Radiography constitutes the initial imaging modality for characterizing primary bone tumors and tumor-like conditions. This modality determines the location of the lesion within the bone and assesses its interface with the adjacent normal bone, along with the presence and appearance of both mineralized matrix and periosteal reaction [5]. Benign neoplasms usually grow more slowly and have well-defined and often sclerotic borders. Slower-growing tumors result in cortical expansion and may produce solid periosteal reaction. Malignant lesions usually show poor marginal definition, along with cortical destruction, a soft tissue mass, and a spiculated or interrupted periosteal reaction. However, these general characteristics are not absolute.

Periosteal reaction may be classified as either linear (single or multiple), solid, or spiculated [6]. Single linear reaction, appearing as a thin layer of new bone adjacent to the cortex, may be seen with benign periosteal reaction of infancy, trauma, osteomyelitis, and benign or malignant tumors, including metastases (Fig. 21.1a). Multiple layers of linear or lamellated reaction, known as “onion skin,” are frequently seen with aggressive tumors such as Ewing sarcoma or osteosarcoma but may also be seen with Langerhans cell histiocytosis and osteomyelitis (Fig. 21.1b). Spiculated periosteal reaction, oriented in the direction of rapid tumor growth, forms along vascular channels and Sharpey bands and appears in either a “hair on end” or “sunburst” form (Fig. 21.1b, c). Hair on end periosteal reaction is associated with Ewing sarcoma; a sunburst appearance is most often seen with osteosarcoma. The Codman triangle, or interrupted periosteal reaction, occurs when tumor, pus, or hemorrhage lifts or interrupts the periosteum. This may be seen with aggressive tumors (Fig. 21.1d), infection, or sometimes healing fractures. However, periosteal reaction has a broad range of appearances, and even with malignant tumors, it is not always visible.

Tumors may produce osseous, chondroid, or fibrous matrix (Fig. 21.2). Osseous matrix is usually cloud-like, while chondroid matrix is typically characterized by stippled rings and arcs [5]. Destruction, replacement, or resorption of bone by the tumor may produce a lytic appearance. This tends to be well marginated, or geographic, with benign tumors, and more aggressive (permeative or moth-eaten) with malignant tumors.

Ultrasound can assess associated soft tissue masses but has an otherwise limited role in imaging bone tumors.

Technetium-99m-methylene diphosphonate ([99m]Tc-MDP) bone scintigraphy and positron emission tomography (PET) have a limited role in the evaluation of solitary benign-appearing bone lesions as the patterns of uptake are usually not specific. However, in the initial staging and treatment evaluation of osteosarcoma and Ewing sarcoma, scintigraphy can assess for bone metastases, synchronous tumor, and skip lesions [7, 8]. It is also useful for multifocal processes, such as fibrous dysplasia, Langerhans cell histiocytosis (LCH), and other metastatic diseases [5]. 18F-fluorodeoxyglucose positron emission tomography (FDG-PET) CT is a promising means to evaluate patients with bone sarcomas [9, 10]. This technique may help predict tumor grade and assess treatment response when coupled with histologic evaluation [9].

Computed tomography (CT) offers greater spatial and contrast resolution than radiography and thus provides more detailed information about the intra- and extraosseous extent of tumor, the configuration of mineralization within the lesion, the type and location of tumor matrix, and the resultant cortical and periosteal changes [1, 5]. Furthermore, fat and fluid–fluid levels may sometimes be detected within the lesion, and the extent of contrast enhancement may be delineated [11]. CT may demonstrate periosteal reaction earlier than plain radiography [6].

Magnetic resonance imaging (MRI) exquisitely assesses the extent of cortical, intramedullary, joint, and soft tissue involvement by the neoplasm [8]. Important features such as fluid–fluid levels and tumor composition may lead to a more accurate diagnosis [11].

The primary tumor, adjacent soft tissues, and vascular, nerve, and joint involvement must be assessed. The MRI examination must include the entire bone to evaluate for skip lesions and assess the longitudinal extent of the tumor [8, 12]. T1-weighted (T1-W) images show the highest specificity for evaluating tumor extent: intraosseous tumor is typically hypointense and distorts the normal marrow architecture [12]. Short tau inversion recovery (STIR) images detect tumor with the highest sensitivity (the tumor is typically hyperintense). Small field-of-view phased-array coil axial fast spin echo (FSE) fluid-sensitive and fat-suppressed (FS) sequences help determine the relationship of tumor to adjacent muscle, nerve, and vascular structures. Gradient echo sequences can show flow within areas of neovascularity. Gadolinium-enhanced (GE) T1-W FS images define areas of viable tumor and areas of tumor necrosis [8]. Hypointensity on both T1-W and T2-weighted (T2-W) images may suggest either osteoblastic/fibrous matrix or chronic hemorrhage [13].

Peritumoral edema may be seen with either benign or malignant tumors and may be intraosseous or extraosseous [5, 13]. The signal characteristics of peritumoral edema resemble those of tumor, and care must be taken to distinguish between the two. Edema does not demonstrate mass effect or alter normal architecture [13]. Its margins within the bone are poorly defined, and it has a feathery appearance that follows muscle and fascial planes. Tumor margins are, in contrast, usually sharp. Like many tumors, signal intensity of marrow edema is intermediate between fat and muscle on T1-W sequences and high on T2 or STIR images. However, T2 hyperintensity due to marrow edema tends to be more heterogeneous than does that associated with tumor. The enhancement pattern also helps differentiate, as edema enhances less than tumor.

Fluid–fluid levels were first described in aneurysmal bone cysts (ABC) but occur in other benign and malignant tumors as well, including unicameral bone cyst, giant cell tumor, osteoblastoma, chondroblastoma, and telangiectatic osteosarcoma [5]. They are best evaluated with T2-W MRI (Fig. 21.3) [14]. Fluid–fluid levels in aneurysmal bone cysts result from separation of blood and serum in the cavernous spaces [13]. In other lesions, they may result from hemorrhage with breakdown of blood products or from tumor necrosis with sedimentation of cells and tissue fluid. The MRI appearance depends on the pulse sequences used as well as the composition of the fluid layers. On T1-W sequences, a high signal layer suggests methemoglobin. A layer that is isointense to slightly hyperintense to muscle on all sequences generally results from layering cellular debris or blood [13]. Although there are exceptions, as a general rule if more than two thirds of a lesion consists of fluid–fluid levels, it is benign, whereas if less than one third of it consists of fluid–fluid levels, it is malignant [15].

There are two major staging systems for malignant bone tumors, a surgical system developed by Enneking (Table 21.4) and a system defined by the American Joint Commission on Cancer [16, 17] (Table 21.5) (AJCC). The former is useful for surgical planning, while the latter categorizes patient and monitor survival.