|Home | About | Journals | Submit | Contact Us | Français|
Although MRI is the technique of choice for evaluating most soft-tissue masses, CT often provides valuable complementary information. Specifically, there are distinguishing CT characteristics that can suggest a specific diagnosis, including the lesion’s mineralization pattern, density, pattern of adjacent bone involvement, and degree and pattern of vascularity.
This article provides an overview of the CT evaluation of soft-tissue masses, emphasizing a differential diagnosis based on these CT features.
Soft-tissue tumors are defined as mesenchymal proliferations that occur in extraskeletal nonepithelial tissues of the body, excluding the viscera, meninges, and lymphoreticular system [1, 2]. CT has long been used to characterize the composition and anatomic location of soft-tissue masses [3-5] and has been known for several decades to be able to distinguish benign from malignant processes [6, 7]. Recently, MRI has become the diagnostic technique of choice because of its excellent soft-tissue contrast for this large and heterogeneous group of tumors with many overlapping features [8-11]. However, the ubiquity of CT, its faster examination times, and its superior patient tolerance compared with MRI have contributed to its exponential growth in utilization, even with regard to musculoskeletal examinations . This fact dictates that the radiologist be familiar with CT features of soft-tissue tumors, as well as mimickers of tumors; moreover, the radiologist must be aware of what distinctive information is provided by CT compared with MRI.
One of the most important roles of CT is in providing useful clues for the characterization of soft-tissue lesions. CT has been shown to provide a more comprehensive assessment of soft-tissue tumors with regard to patterns of matrix mineralization and patterns of cortical and marrow involvement [13-15]. With 3D reconstructions, CT can also be a useful adjunct in the characterization of lesion vascularity . In this article, we briefly review technical considerations for performing CT for the evaluation of soft-tissue masses, outline the role that CT plays for the diagnosis of these masses, and delineate what information may be gained for treatment planning. Four distinguishing features that can be used to characterize soft-tissue masses by CT are presented, including the mineralization pattern, density, pattern of bone involvement, and lesion vascularity.
Although the use of older generation scanners is adequate, the advent of advanced MDCT with isotropic resolution data sets allows multiplanar reformatted thin-section images and the creation of 3D CT images to provide comprehensive information about the internal architecture of a mass. For the cases described here, IV contrast material was administered using 120 mL of nonionic contrast medium (iohexol [Omnipaque 350, GE Healthcare]) at a rate of 2–4 mL/s with a typical scan delay of 30–80 seconds. After initial axial acquisition at 0.75-mm slice thickness, 2D multiplanar reconstructions and maximum intensity projections for CT angiography images were created. In all cases where an isotropic imaging data set was obtained, 3D imaging was created on a workstation (Leonardo, Siemens Healthcare) using InSpace software (Siemens Healthcare).
Mineralization within soft-tissue masses can result from ossification or calcification or both  and produces the appearance of high-density material on radiographs or CT scans. The pattern and morphologic characteristics of mineralization can be a clue to a soft-tissue mass’s cause, and hence, suggest a histologic diagnosis. With MRI, the identification of mineralization within a soft-tissue mass is often limited because of the variable signal intensity of calcium [17, 18]. Also, subtle matrix mineralization that is not detectable by radiography is made apparent by CT [19-21]. Although it is not always possible with imaging, an attempt should be made to distinguish calcification, which is due to dystrophic or metabolic deposition of insoluble calcium phosphate salts in the soft tissues, from ossification, which marked by trabecular bone formation [19, 22] (Table 1).
Soft-tissue calcification has a broad differential diagnosis, although most often it is the result of dystrophic calcification in damaged or inflamed soft tissues or an underlying metabolic abnormality (known as metastatic calcification). The myriad causes of dystrophic calcification include vascular (within thrombus), infectious, traumatic, autoimmune, and neoplastic causes . Vascular calcifications, or phleboliths, are very common in asymptomatic individuals but can also be a feature suggestive of hemangiomas and venous vascular malformations [24-26]. Radiographs often show a radiolucent center, although this has been shown to be unreliable on CT, possibly because of differences in kilovoltage . The association with a soft-tissue tail sign representing the vein or venous plexus in which it is located has been reported to be a specific CT feature of phleboliths, although it lacks sensitivity .
Sheetlike patterns of calcification in the skin, subcutaneous tissue, and fascial planes (calcinosis universalis) can be seen in association with autoimmune connective tissue disorders, such as polymyositis or dermatomyositis [29, 30], as shown in Figure 1. Periarticular deposits of calcium due to metastatic calcification are commonly seen in chronic renal failure (Fig. 2) but can also be seen after traumatic or neurologic insult . Gouty tophi composed of monosodium urate crystals can present as soft-tissue masses with mean attenuation values of approximately 160 HU  (Fig. 3), and metastatic or dystrophic calcification within these tophi are best seen on CT .
