteoma of the femoral neck, in which the lesions were evaluated with bone scintigraphy, CT, and MRI. Although in all cases abnormal findings were apparent in the MR images, the nidi could not be identified prospectively. On the basis of MRI findings of secondary bone marrow edema or synovitis, several incorrect diagnoses were made, which included Ewing sarcoma, osteonecrosis, stress fracture, and juvenile arthritis. In these cases, it is noteworthy that the correct diagnoses were made only after review of the radiographs and thin-section CT studies. Another report by Woods et al. (154) involved three patients with a highly unusual association of osteoid osteoma with a reactive soft tissue mass. In these cases, MRI studies might have led to confusion of osteoid osteoma with osteomyelitis or a malignant tumor. Moreover, in each case the nidus displayed different signal characteristics. In one case the intensity of signal was generally low on all pulse sequences but mild enhancement was seen after administration of gadolinium. In another case, the signal was of intermediate intensity and administration of gadolinium revealed inhomogeneous enhancement of the nidus. For the third case in which plain films showed the nidus to be intracortical, MRI could not identify the nidus distinctly. More recently, Davies et al. analyzing the MR imaging findings in 43 patients with osteoid osteoma concluded that reliance on MRI alone may lead to misdiagnosis (95). They suggested additional application of skeletal scintigraphy and CT in unclear cases, particularly when unexplained areas of bone marrow edema are encountered.

However, some reports do suggest the effectiveness of MRI for demonstrating the nidus of osteoid osteoma (84,104,156) (Figs. 2-40 and 2-41). Bell et al. (87) clearly demonstrated an intracortical nidus on MRI that had not been seen on scintigraphy, angiography, or CT scans. The nidus exhibits high signal intensity on T2-weighted sequences, most likely due to abnormal Cox-1 or Cox-2 levels (116). Recently, Liu et al. (126) advocated imaging of osteoid osteoma with dynamic gadolinium-enhanced MRI for greater conspicuity.

In lesions of osteoid osteoma, the nidus is small, well circumscribed, and self-limited. It is composed of osteoid tissue or mineralized immature woven bone (Figs. 2-42A–C). The osteoid matrix and bone form small and irregular trabeculae, with a thickness ranging from thin and delicate to broad and sclerotic (120). The trabeculae are surrounded by a highly vascular fibrous stroma that exhibits prominent osteoblastic and osteoclastic activity (Fig. 2-42D). The sclerosis surrounding the lesion is composed of dense bone that displays a variety of maturation patterns. From the ultrastructural viewpoint, the morphology of osteoblasts of osteoid osteoma is


generally similar to that of normal osteoblasts. However, these osteoblasts possess irregular, indented nuclei (indicative of high metabolic activity), glycogen particles, an abundance of fine intracytoplasmic fibers, and occasional iron-containing lysosomes (147). Moreover, some of these osteoblasts possess atypical mitochondria with a lobulated or “honeycomb” appearance. The areas of mineralized matrix exhibit the morphology of coarse woven bone. In two of the cases reported by Steiner (147), the osteoid contained, in addition to collagen, fine granular material, probably representing polysaccharides. Applying immunohistochemistry, nerve fibers positive for S-100, PGP 9.5 and/or neurofilament could be demonstrated in the fibrous zone and also in the nidus of osteoid osteoma, but not in other tumors including osteoblastoma (108,133). This finding in combination with prostaglandin and Cox-1/Cox-2 synthesis may explain the almost pathognomonic clinical presentation of this lesion.

Like osteoid osteoma, osteoblastomas are most often found in patients in the first to third decades with the peak incidence in the second decade (184); “aggressive” tumors are usually seen in an older age group (average age 33 years) (180,187). The male-to-female ratio appears to be approximately 2:1 (122,175). The long tubular bones are frequently affected, but the lesion shows a distinct predilection for the axial skeleton in general and the vertebral column in particular (13,34,164,180,181) (Fig. 2-52). Approximately 30% to 40% are localized in the spine (131), more or less equally in the cervical, thoracic, and lumbar segments, mainly in the posterior elements including the arch and spinous processes (186). In one series of 123 tumors, 39 (32%) were located in the spine and 23 (19%) in the jaw; the remaining 61 tumors were distributed throughout the appendicular skeleton with a predilection for the femur and tibia (187). Periosteal osteoblastomas have occasionally been reported (190), but multifocal lesions are quite rare (158).

