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Radiologic Evaluation of Soft-Tissue Masses

A Current Perspective

¹ Department of Radiology, Mayo Clinic, 4500 San Pablo Rd., Jacksonville, FL 32224-3899.
² Department of Radiologic Pathology, Armed Forces Institute of Pathology, Walter Reed Army Medical Center, Bldg. 54, Alaska and Georgia Aves., Washington, DC 20306-6000.
³ Department of Radiology and Nuclear Medicine, Uniformed Services University of the Health Sciences, 4301 Jones Bridge Rd., Bethesda, MD 20814-4799.
⁴ Department of Radiology, University of Maryland School of Medicine, 22 South Greene St., Baltimore, MD 21201-1595.

Received March 7, 2000; accepted after revision April 12, 2000.

 
Honoring Lewis G. Cole, MD and Willis F. Manges, MD

This is the ninth in a series of Centennial Dissertations that the AJR is publishing this year in honor of the former presidents of the American Roentgen Ray Society, two of whom are pictured above.

The opinions or assertions contained herein are the private views of the authors and are not to be construed as official or as reflecting the views of the Department of the Army or the Department of Defense.

Address correspondence to M. J. Kransdorf.

Introduction

Top Introduction General Concepts Preliminary Evaluation Summary References

 
The radiologic evaluation of soft-tissue masses has changed dramatically within the last two decades. Before the advent of computer-assisted imaging, assessment of clinically suspicious soft-tissue masses was usually limited to radiographs. Although radiographs were sensitive to the identification of adipose tissue and soft-tissue mineralization, they provided little other diagnostic information. When lesions were small, radiologists of those dark days were happy just to confirm the presence of a mass, much less give a confident diagnosis. The emergence of CT improved this situation dramatically. Masses could be not only delineated with great confidence but well staged with excellent depiction of anatomic detail. However, diagnosis remained problematic, with images sufficiently characteristic to suggest the correct histology in only a minority of cases: typically, lipomas and hemangiomas [1]. The introduction of MR imaging was met with great enthusiasm because of the markedly improved soft-tissue contrast and multiplanar image acquisition capabilities. The imaging of soft-tissue masses was now on a par with that of other imaging-intense radiologic subspecialties, with exquisite depiction of anatomic detail. This ability to accurately anatomically characterize masses spurred new interest in the evaluation of soft-tissue tumors. Attempts were made to develop rules analogous to those for bone tumors for differentiating benign and malignant processes on the basis of lesion morphology and signal intensity; however, with few exceptions, these rules proved unreliable [2,3,4,5,6,7].

View larger version (95K): [in this window] [in a new window] [as a PowerPoint slide]   Lewis G. Cole 18th President of ARRS 1917-1918

View larger version (139K): [in this window] [in a new window] [as a PowerPoint slide]   Willis F. Manges 19th President of ARRS 1918-1919

Despite these limitations, most radiologists are comfortable with the use of MR imaging in the evaluation of soft-tissue lesions. We strongly believe it is the modality of choice and also think that, when it is used in conjunction with a systematic approach, one can correctly diagnose most masses. Accordingly, this review will present a systematic approach to the evaluation of soft-tissue tumors, highlighting MR imaging in diagnosis and in differentiating benign from malignant soft-tissue lesions. In addition, an approach is provided for establishing a differential diagnosis for those lesions with a nonspecific imaging appearance, as well as indications for contrast-enhanced imaging.

General Concepts

Top Introduction General Concepts Preliminary Evaluation Summary References

 
Soft tissue is derived primarily from mesenchyme and, by convention, consists of skeletal muscle, fat, fibrous tissue, and the serving vascular structures as well as the associated peripheral nervous system [9]. Soft-tissue tumors are classified histologically on the basis of the adult tissue they resemble [9, 10]. The designation of liposarcoma, for example, does not indicate a lesion arose from fat, but rather that it is a malignant mesenchymal tumor that has differentiated into tissue that microscopically resembles normal adult fat [9]. Many sarcomas are poorly differentiated and, consequently, lack the microscopic features required to make a specific diagnosis. In such cases, immuno-histochemical stains have aided pathologists in identifying their pattern of differentiation, allowing accurate classification. Despite the pathologist's best efforts, however, approximately 5-15% of soft-tissue sarcomas cannot be further classified [11,12,13,14].