Periarticular calcification is relatively nonspecific and can also be seen after traumatic or neurologic insult. A dense homogeneous calcified mass with interspersed fibrous septae is suggestive of idiopathic tumoral calcinosis, which can also reveal fluid-calcium levels on CT [29, 33]. Extraarticular deposition of calcium hydroxyapatite crystals in the tendons, commonly called calcific tendinitis, is usually evident on radiographs around more mobile joints, although an atypical location or associated cortical erosion identified on CT can lead to unnecessary workup [34, 35]. When located intraarticularly, round symmetric calcified bodies that are associated with cortical scalloping suggest synovial chondromatosis (Fig. 4).
Benign heterotopic ossification in the soft tissues can be seen after localized trauma, neurologic injury, or, rarely, in hereditary forms, such as myositis ossificans progressiva (now referred to as fibrodysplasia ossificans) . The clinical history and distribution of the disease are important etiologic clues, with the hip, shoulder, knee, and elbow being the most commonly affected joints in the neurogenic form. Heterotopic ossification following trauma (myositis ossificans traumatica) is characterized initially by the appearance of floccular calcifications approximately 3 weeks after injury. After 6–8 weeks, lamellar bone with well-defined cortex forms . Intermediate heterotopic ossification often shows a distinctive peripheral rim of dense mineralization, the so-called eggshell ossification , as seen in Figure 5. More mature heterotopic ossification often shows a well-defined peripheral cortex and inner trabecular pattern of mineralization.
When soft-tissue tumors generate an osteoid or chondroid extracellular matrix, distinctive mineralization patterns may suggest a histologic diagnosis. Cloudlike matrix mineralization in a smooth pattern that progressively increases in density toward the center is typical of osteoid matrix (Fig. 6), although a ground-glass appearance can also be seen. In contrast, calcifications in rings and arcs, with or without dense punctate, stippled, or flocculent calcification, are classic for chondroid mineralization  (Fig. 7). The rings and arcs pattern of mineralization is due to calcification developing around lobules of cartilage. It should be noted that both of these patterns are associated with benign and malignant entities. In addition, other malignant soft-tissue tumors, particularly synovial cell sarcoma (Fig. 8), may exhibit metaplastic bone formation, though production of definitive osteoid matrix in such tumors is rare [29, 39].
The internal composition of a soft-tissue mass may be surmised from its CT density, with fat measuring −130 to −70 HU, fluids measuring 0–30 HU, and muscle and soft tissues measuring 40–60 HU .
Lesions containing fat density include benign and malignant lipomatous tumors, hemangiomas and vascular malformations, and peripheral nerve sheath tumors. The macroscopic fatty components of classic lipomas have CT values typically measuring between −65 and −120 HU (Fig. 9), similar to the subcutaneous fat . Although the appearance of well-differentiated liposarcomas may overlap with lipoma variants (angiomyolipoma, spindle cell, pleomorphic, and chondroid lipomas), characteristics that increase the likelihood of malignancy include increased patient age, large lesion size (> 10 cm), thick septations (> 2 mm), nodular or globular nonadipose mass-like areas, and decreased percentage of fat composition [41-44] (Figs. 10 and and11).11). However, it is important to remember that many higher grade liposarcomas, including myxoid, pleomorphic, and round cell subtypes, may contain little or no radiologically visible fat . Large lipomatous tumors of infancy, such as lipoblastomas (Fig. 12), are rare [41, 45]. Hemangiomas are tumors of childhood that undergo proliferative and involuting phases, with increased fat seen during the later phases [24, 46]. Finally, deep-seated soft-tissue masses in the extremities that are fusiform in shape and have a fatty rim of tissue, which often is best seen in coronal or sagittal planes, suggest a localized peripheral nerve sheath tumor [47, 48]. This has been termed the “splitfat sign,” and its appearance owes to the displaced but intact surrounding layer of intermuscular fat that normally covers the neurovascular bundle, a finding indicative of a noninfiltrating underlying neurogenic mass [49, 50]. Also suggestive of peripheral nerve sheath tumors is the target sign, which is seen on CT as a central area of higher attenuation (representing fibrous collagenized tissue) surrounded by a halo of lower attenuation (representing more myxoid tissue). Classically associated with neurofibromas, the target sign can also be found with schwannomas and, unlike the split-fat sign, arises as a result of intrinsic properties of the neurogenic tumor itself .
Lesions containing fluid include simple cysts, as well as cystic-appearing solid masses that are distinguished by the administration of IV contrast material. True cystic lesions, such as ganglion cysts, abscesses, seromas, lymphoceles (Fig. 13), and synovial cysts, may show a thin rim of enhancement in the cyst wall but no internal enhancement. In contradistinction, cystic soft-tissue tumors may have dense fluid (relative to water), possibly due to hemorrhagic, mucoid, or necrotic material, but show enhancement of their solid-tumor components after contrast administration. Rapidly growing or treated tumors exhibit hypodense regions of tumor necrosis that can usually be distinguished from simple fluid components by margin irregularity; diffusion-weighted MRI may allow improved discrimination in more difficult cases . Because of their high mucin content, myxoid tumors, such as intramuscular myxomas, myxoid malignant fibrous histiocytomas (myxofibrosarcomas), and myxoid liposarcomas (Fig. 14), often have a pseudocystic appearance [41, 52]. The recognition that synovial cysts, bursae, and ganglia occur in typical periarticular locations in the dorsal wrist and popliteal regions and are intermuscular serves to distinguish them from myxomas, which are typically intramuscular . Fluid–fluid levels are a nonspecific result of intratumoral hemorrhage and may be revealed by CT as well as MRI; among soft-tissue lesions, they have been described in synovial sarcomas, venous vascular malformations, and peripheral nerve sheath tumors [53-56].