The clinical presentation of osteoblastoma is different from that of osteoid osteoma (159,176). Some patients
are asymptomatic, and the lesion is found incidentally (183). Symptomatic patients in one study most often experienced a dull, localized pain, with a median duration of symptoms of 6 months before they presented for treatment. The pain very rarely interfered with sleep. Although the use of salicylates for pain relief is not mentioned in the study by Kroon and Schurmans (180), the pain of osteoblastoma does not appear to be as readily relieved by salicylates as the pain in osteoid osteoma. Tenderness at the tumor site is a consistent physical finding (186), and swelling may be present, particularly when the lesion is near the surface (180).

Patients with lumbar vertebral lesions will very commonly present with scoliosis. Scoliosis also commonly occurs in patients whose tumors arose in the ribs (169,172). Spinal lesions in particular may cause neurologic deficits ranging from muscle weakness to paraplegia.

Toxic osteoblastoma, a rare variant of this tumor, has recently been recognized (166). It is associated with systemic manifestations, including diffuse periostitis of multiple bones, fever, and weight loss (191).

Osteoblastomas rarely extend into the soft tissues and do not metastasize, although they do tend to recur locally.

Conventional radiography is the basis for the diagnosis of osteoblastoma. In a review of the spectrum of osteoblastoma in 123 patients by McLeod et al. (187), most tumors (75%) were either spherical or slightly oval, with the remainder being longitudinal; the tumor margin was well defined (83%). In half of the appendicular tumors, there was considerable reactive sclerosis; 13% of these tumors, however, had only a sclerotic rim, and 37% showed no surrounding sclerosis. The interior of the lesions was most often radiolucent (64%), whereas various degrees of ossification were present in the remaining cases. The authors speculated that lesions tend to be radiolucent when they are younger, ossifying as they mature. In the great majority of tumors, the cortex was either perfectly normal or intact, although expanded and thinned (75%); the cortical expansion was usually eccentric. Destruction or penetration of the cortex suggesting malignancy was present in a minority (20%) of tumors. The great majority of osteoblastomas showed periosteal new bone formation, which in some cases was extremely prominent. The periosteal reaction was predominantly benign and of the solid type (86%), whereas the remainder showed spiculation or multilamination, suggesting a more aggressive lesion.

In a review of the range of manifestations of osteoblastoma by Marsh et al. (186), the majority of tumors showed a shell of reactive bone separating the tumor from the surrounding normal bone. Osteoblastomas of the spongy bone of the spine, ilium, or talus exhibited less prominent reactive bone formation, and those arising in the cortex more pronounced reactive bone, than is seen in osteoid osteoma.

Four distinctive types of osteoblastomas can be identified, based on the radiographic findings (23). (a) The lesion may be almost identical to osteoid osteoma, although much larger (usually >2 cm in diameter). This type occasionally exhibits less reactive sclerosis than osteoid osteoma, but the periosteal reaction is more prominent. (Fig. 2-53). (b) Osteoblastoma may also appear as a blow-out expansive lesion similar to an aneurysmal bone cyst, with small radiopacities in the center. This pattern is particularly common in tumors involving the spine (Fig. 2-54). (c) The tumor may occasionally appear as an aggressive lesion, simulating a malignant neoplasm (Fig. 2-55). Many of the so-called aggressive osteoblastomas
belong to this group. (d) Exceptionally rare, osteoblastoma is juxtacortical (periosteal) in location, as described in 6 of 42 tumors reported by Schajowicz and Lemos (44) and in 2 of 20 tumors reported by Lichtenstein and Sawyer (31). These lesions generally lacked perifocal bone sclerosis and in addition, a thin shell of newly formed periosteal bone was noted covering the lesion (19,34,44) (Fig. 2-56; see also Fig. 2-10).

Conventional tomography, although nowadays rarely used, may be valuable for determining the extent of the tumor and whether and to what degree intralesional bone formation has occurred. It is particularly useful for spinal lesions, which may be difficult to interpret on routine radiographs. Tomography may also help gauge the size of the nidus, thus facilitating the distinction from osteoid osteoma (Fig. 2-57).