Soft-tissue sarcomas, unlike benign soft-tissue lesions, are relatively uncommon and are estimated to represent about 1% of all malignant tumors [15, 16]. Hajdu [15] noted that the incidence in the United States is about the same as that of multiple myeloma or carcinoma of the thyroid. Soft-tissue sarcomas are two to three times as common as primary malignant bone tumors [9, 16]. Benign soft-tissue tumors are more common, although it is difficult to estimate the annual incidence because many lipomas, hemangiomas, and other benign lesions are not biopsied. The annual clinical incidence of benign soft-tissue tumors is estimated at 300 per 100,000, and these tumors are about 100 times more common than malignant soft-tissue tumors [9, 12].

Preliminary Evaluation

Top Introduction General Concepts Preliminary Evaluation Summary References

 
The initial evaluation of a patient with a soft-tissue mass begins with a thorough clinical history and radiologic evaluation.

Clinical History
As radiologists, we are trained to evaluate cases as unknowns. As we complete our training, we are cautioned that Board examiners will require an explanation if additional information is sought. In daily practice, histories are often scant, with little substantive data. The concept that we should take a clinical history requires a significant change in paradigm. The clinical history is an important factor in establishing an accurate diagnosis. In many circumstances it may provide key information that will allow a specific diagnosis when imaging is nonspecific. Is there a history of a previous lesion or underlying malignancy? Has there been previous surgery or radiation? It is essential to know how the patient presented: Is the lesion painful or did the patient note a painless mass? A painful mass always requires that an inflammatory process be included in the differential diagnosis. Is there a history of notable trauma or use of anticoagulants? Has the lesion remained stable over a long period of time or varied in size, or is it growing? A history of continued growth is always suggestive of malignancy. Unlike bone tumors, however, a slowly growing soft-tissue mass is not invariably indicative of a benign process. Variation in lesion size with time or activity would be exceedingly unusual for a malignancy and suggests a process such as a ganglion or hemangioma.

Is there more than one lesion? Soft-tissue tumors are typically solitary, and the identification of multiple lesions markedly limits the differential diagnosis. Multiple lipomas are seen in 5-15% of patients presenting with a soft-tissue mass [17,18,19]. The diagnosis in these cases can be made confidently on the basis of signal intensity on MR imaging. Aggressive fibromatosis is multifocal in 10-15% of patients [20, 21]. A second soft-tissue mass in a patient with a previously confirmed desmoid tumor should be regarded as a second desmoid tumor until proven otherwise [22]. Patients with neurofibromatosis have multiple lesions, and although the diagnosis is often known or suspected, such is not always the case (Fig. 1). The diagnosis may be suggested on the basis of imaging findings by the identification of multiple lesions in a major nerve distribution.

View larger version (122K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 1. —36-year-old woman with neurofibromatosis and multiple neurofibromas. Coronal contrast-enhanced T1-weighted spin-echo MR image (500/15, TR/TE) shows multiple left paraspinal masses with cystic change.

Angiomatous lesions are quite common and are multiple in as many as 20% of patients [9]. In such cases, superficial and deep lesions may coexist. Multiple lesions may also be seen with metastatic disease. Soft tissue is relatively resistant to metastasis, and although soft tissue comprises about 40% of total body weight, soft-tissue metastases are quite rare. The skin and subcutaneous tissue is also a frequent site of extraosseous involvement in patients with multiple myeloma, with involvement typically in the form of multiple subcutaneous nodules [23]. Extraosseous manifestations are found in less than 5% of patients with multiple myeloma and are associated with a more aggresive clinical course [23]. Metastatic melanoma may display a similar pattern of multiple nodular subcutaneous metastases [24] (Fig. 2A,2B). These metastases are seen in more than 30% of patients with melanoma metastatic disease, usually in patients with Clark level IV or V disease (depth of tumor invasion into the deep dermis or through the dermis into the subcutaneous fat) and may be the only radiologic manifestation of metastases [24]. Finally, multiple myxomas may be seen in association with fibrous dysplasia of bone (Mazabraud syndrome) [25]. The myxomas are usually intramuscular and the association is most typically with polyostotic disease [25, 26].