Lesions predominantly composed of fibrous tissue are usually of intermediate density similar to that of muscle. Benign fibrous masses include nodular fasciitis, fibromas, and fibromatoses (Fig. 15), the last of which can recur and be locally aggressive [57, 58]. Uncommon in the extremities, solitary fibrous tumors are a type of spindle cell neoplasm originally described in the pleura but now recognized to be anatomically ubiquitous and of uncertain malignant potential ; their CT appearance is typically that of a well-defined mass nearly isodense to muscle  (Fig. 16). Of malignant lesions, malignant fibrous histiocytoma (termed highgrade undifferentiated pleomorphic sarcoma in the new World Health Organization classification) is the most commonly encountered . It should be suspected in any deep intramuscular unencapsulated mass in a patient older than 50 years, particularly when cortical erosion is seen .
CT is excellent in revealing the relationship between a soft-tissue tumor and the adjacent bone, especially in complex anatomic areas such as the spine and pelvis [14, 62]. The use of 3D imaging is particularly valuable for delineating the nature of bony involvement and its extent for preoperative planning [16, 63].
Benign soft-tissue masses tend to grow slowly over a long period of time and, therefore, frequently display smooth pressure erosion or scalloping of adjacent bone (Fig. 17). Intraarticular soft-tissue masses, such as those in synovial chondromatosis (Fig. 3), classically result in scalloping of the intraarticular portions of the adjacent bone. Malignant masses frequently show aggressive behavior, as evidenced by infiltration and osseous destruction (Fig. 18). Malignant fibrous histiocytoma, for example, commonly shows adjacent cortical destruction . It should be noted, however, that exceptions to these generalizations will occur, as evidenced by synovial sarcomas that can be associated with benign-appearing bony erosions  and slow-growing metastases that scallop or splay underlying bony structures (Fig. 19).
CT angiography may aid in the characterization of soft-tissue masses by revealing arterial or venous lesion vascularity, which is characteristic of certain tumors or vascular malformations. CT angiography is especially useful for preoperative planning by depicting vascular structures with a high degree of spatial resolution in multiple planes and in 3D reconstructions [16, 25, 63, 65] (Figs. 8 and and2020).
CT of benign nonneoplastic vascular lesions typically shows a mottled low-density pattern caused, in part, by the fatty, fibrous, and vascular tissue components. Venous malformations, which are characterized by slow flow and the pooling of blood, show a soft-tissue mass with serpentine vascular components that enhance with IV contrast medium unless they are thrombosed, along with phleboliths, as discussed in a previous section [25, 66]. High-flow vascular malformations, such as arteriovenous fistulas or arteriovenous malformations (Fig. 20), will show large feeding arteries in addition to draining vessels , although these can occasionally be seen in large low-flow malformations or noninvoluting hemangiomas as well (Fig. 21). CT venography has been recently reported to be of particular value in patients with mixed capillary and venous vascular malformations by revealing the anatomy and extent of aberrant venous drainage patterns [68, 69].
With regard to neoplastic conditions, benign soft-tissue tumors generally show well-defined homogeneous contrast enhancement, whereas malignant tumors are usually irregular and show heterogeneous enhancement. Arteriovenous shunting can be observed within malignant tumors and has been reported to occur in up to 24% of synovial sarcomas . In addition, tumor involvement of neurovascular structures has well-known prognostic implications for soft-tissue tumors, as has been shown for synovial cell sarcoma . It should be noted, though, that malignant soft-tissue tumors rather infrequently invade the neurovasculature; Panicek et al.  reported a prevalence of 4.5% for vascular involvement and 6.8% for neural involvement.
The primary role of CT in the evaluation of soft-tissue masses is adjunctive to that of MRI for the characterization of the masses. With CT, subtle areas of matrix mineralization may be detected that are diagnostic for a specific entity when minute areas of ossification or calcification are undetectable by MRI or radiography. Lesion density can suggest a histologic diagnosis, and careful evaluation of the adjacent bone often reveals clues regarding the potential for aggressive behavior. The techniques of CT angiography and 3D post-processing are particularly useful in the evaluation of soft-tissue masses and their relationship with adjacent osseous and neurovascular structures and often provide essential anatomic information for the referring physician in designing an optimal surgical approach. Thus, CT is often complementary to other imaging techniques in the radiologic evaluation of patients presenting with soft-tissue masses.
This work was supported by the National Institutes of Health (grant number 1T32EB006351). Its contents are solely the responsibility of the authors and do not necessarily represent the official views of the National Institutes of Health.