Skeletal scintigraphy invariably demonstrates intense focal accumulation of bone-seeking radiopharmaceutical agents (Fig. 2-58). Angiography is infrequently performed as part of the evaluation of osteoblastoma because of its questionable importance as a preoperative assessment tool (180).

CT, as in osteoid osteoma, is the most important imaging modality for facilitating the diagnosis of osteoblastoma and demonstrating its exact size and location (174) (Fig. 2-59). In addition, areas of calcifications and ossifications within the lesion (Fig. 2-60), as well as cortical destruction and soft tissue extension, are well delineated on CT sections (180). CT is particularly helpful in working up lesions of the spine (159) (Fig. 2-61).

Very few reports on the usefulness of MRI in the diagnosis and evaluation of osteoblastoma can be found in the literature (180). The strength of MRI would appear to be similar to that of CT, namely, defining the extent of bone and soft tissue involvement. Most osteoblastomas demonstrate signal intensity patterns on MR scans
similar to those of the majority of bone neoplasms: spin-echo T1-weighted images show a low to intermediate intensity signal, whereas with T2-weighting signal intensity is intermediate to high. More sclerotic lesions are of low signal intensity on all pulse sequences (Fig. 2-62). MRI effectively reveals peritumoral edema in the adjacent marrow and in soft tissues. A recent report by Crim et al. (165) of a 19-year-old man with osteoblastoma described an unusual “flare phenomenon” that caused a misleading appearance on MR scans, simulating a malignant process such as lymphoma or Ewing sarcoma. The phenomenon represented a widespread inflammatory response to the lesion that led to diffuse, reactive inflammatory infiltrates in two vertebrae, the adjacent ribs, and the paraspinal soft tissues. It is noteworthy that only CT-myelography was diagnostic in this case.

Osteoblastoma is marked by a very active new formation of osteoid and immature bone trabeculae produced by compact masses of large osteoblasts (44). Osteoid production is generally greater in osteoblastoma than in osteoid osteoma, and the lesion is more vascularized (185) (Fig. 2-63). Furthermore, osteoblastoma exhibits a less organized pattern of osteoid and trabecular bone distribution than the nidus of osteoid osteoma, which invariably appears as a circumscribed, organized structure maturing toward its periphery; in effect, the lesion of osteoblastoma is at the same stage of development throughout (44). Very rarely, cartilaginous matrix may

be present (161). Occasionally, spindle-shaped hyperchromatic cells with uniform nuclei and irregular eosinophilic cytoplasm may be interdispersed among bony trabeculae (173). In contrast to most forms of osteosarcoma, osteoblastomas characteristically show variable numbers of osteoclasts at the surfaces of the bone trabeculae and seams of osteoblasts rimming the periphery of the lesion (Figs. 2-64A–C). In addition, they do not infiltrate or permeate the normal trabeculae at the interface with the lesion, as occurs in osteosarcoma (34), but usually are separated from them by a narrow but distinct layer of bone-free fibrous tissue. Some atypical osteoblastomas may, however, closely resemble an osteosarcoma (Fig. 2-64D).

Some osteoblastomas contain “epithelioid” osteoblasts, twice the size of ordinary osteoblasts (171). The cells are rounded and have large nuclei containing one or more prominent nucleoli; their cytoplasm is usually abundant. Bone trabeculae are wider and more irregular than in conventional osteoblastoma, and cement lines are usually absent (179) (Fig. 2-65). These variants have been denoted as “aggressive” osteoblastomas. Della Rocca and Huvos (168) stated recently that a clinically aggressive behavior of osteoblastomas is not correlated with histologic qualities but with a localization within the skeleton that is more difficult to approach surgically. Schajowicz and Lemos (195) described eight patients with “malignant” osteoblastoma whose tumors
appear to be similar to “aggressive osteoblastoma.” They noted on microscopic examination more frequent atypical mitoses and increased numbers of osteoclast-like giant cells. In addition, there were bone spicules of regular shape (dark blue staining with hematoxylin), closely resembling those observed in osteosarcoma. Another pseudomalignant or pseudoanaplastic variant of osteoblastoma contains bizarre multinucleated cells completely devoid of mitotic activity (160,187,189).