View larger version (137K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 2A. —35-year-old woman with metastatic melanoma and multiple metastases. Coronal T1-weighted spin-echo MR image (600/20, TR/TE) shows lobulated nodule (asterisk) in subcutaneous adipose tissue of buttocks. Note subtle intraosseous metastasis (arrow) in ischium.

View larger version (137K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 2B. —35-year-old woman with metastatic melanoma and multiple metastases. Axial conventional T2-weighted MR image (2500/80) shows fluid-fluid level (arrow) indicative of previous hemorrhage.

Radiography
Despite dramatic technologic advances in our ability to image soft-tissue tumors, the radiologic evaluation of a suspected soft-tissue mass must begin with the radiograph. Although frequently unrewarding, it is impossible to predetermine those cases in which radiographs will be critical for diagnosis. Radiographs may be diagnostic of a palpable lesion caused by an underlying skeletal deformity (such as exuberant callus related to prior trauma) or exostosis, which may masquerade as a soft-tissue mass. Radiographs may also reveal soft-tissue calcifications, which can be suggestive and, at times, very characteristic of a specific diagnosis. For example, they may reveal phleboliths within a hemangioma (Fig. 3A,3B), juxtaarticular osteocartilaginous masses of synovial chondromatosis, peripherally more mature ossification of myositis ossificans, or characteristic bone changes of other processes with associated soft-tissue involvement. When not characteristic of a specific process, soft-tissue calcification can suggest certain diagnoses. For example, nonspecific dystrophic calcifications in a slowly growing lower extremity mass in a young adult should suggest a synovial sarcoma as the diagnosis of exclusion (Fig. 4A,4B).

View larger version (97K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 3A. —52-year-old man with hemangioma of hypothenar eminence of hand. Radiograph shows multiple small, smooth, rounded calcifications (solid arrow), more opaque peripherally, characteristic of phleboliths. Note small nonspecific calcifications (open arrow).

View larger version (110K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 3B. —52-year-old man with hemangioma of hypothenar eminence of hand. Corresponding intraoperative photograph shows multiple phleboliths within interstices of hemangioma.

View larger version (135K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 4A. —17-year-old girl with synovial sarcoma of foot who presented with slowly growing painless mass. Axial conventional T2-weighted spin-echo MR image (1800/80, TR/TE) shows well-defined nonspecific soft-tissue mass (asterisk).

View larger version (115K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 4B. —17-year-old girl with synovial sarcoma of foot who presented with slowly growing painless mass. Corresponding radiograph shows peripheral and central clacification. This radiographic appearance (calcified soft-tissue mass) in context of slowly growing juxtaarticular mass in young adult strongly suggests appropriate diagnosis of synovial sarcoma.

In addition, radiographs are the best initial method of assessing coexistent osseous involvement, such as remodeling, periosteal reaction, or overt osseous invasion and destruction [27]. However, unlike bone tumors, the biologic activity of a soft-tissue mass cannot be reliably assessed by its growth rate. A slowly growing soft-tissue mass that may remodel adjacent bone (causing a scalloped area with well-defined sclerotic margins) may still be highly malignant on histologic examination [9].

A soft-tissue mass may also be the initial presentation of a primary bone tumor or inflammatory process. In such cases, the radiograph may be useful in identifying the osseous origin of the lesion. The diagnosis of a malignant bone tumor such as Ewing's sarcoma or primary lymphoma of the bone should be considered when there is a large circumferential soft-tissue mass in association with an underlying destructive permeative bone lesion. A subtle radiologic feature, which may help to separate inflammatory and neoplastic processes, is that an inflammatory process typically obliterates fascial planes rather than displaces them. CT may be a useful adjunct in specific circumstances. We generally reserve CT for patients in whom radiographs do not adequately depict the lesion, its pattern of mineralization, or its relationship to the host. This inadequacy typically occurs in areas in which the osseous anatomy is complex, such as the pelvis, shoulder, and paraspinal regions.

