Abstract and Introduction
With bone tumor findings in musculoskeletal radiology, the radiologist's main goal is to assess whether the lesion is benign or aggressive and to determine if further workup is required. The authors demonstrate how contrast-enhanced MRI helps determine tumoral extent and risks, how CT may reveal occult, pathologic fractures, and how scintigraphy may be used to distinguish between monostotic and polyostotic lesions.
Bone tumors are a relatively infrequent finding in musculoskeletal radiology. When evaluating osseous lesions, the radiologist's main goal is to assess whether the lesion is benign or aggressive in appearance and whether further workup is required. The list of potential osseous lesions is extensive; this review of bone tumors does not include metabolic or degenerative lesions. To provide a meaningful differential diagnosis to the referring clinician, several characteristics of every osseous lesion should be routinely assessed.
Classically, 10 radiographic features of a bone lesion should be examined. Five of these include: zone of transition (ZOT), presence or absence of periostitis, location in the bone, pattern of osseous destruction, and age of the patient, along with associated symptoms.[1–3] Additionally, determining if the process is mono-ostotic or polyostotic can be very helpful. Because these characteristics are so important to properly classifying bone tumors, more should be said about each.
The zone of transition is that outer margin of the lesion that represents the change from pathologic to normal bone. A wide ZOT is said to be present when the lesion cannot be clearly circumscribed; this is usually associated with an aggressive lesion. Malignancies such as Ewing's sarcoma and osteosarcoma typically show this pattern of involvement; however, infection and other benign processes, such as eosinophilic granulomas (EG) (Figure 1), can have wide margins. A wide ZOT does not equate to malignancy, but it is very rare for a narrow ZOT (a geographic lesion) to be associated with anything other than a benign lesion.[3,4]
Eosinophilic granuloma. (A) Frontal radiograph and (B) coronal reformatted CT images from a 2-year-old boy with a high ESR and no fever. The lesion has ill-defined margins and an associated periosteal reaction.
The location of the lesion in the bone, both transversely and longitudinally, can also be useful in narrowing the differential. For example, a select group of lesions is eccentrically located and involves the cortex (osteoid osteoma, parosteal osteosarcoma, and nonossifying fibroma) (Figure 2). Other lesions almost always involve the epiphysis (giant cell tumor after physeal closure (Figure 3), chondroblastoma). Some lesions, such as solitary bone cysts (Figure 4), enchondromas (Figure 5), EG and Ewing's sarcoma, tend to be centrally located.[1,3]
Nonossifying fibroma with secondary aneurysmal bone cyst formation. Lateral radiograph (A) and (B) axial T2-weighted MRI with fat saturation of the distal tibia in a 14-year-old boy. The radiograph shows a mildly expansile lytic eccentric metaphyseal lesion posteriorly in the distal tibia. The MRI shows the lesion to be mildly loculated with central low signal and a thin rim of high signal, and a single lobule with a fluid-fluid level due to secondary ABC formation.
Giant cell tumor of bone. (A) Frontal radiograph, (B) surface-rendered 3D CT and (C) repeat frontal radiograph taken 3 months later of the distal forearm in a 27-year-old woman. The expansile lytic subchondral nature of the lesion and in this case its very aggressive increase in size over 3 months is well demonstrated.
Simple bone cyst. (A) Frontal and (B) axial radiographs of the proximal humerus in a 5-year-old boy. There is a well-defined lucency of the proximal humeral metaphysis with a pathologic fracture. In this case there is no fallen fragment sign as can occur when a fracture fragment migrates through the fluid to lie dependently.
Enchondroma. (A) Frontal and (B) oblique radiographs of the distal femur. The lesion has the classic appearance for this location with a rings-and-arcs pattern of calcification in a central metaphyseal position.
The number of osseous lesions can also provide clues to etiology. Several lesions that can be polyostotic include multiple hereditary exostoses, enchondromas, fibrous dysplasia and, occasionally, eosinophilic granulomas. Of course, metastatic disease and multiple myeloma (Figure 6) are common causes of multiple lesions, but a few bony metastases can present as solitary lesions—especially in renal or thyroid carcinoma.[3,5]
Multiple myeloma. Axial CT image through the pelvis of a 68-year-old woman. Note the diffuse permeative pattern of bone destruction. The propensity of lesions should suggest myeloma or metastases in this age group.
