On radiographs, soft tissue masses often have subtle or no visible findings. However, several situations exist where radiographs can be helpful. For example, secondary bone involvement may manifest as extrinsic bone erosion, cortical destruction, and/or periosteal reaction, all of which are well-depicted by radiographs. Lipomatous soft tissue masses may exhibit lower density than surrounding soft tissues, or similar density to subcutaneous fat, and therefore the fatty content of these lesions may be detected on radiographs ( Fig. 3.1 ).

A 61-year-old female with a parosteal lipoma. Lateral radiograph of the left femur demonstrates a mass along the surface of the femur with decreased density compared to the surrounding muscles ( arrow ) that contains an osseous excrescence ( arrowhead ) attached to the cortical surface. The radiographic appearance is pathognomonic for a parosteal lipoma.

Matrix or mineralization is also well seen on radiographs; for example, osteoid matrix in extraskeletal osteosarcoma ( Fig. 3.2 ), chondroid matrix in synovial chondromatosis, peripheral zonal ossification in myositis ossificans, and phleboliths in venous malformations. Radiographs can also be helpful to correlate findings with other modalities, such as clarifying whether T2 hypointensity on MRI is secondary to mineralization, as opposed to hemosiderin or fibrous tissue. As such, radiographs can play an important role in the radiological evaluation of soft tissue tumors.

A 62-year-old male with an extraskeletal osteosarcoma. Frontal radiograph of the right hip demonstrates a soft tissue mass containing amorphous mineralization ( arrow ) typical of osteoid matrix.

In recent years, ultrasound has gained popularity for the evaluation of soft tissue neoplasms, particularly in Europe and Asia, where ultrasound is the preferred modality for the initial evaluation of palpable soft tissue masses. Ultrasound has several advantages, including being readily available, noninvasive, cost-effective, and free from ionizing radiation. It is useful to evaluate the size and location of lesions, and to differentiate between cystic and solid soft tissue lesions ( Fig. 3.3 ). While some studies have demonstrated that certain ultrasound characteristics, including a longest diameter of >46 mm, hypervascularity with internal vessels, ill-defined margins, and deep location are more suggestive of malignant soft tissue lesions, ultrasound has overall low specificity for further characterizing soft tissue masses. However, it may be useful to guide percutaneous biopsy, especially for superficial lesions.

A 77-year-old female with an undifferentiated pleomorphic sarcoma. Doppler ultrasound image demonstrates a hypoechoic mass within the superficial soft tissues of the right thigh ( arrow ) with internal blood flow, confirming that the lesion is a solid mass.

The advent of multidetector CT has allowed for multiplanar reformatted thin-slice imaging and three-dimensional (3D) rendering of images. Faster examination times with CT than with MRI lead to improved patient tolerance and decreased patient motion artifact. Additionally, CT has a high sensitivity for detecting calcifications and ossification that may be missed on MRI or may be too subtle to detect on radiographs. CT is superior to MRI in detecting cortical invasion ( Fig. 3.4 ), but less accurate in determining medullary involvement. CT angiography is also invaluable for evaluating the vascular anatomy and has supplanted conventional angiography for this purpose. A disadvantage of CT is its exposure of patients to ionizing radiation. In addition, CT has lower soft tissue contrast resolution than MRI, which may limit the assessment of fine anatomic detail (for example, a tumor’s relationship to nearby neurovascular structures).

An 80-year-old female with an undifferentiated pleomorphic sarcoma. Coronal T2-weighted, fat-saturated MRI ( A ) demonstrates an infiltrative soft tissue mass ( arrows ) along the lateral right knee with focal increased T2 signal in the subjacent tibia ( arrowhead ). This finding could be seen with osseous invasion or reactive bone marrow edema. Corresponding coronal CT image ( B ) demonstrates focal bone destruction in the lateral tibia ( arrowhead ) subjacent to the mass ( arrow ), consistent with early osseous invasion. Findings were confirmed on surgical resection.