MR Imaging
MR imaging has emerged as the preferred modality for evaluating soft-tissue lesions [2,3,4, 28,29,30,31,32,33,34,35]. It provides superior soft-tissue contrast, allows multiplanar image acquisition, obviates iodinated contrast agents and ionizing radiation, and is devoid of streak artifacts commonly encountered with CT [2, 28,29,30]. Although initial investigations maintained that CT was superior to MR imaging in detecting destruction of cortical bone [29, 30, 35], later studies suggest that these two modalities are comparable in this regard [28, 36].

Technique
Lesions should be imaged in at least two orthogonal planes, using conventional T1-weighted and T2-weighted spin-echo MR pulse sequences in at least one of these planes. Standard spin-echo MR images are most useful in establishing a specific diagnosis, when possible, and this technique is both the most reproducible technique and the one most often referenced in the tumor imaging literature. It is the imaging technique with which we are most familiar for tumor evaluation, and it has been established as the standard by which other imaging techniques must be judged [37]. The main disadvantage of spin-echo MR imaging remains the relatively long acquisition times, especially for double-echo T2-weighted MR imaging sequences [37]. Radiologists are most familiar with conventional axial anatomy, and axial T1- and T2-weighted spin-echo MR images should be obtained in almost all cases. The choice of an additional imaging plane or planes varies with the involved body part, the lesion location, and the lesion's relationship to crucial structures. In general, the additional plane is sagittal with anterior or posterior masses and coronal with medial or lateral lesions. Oblique planes may also be a useful adjunct. In these additional planes, it is useful to use a combination of conventional T1- and T2-weighted spin-echo MR images, turbo (fast) spin-echo MR images, gradient MR images, and short tau inversion recovery (STIR) images, as the case requires.

Fast scanning techniques may be useful in the evaluation of soft-tissue masses. They allow shorter imaging times, decreased motion artifacts, and increased patient tolerance, as well as patient throughput [37, 38]. They may add additional information and be helpful in specific instances, although fast scanning techniques have not replaced standard spin-echo imaging. Gradient-echo imaging may be a useful supplement in revealing hemosiderin because of the greater magnetic susceptibility of hemosiderin. In general, susceptibility artifacts related to metallic material, hemorrhage, and air are accentuated on gradient-echo MR images [38] (Fig. 5A,5B,5C,5D). Gradient-echo MR images may also be better, in some instances, for showing the lesion—fat interfaces and depicting small surrounding vessels [39].

View larger version (145K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 5A. —27-year-old woman with foreign body and associated abscess. Oblique radiograph of foot shows irregular opacity (arrow), initially interpreted as calcification.

View larger version (134K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 5B. —27-year-old woman with foreign body and associated abscess. Coronal T1-weighted spin-echo MR image (600/15, TR/TE) shows prominent signal void (asterisk), with "parenthetic" artifact, compatible with foreign body.

View larger version (151K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 5C. —27-year-old woman with foreign body and associated abscess. Corresponding conventional T2-weighted spin-echo MR image (2500/80) shows foreign body (asterisk) with associated inflammatory change.

View larger version (150K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 5D. —27-year-old woman with foreign body and associated abscess. Gradient-echo MR image (15/12, 15° flip angle) shows "blooming" (asterisk) caused by greater magnetic susceptibility.

STIR imaging can be an adjunct in selective cases. STIR imaging produces fat suppression and enhances the identification of abnormal tissue with increased water content and, as a result, is useful to confirm subtle areas of soft-tissue abnormality [40]. This technique increases lesion conspicuity [40, 41], but it typically has a lower signal-to-noise ratio than does spin-echo imaging and is also more susceptible to degradation by motion [37, 40]. Lesions are generally well seen on standard imaging, and, in our opinion, STIR imaging tends to reduce the variations in signal intensities identified on conventional spin-echo MR imaging that are most helpful in tissue characterization.

Fat suppression on T2-weighted MR images is useful to increase lesion-to-background signal intensity differences for high-signal-intensity lesions within the marrow or fatty soft tissue [37]. Fat-suppression imaging is also useful in decreasing or eliminating the MR signal from fat, allowing increased conspicuity of lesions containing paramagnetic substances (such as methemoglobin) on T1-weighted MR images, and revealing contrast enhancement. As with STIR techniques, fat-suppressed T2-weighted MR imaging decreases variations in tumor signal intensities, and we do not use this technique in place of conventional T2-weighted MR imaging.