Elucidating information from periosteal changes can be relatively more difficult. Periostitis is often subtle and can mislead the radiologist attempting to classify a lesion as benign or aggressive. Classic, aggressive-appearing periostitis is described as having an "onion-skin," "sunburst" (Figure 7), or "hair-on-end" appearance.[2,3] A Codman triangle pattern is another aggressive configuration. Benign patterns are those that, in theory, have had sufficient time to organize and, thus, show solid thick or wavy unilamellar periosteal changes. Use caution when assessing periosteal reactions, as many benign lesions such as infection, EG (Figure 1), and aneurysmal bone cysts can result in an aggressive-appearing periostitis. Regardless, recognizing periosteal reaction of any type remains important, as this effectively excludes several lesions from the differential. If periostitis is present, fibrous dysplasia, solitary bone cyst, nonossifying fibromas, and enchondromas can be removed from consideration unless complicated by fracture.[3,4]
Osteogenic sarcoma. (A) Frontal and (B) lateral radiographs of the tibia/fibula in a 15-year-old boy demonstrate a proximal tibia diaphyseal aggressive bone-forming tumor with a large soft-tissue mass. The periosteal reaction has an aggressive "Sun ray spiculation" pattern.
Two additional pieces of information that can be extremely helpful are the age of the patient and the presence or absence of pain. Some lesions, such as Ewing's sarcoma and primary osteosarcoma, are seen overwhelmingly only in the pediatric, adolescent, and young adult populations. Likewise, EG, chondroblastoma, solitary and aneurysmal bone cysts are rarely seen in adults >30 yrs. Moreover, while infection, metastatic lesions, and aneurysmal bone cysts typically present with pain, discomfort is rare (in the absence of trauma) with fibrous dysplasia, enchondromas, and solitary bone cysts. Several other characteristics, such as the presence of sclerotic margins, soft tissue involvement, a pattern of bony destruction, endosteal scalloping, and the pattern of matrix calcification, can also aid in diagnosis.
Although plain radiographs, computed tomography (CT), magnetic resonance imaging (MRI), and radionuclide studies may each provide additional information, suspected soft-tissue extension of an osseous lesion should be evaluated further with contrast-enhanced MRI to determine not only tumoral extent but also the risk of complications like neurovascular compromise. Also, MRI may help narrow the list of differential considerations by demonstrating cystic or necrotic components, encapsulation, contrast enhancement and the presence of fluid levels (Figures 2, 8, 9) or peritumoral edema on MRI. Dedicated CT may show occult, pathologic fracture in an otherwise benign-appearing but painful lesion.
Aneurysmal bone cyst. (A) Lateral radiograph, (B) axial bone and (C) soft-tissue window CT and (D) axial T2 with fat saturation MRI of the distal left tibia in a 25-year-old man with aneurysmal bone cyst. Note the expansile nature of the lytic lesion with a thin rim of intact bone and the multiple fluid-fluid levels from the stagnant pooling of blood on the soft tissue CT and MRI.
Telangiectatic osteosarcoma. (A) Frontal radiograph, (B) MDP bone scan and (C) axial T2-weighted MRI with fat saturation show a lesion in the lateral aspect of the right distal femur meta-epiphysis of a 19-year-old man. This version of osteosarcoma is relatively lytic compared to the conventional osteosarcoma, but it shows marked uptake on MDP. The axial MRI shows multiple fluid levels, strongly suggestive of the diagnosis.
Scintigraphy with Tc99m–MDP bone scan can determine whether a lesion is mono-ostotic versus polyostotic in nature. The following is a brief description of each of the 15 most common benign and malignant osseous lesions the radiologist is most likely to encounter.
Appl Radiol. 2012;41(10):6-15. © 2012 Anderson Publishing, Ltd.