MRI plays an invaluable role in the evaluation of soft tissue tumors and has become the modality of choice for orthopedic oncologic imaging. MRI provides an accurate assessment of the anatomic extent of a lesion, by defining fascial planes and demonstrating a tumor’s relationship to bone and neurovascular structures. MRI also provides superior soft tissue contrast resolution compared to CT, and unlike CT, it does not expose patients to ionizing radiation. Although, in many instances, MRI does not provide the ability to make a specific histologic diagnosis, it can provide a reasonable differential diagnosis based on signal characteristics, morphology, multifocality, and the anatomic location of the lesion, especially when considered in combination with the relevant clinical history. The appearance of benign and malignant soft tissue tumors may be similar on MRI, although a lesion should not be classified as “benign” unless it can be definitively characterized based on specific MRI criteria. When a soft tissue mass cannot be definitively characterized based on MRI features, the location of the mass, and patient age, it must be classified as an “indeterminate mass” and clinically managed as a malignancy until proven otherwise.

MRI is also useful for surgical planning and follow-up. In patients with soft tissue sarcomas, MRI accurately delineates the tumor margins, which provides the necessary information to optimize the surgical approach. Following treatment, MRI provides an accurate assessment of postoperative complications and is sensitive for detecting local recurrence and regional metastatic disease. However, the overlapping appearance of tumor recurrence, edema, hemorrhage, and postoperative inflammatory change on MRI may complicate the interpretation of follow-up images. Another shortcoming of MRI is its unreliability in detecting mineralization, which is why it is important to interpret MRI in concert with radiographs or CT.

Many pulse sequences are utilized to evaluate soft tissue tumors on MRI. T1- and T2-weighted fast spin echo (FSE) sequences are foundational to all MRI protocols for soft tissue lesion evaluation. Fat suppression is an important tool in the MRI evaluation of soft tissue tumors. Fat suppression can be used to characterize fat-containing tumors, improve the dynamic range for soft tissue contrast display, and more accurately discriminate between signal differences for different tissues on T1- and T2-weighted images. This can be accomplished through a variety of means, most commonly chemical fat-saturation, or Dixon-based fat suppression techniques. A short tau inversion recovery (STIR) sequence can also be used to suppress signal from fat. STIR sequences also eliminate chemical shift artifact at fat–water interfaces, which can lead to improved lesion-to-background contrast. When fat suppression techniques are utilized, a non-fat-suppressed T1-weighted image should also be acquired; this combination of sequences can be used to differentiate between intralesional fat and hemorrhage and to identify and characterize marrow signal changes in the adjacent bones.

There is significant overlap in the signal characteristics of many soft tissue tumors on MRI. Most soft tissue tumors demonstrate signal intensity similar to or lower than muscle on T1-weighted images and signal intensity higher than muscle on T2-weighted images ( Fig. 3.5 ). In cases where a tumor demonstrates signal characteristics specific to that lesion, the signal characteristics are a direct reflection of the histologic composition of the mass. The ability to make a tissue-specific diagnosis is more common for benign lesions, but characteristic imaging features can also be seen with a few malignant soft tissue tumors. Gadolinium-based contrast can be helpful for characterizing soft tissue masses. For example, gadolinium can be utilized to differentiate between solid and cystic lesions, assess the degree of necrosis in a mass, and enable image-guided biopsy to avoid necrotic regions.

A 56-year-old female with a high-grade, undifferentiated pleomorphic sarcoma in the lateral left thigh. Axial T1-weighted ( A ), T2-weighted, fat-saturated ( B ), and postcontrast fat-saturated ( C ) MRI demonstrates a large soft tissue mass that is T1 isointense, heterogeneously T2 hyperintense, and heterogeneously enhancing ( arrows ). Note the perilesional edema laterally ( open arrowhead , B ) and focal invasion through the muscular fascia medially ( solid arrowheads , B and C ). The imaging features are suggestive of a high-grade sarcoma, but not specific for a particular histologic diagnosis.