Field of view is dictated by the size and location of the lesion. In general, a small field of view is preferred; however, the field of view must be large enough to allow evaluation of the lesion and appropriate staging. When an extremity is being evaluated, it is not usually necessary to examine the contralateral extremity for comparison, unless no lesion is detected on initial sequences. It is useful to place a marker over the area of clinical concern to ensure it is appropriately imaged. This becomes important in evaluation of a lesion such as a subcutaneous lipoma or lipomatosis in which the lesion may not be appreciated as distinct from the adjacent adipose tissue (Fig. 6). When small superficial lesions are being evaluated, care should be taken to ensure the marker or patient position does not compress the mass.

View larger version (92K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 6. —54-year-old woman with mild lipomatosis of right lower extremity, who presented with "fullness" around knee. Axial T1-weighted spin-echo MR image (700/16, TR/TE) of both distal thighs shows increased adipose tissue on right as compared with contralateral side. Images of both distal thighs were obtained after no cause for clinical findings was found on axial images of right knee.

MR Imaging Contrast Enhancement
Although there is general agreement on the value of MR imaging in the detection, diagnosis, and staging of soft-tissue tumors and tumorlike lesions, the use of IV contrast material in their evaluation remains controversial. In general, MR imaging contrast agents enhance the signal intensity of many tumors on T1-weighted spin-echo MR images, in some cases enhancing the demarcation between tumor and muscle and tumor and edema as well as providing information on tumor vascularity [42, 43]. In actuality, differentiation between tumor and muscle is usually quite well delineated without contrast-enhanced imaging on T2-weighted MR images, and the accurate distinction between tumor and edema is probably of little practical value. Edema, which is infrequent without superimposed trauma or hemorrhage, is considered to be part of the reactive zone around the neoplasm and, as a result, is removed en bloc with the tumor [44].

Information on tumor enhancement is not without a price. The use of IV contrast material substantially increases the length and cost of the examination. Although contrast-enhanced MR imaging may provide some additional information, it has not been shown to increase lesion conspicuity or to replace conventional T2-weighted MR imaging [45]. Moreover, although the incidence of untoward reaction as a result of contrast material administration is small, it is present. Severe reactions have been reported with both gadopentetate dimeglumine (Magnevist; Berlex Laboratories, Wayne, NJ) and gadoteridol (ProHance; Squibb Diagnostics, Princeton, NJ), including hypotension, laryngospasm, bronchospasm, anaphylactoid reaction, and anaphylactic shock [46,47,48,49,50], as well as a full spectrum of less serious reactions. Jordan and Mintz [51] described a fatal reaction to gadopentetate dimeglumine that was presumed to be caused by anaphylactic reaction with associated bronchospasm. Consequently, gadolinium-enhanced imaging should be reserved for cases in which the results would influence patient care.

One specific circumstance in which gadolinium-enhanced imaging is useful is in the evaluation of hematomas. In the case of hematoma, contrast-enhanced imaging may reveal a small tumor nodule that may have been inapparent within the hemorrhage on conventional MR imaging [52, 53]. Caution is required, however, because the fibrovascular tissue in organizing hematomas may show enhancement [54]. Gadolinium-enhanced imaging has also been used to differentiate solid from cystic (or necrotic) lesions or to identify cystic or necrotic areas within solid tumors, with these necrotic or cystic areas showing no enhancement [42]. This distinction is especially important to guide biopsy and may be difficult or impossible to make on conventional T2-weighted MR images when both tumor and fluid show high signal intensity, well-defined margins, and homogeneous signal intensity. Note, however, that myxoid lesions such as intramuscular myxoma or myxoid liposarcoma, and hyaline cartilage lesions such as synovial chondromatosis may show little or mild enhancement and may mimic cysts or lesions with cystic components (Fig. 7A,7B,7C,7D). In general, sonography is fast and inexpensive and is an ideal method for differentiating solid and cystic lesions when the lesion is in an anatomic location accessible to sonographic evaluation.

View larger version (101K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 7A. —35-year-old man with extra articular synovial chondromatosis, mimicking loculated fluid. Axial T1-weighted spin-echo MR image (763/18, TR/TE) shows large lobulated mass, with signal intensity similar to that of skeletal muscle, in adductor compartment (asterisk).

View larger version (105K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 7B. —35-year-old man with extra articular synovial chondromatosis, mimicking loculated fluid. Corresponding conventional T2-weighted spin-echo MR image (2912/80) shows lesion to have signal intensity greater than that of fat.

View larger version (104K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 7C. —35-year-old man with extra articular synovial chondromatosis, mimicking loculated fluid. Fat-suppressed axial T1-weighted spin-echo MR image (475/18) after contrast material administration shows peripheral and septal enhancement, suggesting loculated fluid.

View larger version (113K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 7D. —35-year-old man with extra articular synovial chondromatosis, mimicking loculated fluid. Radiograph shows nonspecific calcifications (arrowheads) within mass.

MR Imaging Diagnosis
Despite the superiority of MR imaging in delineating soft-tissue tumors, it remains limited in its ability to precisely characterize them, with most lesions showing a nonspecific appearance with prolonged T1 and T2 relaxation times. Consequently, a correct histologic diagnosis is reached solely on the basis of MR imaging studies in only 25-35% of cases [5,6,7]. There are instances, however, in which a specific diagnosis may be made or strongly suspected on the basis of MR imaging features. This diagnosis is usually made on a basis of lesion signal intensity, pattern of growth, location, and associated signs and findings (Figs. 8,9A,9B,10A,10B). The MR imaging appearances of these lesions have been well reported and is not reviewed here. They are listed in Appendix 1.

View larger version (90K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 8. —36-year-old woman with intramuscular lipoma. Coronal T1-weighted spin-echo MR image (600/14, TR/TE) shows mass in right neck, with signal intensity identical to that of fat on all pulse sequences. Diagnosis can be made with confidence on basis of signal intensity.

View larger version (152K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 9A. —41-year-old woman with hemangioma of foot. Axial T1-weighted spin-echo MR image (600/15, TR/TE) shows large lobulated mass with signal intensity similar to that of skeletal muscle infiltrating soft tissue of first interspace and fat (arrow) within interstices of lesion.

View larger version (135K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 9B. —41-year-old woman with hemangioma of foot. Corresponding conventional T2-weighted spin-echo MR image (2500/80) shows lesion to have signal intensity greater than that of fat. Infiltrating pattern of growth, interspersed adipose tissue, and signal intensity are highly characteristic of diagnosis.

View larger version (102K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 10A. —22-year-old man with schwannoma in upper arm. Axial T2-weighted spin-echo MR image (2000/80, TR/TE) shows large mass associated with neurovascular bundle with target sign, characteristic of peripheral nerve sheath tumor.

View larger version (115K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 10B. —22-year-old man with schwannoma in upper arm. Corresponding coronal T1-weighted spin-echo MR image (650/20) shows lesion is contiguous with median nerve (arrow). Signal intensity of lesion, target sign, and location allow diagnosis of peripheral nerve sheath tumor to be made with confidence.

More commonly, MR imaging may reveal a nonspecific appearance. In such cases, it is often not possible to establish a meaningful differential diagnosis or reliably determine if a lesion is benign or malignant. In such situations, it is useful to formulate a suitably ordered differential diagnosis on the basis of a knowledge of tumor prevalence, patient age, and lesion anatomic location. This diagnosis can be further refined by considering clinical history and radiologic features, such as pattern of growth, signal intensity, and localization (subcutaneous, intramuscular, or intermuscular). The most common malignant and benign lesions, by tumor location and patient age, have been previously published [13, 14]. Appendix 2 includes the localization of common lesions (Fig. 11A,11B,11C).

View larger version (97K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 11A. —34-year-old man with malignant fibrous histiocytoma of thigh, who presented with slowly growing painless mass. Axial conventional T2-weighted spin-echo MR image (1800/80, TR/TE) shows relatively well-defined mass in anterior compartment of thigh with nonspecific high signal intensity.