Although anatomically based imaging modalities are considered the standard of care for the diagnosis of soft tissue neoplasms, functional imaging such as PET plays an increasingly important role. With PET imaging, a radiotracer is administered, most commonly [ ¹⁸ F] fluorodeoxyglucose (FDG), which is an analogue of glucose that accumulates to a greater degree in malignant cells than in noncancerous cells. In PET/CT, CT is used for attenuation correction and anatomic localization, allowing the PET and CT images to be overlaid and fused, which allows a combination of functional and morphologic information. Standardized uptake value (SUV) is often used to characterize lesions on PET/CT; a higher SUV indicates increased metabolic activity. However, many benign conditions including fractures, infectious or inflammatory processes, and several benign neoplasms may demonstrate increased uptake on PET, simulating malignant lesions. The primary benefit of PET/CT is its whole-body functional evaluation. As such, PET/CT is ideal for staging malignant soft tissue tumors. PET/CT is also helpful in assessing response to therapy and monitoring for residual or recurrent disease after treatment ( Fig. 3.6 ). In general, effective therapy leads to decreased metabolic activity, which results in decreased FDG uptake.

A 65-year-old male with an undifferentiated pleomorphic sarcoma in the posterior right chest wall. Maximum-intensity projection FDG PET image before therapy ( A ) demonstrates intense FDG uptake within the mass ( arrow ). FDG PET MIP image after neoadjuvant chemoradiation ( B ) demonstrates decreased size and reduction of FDG uptake in the mass ( arrow ) consistent with positive treatment response.

PET/MRI allows for simultaneous PET and MRI acquisition and fusion. Potential advantages are similar to those afforded by PET/CT, with additional benefits including decreased exposure to ionizing radiation (by replacing CT with MRI) and higher soft tissue contrast. Decreased radiation exposure may be particularly important for pediatric patients and young adults, who may require long-term oncologic follow-up and multiple radiologic studies. Perhaps most importantly, the simultaneous acquisition of both PET and MRI data allows for a powerful evaluation of multiple functional and morphologic imaging parameters from both PET and MRI, with perfect spatial and temporal alignment.

Lipomas are the most common soft tissue neoplasm, and thus frequently encountered in clinical practice. Most simple lipomas occur in the subcutaneous tissues, but lipomas may also occur in deeper locations, including intramuscular, intermuscular, and parosteal types. On MRI, a typical lipoma will demonstrate homogeneous fat signal intensity on all pulse sequences, with signal characteristics similar to subcutaneous fat, complete saturation of signal on fat-suppressed sequences, and no postcontrast enhancement ( Fig. 3.8 ). A subtle capsule and thin septations (<2 mm) are frequently seen on MRI. CT shows homogeneous fat attenuation and may also demonstrate thin septations and a capsule. Intramuscular lipomas can present a diagnostic challenge because interdigitating muscle fibers can mimic the thickened septations that are characteristic of atypical lipomatous tumors. However, the fibers may be recognized by the fact that they follow the signal intensity of muscle on all sequences, and importantly do not display postgadolinium enhancement. The preserved muscle fibers are also usually well-recognized on longitudinal planes of imaging, given that their orientation parallels the surrounding muscle ( Fig. 3.9 ).

A 55-year-old female with a right pelvic lipoma. Axial contrast-enhanced CT ( A ) and axial MRI, including T1-weighted ( B ), T2-weighted, fat-saturated ( C ), and postcontrast fat-saturated ( D ) sequences demonstrate a well-circumscribed mass comprised entirely of fat ( arrows ). Note the low attenuation on CT and hyperintense T1 MRI signal, with corresponding suppression on the fat-saturated images ( C , D ). There are no areas of nonlipomatous signal or enhancement within the mass.

A 70-year-old female with an intramuscular lipoma in the biceps muscle. Axial ( A ) and sagittal ( B ) T1-weighted MRI demonstrates a T1 hyperintense fatty mass within the biceps muscle ( arrows ) with preserved T1 hypointense muscle fibers coursing through the mass. On axial sequences, the muscle fibers can appear somewhat nodular ( arrowhead , A ), and are easier to recognize on the longitudinal plane ( arrowhead , B ).