View larger version (105K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 11B. —34-year-old man with malignant fibrous histiocytoma of thigh, who presented with slowly growing painless mass. Corresponding unenhanced (B) and contrast-enhanced (C) axial T1-weighted spin-echo MR images (400/20) show uniform enhancement. Although appearance of lesion is nonspecific, malignant fibrous histiocytoma is statistically most likely diagnosis based on patient age and lesion location.

View larger version (101K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 11C. —34-year-old man with malignant fibrous histiocytoma of thigh, who presented with slowly growing painless mass. Corresponding unenhanced (B) and contrast-enhanced (C) axial T1-weighted spin-echo MR images (400/20) show uniform enhancement. Although appearance of lesion is nonspecific, malignant fibrous histiocytoma is statistically most likely diagnosis based on patient age and lesion location.

Benign Versus Malignant
There is general agreement on the diagnostic value of MR imaging in many cases, but the issue of whether MR imaging can reliably distinguish benign from malignant is much less clear. One study has suggested that MR imaging can differentiate benign from malignant masses in more than 90% of cases on the basis of the morphology of the lesion [6]. Criteria used for benign lesions included smooth well-defined margins, small size, and homogenous signal intensity, especially on T2-weighted MR images. Other studies, however, note that malignant lesions may appear as smoothly marginated homogeneous masses and MR imaging cannot reliably distinguish benign from malignant processes [2,3,4,5, 7, 42]. This discrepancy likely reflects differences in the studied populations.

When the MR images of a lesion are not sufficiently characteristic to suggest a specific diagnosis, a conservative approach is warranted. Malignancies, by virtue of their very nature and potential for autonomous growth, are generally larger and more likely to outgrow their vascular supply with subsequent infarction, necrosis, and heterogeneous signal intensity on T2-weighted spin-echo MR imaging. Consequently, the larger a mass is, the greater its heterogeneity, the greater is the concern for malignancy. Only 5% of benign soft-tissue tumors exceed 5 cm in diameter [55, 56]. In addition, most malignant tumors are deep, whereas only about 1% of all benign soft-tissue tumors are deep [55, 56]. Although these figures are based on surgical series, these trends are likely still valid for radiologists.

When sarcomas are superficial, they generally have a less aggressive biologic behavior than do deep lesions [57]. As a rule, most malignancies grow as deep space-occupying lesions, enlarging in a centripetal fashion [57], pushing rather than infiltrating adjacent structures (although clearly there are exceptions to this general rule). As sarcomas enlarge, a pseudocapsule of fibrous connective tissue is formed around them by compression and layering of normal tissue, associated inflammatory reaction, and vascularization [57]. Generally, they respect fascial borders and remain within anatomic compartments until late in their course [57]. It is this pattern of growth that gives most sarcomas relatively well-defined margins, in distinction to the general concepts of margins used in the evaluation of osseous tumors.

Although our experience with metastatic carcinoma to soft tissue is limited, we have generally found these lesions appear more infiltrative with ill-defined margins and often violating fascial planes and anatomic compartments. This pattern of growth is quite different from that seen in most primary soft-tissue tumors.

Increased signal intensity in the skeletal muscle surrounding a musculoskeletal mass on T2-weighted spin-echo MR images or other fluid-sensitive sequences (i.e., STIR) has also been suggested as a reliable indicator of malignancy [58, 59]. These results are based on studies in which both bone and soft-tissue lesions were evaluated. Although this increased signal intensity may be seen with malignancy, in our experience, this finding is quite nonspecific. In fact, prominent high signal intensity surrounding a soft-tissue mass more commonly suggests an inflammatory process, abscess, myositis ossificans, local trauma, hemorrhage, biopsy, or the effect of radiation therapy rather than a primary soft-tissue neoplasm (Fig. 12A,12B).

View larger version (120K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 12A. —14-year-old boy with myositis ossificans in forearm. Axial fast spin-echo T2-weighted spin-echo MR image (2600/80, TR/TE) shows poorly defined mass in extensor compartment of forearm and adjacent to ulna. Lesion predominantly involves extensor carpi ulnaris, although there is abnormal signal in and between adjacent muscles.

View larger version (113K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 12B. —14-year-old boy with myositis ossificans in forearm. Corresponding axial T1-weighted spin-echo MR image (650/20) shows only minimal signal alteration with effacement of subcutaneous adipose tissue (arrow).

Gadolinium-enhanced imaging has also been proposed as useful in differentiating benign and malignant soft-tissue lesions, with malignant lesions showing a greater enhancement as well as a greater rate of enhancement [43, 45, 47, 60]. Enhancement reflects tissue vascularity and tissue perfusion, and, in general, the rate of enhancement in malignant lesions is greater than that seen in benign lesions. However, the overlap between benign and malignant is so great that, in our opinion, this finding is of little practical value in any specific case [60]. When a lesion has a nonspecific MR imaging appearance, one is ill-advised to suggest a lesion is benign or malignant solely on the basis of its MR imaging characteristics and rate or degree of enhancement.

DeSchepper et al. [61] performed a multivariate statistical analysis of 10 imaging parameters, individually and in combination. These researchers found that malignancy was predicted with the highest sensitivity when a lesion had a high signal intensity on T2-weighted MR images, was larger than 33 mm in diameter, and had a heterogeneous signal intensity on T1-weighted MR images. Signs that had the greatest specificity for malignancy included tumor necrosis, bone or neurovascular involvement, and mean diameter of more than 66 mm (Figs. 13A,13B and 14A,14B,14C).

View larger version (114K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 13A. —15-year-old girl with rhabdomyosarcoma of leg. Sagittal T1-weighted spin-echo MR image (641/16, TR/TE) shows large mass with bone invasion.

View larger version (131K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 13B. —15-year-old girl with rhabdomyosarcoma of leg. Corresponding contrast-enhanced MR image (641/16) shows nonenhancing area compatible with necrosis. Bone invasion and necrosis are both specific for malignancy. Note nodal involvement (arrows).

View larger version (151K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 14A. —57-year-old woman with liposarcoma of thigh. Axial fast spin-echo T2-weighted MR image (3200/102, TR/TE) shows large mass with mixed intermediate signal intensity.

View larger version (108K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 14B. —57-year-old woman with liposarcoma of thigh. Corresponding coronal unenhanced (B) and contrast-enhanced (C) T1-weighted spin-echo MR images (600/16) show adipose tissue within lesion, compatible with fat differentiation. Enhancement in portions of tumor is extensive. Large size and deep location with adipose differentiation suggest diagnosis of liposarcoma.

View larger version (107K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 14C. —57-year-old woman with liposarcoma of thigh. Corresponding coronal unenhanced (B) and contrast-enhanced (C) T1-weighted spin-echo MR images (600/16) show adipose tissue within lesion, compatible with fat differentiation. Enhancement in portions of tumor is extensive. Large size and deep location with adipose differentiation suggest diagnosis of liposarcoma.

Staging
Simply stated, the purpose of a staging system is to provide a standard manner in which to readily communicate the state of a malignancy, defining the extent of the local and distant tumor. Local staging is best accomplished using MR imaging, which can accurately depict the anatomic spaces (compartments) involved by the tumor [62]. A review of compartment anatomy is beyond the scope of this article, but accurate staging is critical for optimum patient care and planning of percutaneous biopsy. It must be emphasized that coordination with the orthopedic surgeon who will perform the definitive surgery is essential before biopsy.

Summary

Top Introduction General Concepts Preliminary Evaluation Summary References

 
MR imaging is the preferred modality for the evaluation of a soft-tissue mass after radiography. The radiologic appearance of certain soft-tissue tumors or tumorlike processes, such as myositis ossificans, fatty tumors, hemangiomas, peripheral nerve sheath tumors, pigmented villonodular synovitis, and certain hematomas may be sufficiently unique to allow a strong presumptive radiologic diagnosis. It must be emphasized that MR imaging cannot reliably distinguish between benign and malignant lesions and when radiologic evaluation is nonspecific, one is ill-advised to suggest that a lesion is benign or malignant solely on its MR imaging appearance. When a specific diagnosis is not possible, knowledge of tumor prevalence by location and age, with appropriate clinical history and radiologic features, can be used to establish a suitably ordered differential diagnosis.

Top Introduction General Concepts Preliminary Evaluation Summary References

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