Lipoma variants are defined as lipomatous tumors with variable quantities of other tissue types; for example, fibrous tissue (fibrolipoma), blood vessel elements (angiolipoma), and smooth muscle (myolipoma). The MRI appearance of these tumors will vary depending on the proportion and type of nonlipomatous tissue. Unfortunately, lipomas that grow to a large size or that occur in regions prone to trauma or mechanical irritation may develop areas of fat necrosis, which can exactly mimic the nonlipomatous elements in lipoma variants and atypical lipomatous tumors ( Fig. 3.10 ).

An 81-year-old male with a lipoma in the posterior left thigh. Lateral radiograph ( A ) demonstrates a large, low-density soft tissue mass ( arrows ). Coronal T1 ( B ) and T2-weighted, fat-saturated ( C ) MRI demonstrates a predominantly fatty mass containing areas of hypointense reticular and nodular T1 and T2 signal within the superior aspect of the mass ( arrowheads ), which correlates with dystrophic calcification on radiographs ( arrowhead , A ). The features are typical of a lipoma with fat necrosis.

Hibernoma is a lipoma variant in which brown fat predominates, and this composition results in unique imaging features. On MRI, brown fat typically displays slightly decreased T1 signal relative to subcutaneous fat, incomplete signal loss on fat-suppressed images, and variable enhancement ( Fig. 3.11 ). These tumors often exhibit curvilinear branching fibrous or fibrovascular septations, and prominent internal vessels; though the absence of prominent internal vessels does not exclude the diagnosis. On FDG PET/CT, these lesions demonstrate variable FDG uptake ( Fig. 3.11 ), which can be intense, in ranges often seen with high-grade malignant lesions.

A 71-year-old male with a benign hibernoma. Sagittal T1-weighted ( A ), T2-weighted, fat-saturated ( B ), and postcontrast fat-saturated ( C ) MRI demonstrates a lipomatous left inguinal mass ( arrows ). Compared to the overlying subcutaneous fat, the mass exhibits slight decreased T1 signal, incomplete fat suppression, and mild heterogeneous contrast enhancement. Corresponding sagittal fused FDG PET/CT image ( D ) demonstrates heterogeneous FDG activity ( arrow ).

Solitary benign peripheral nerve sheath tumors include two main groups—schwannoma and neurofibroma. Together, these lesions constitute about 10% of all benign soft tissue tumors. Unlike many other soft tissue tumors, neurogenic tumors have characteristic imaging and clinical features that may be helpful for diagnosis.

Schwannomas , which are slightly less common than neurofibromas, are slow-growing tumors arising from the outer sheath of a peripheral nerve, typically eccentric to the nerve fibers, and may be diagnosed prospectively, especially if proximal and distal nerve fibers are visualized on MRI ( Fig. 3.12 ). However, it may be difficult to identify the associated nerve when tumors arise from small nerve branches. These tumors most frequently occur along the flexor surfaces of the extremities, mediastinum, retroperitoneum, and head and neck. Long-standing lesions known as ancient schwannomas may demonstrate cystic changes, calcification, hemorrhage, and fibrosis ( Fig. 3.13 ), which may be mistaken for features of more aggressive tumors on imaging.

A 78-year-old male with a benign schwannoma. Sagittal T1-weighted ( A ), T2-weighted, fat-saturated ( B ), and postcontrast fat-saturated ( C ) MRI demonstrates a well-circumscribed oval mass with intermediate TI and high T2 signal, and avid enhancement ( arrows ) along the right tibial nerve ( arrowhead , A ). While the mass is slightly heterogeneous ( B ), there is no internal cystic change, and the margins are well-circumscribed. There is a thin rim of fat at the superior and inferior margins of the mass ( open arrowheads , A ) consistent with the “split fat” sign.

A 59-year-old male with a benign presacral schwannoma. Axial T1-weighted ( A ), T2-weighted, fat-saturated ( B ), and postcontrast fat-saturated ( C ) MRI demonstrates a large, heterogeneous mass in the presacral space ( arrows ). Note the areas of internal high T1 and high T2 signal without enhancement consistent with cystic change and internal blood products or proteinaceous content. The mass is well-circumscribed, and there is no perilesional edema or enhancement. The size and heterogeneity raise the possibility of a malignant peripheral nerve sheath tumor, but biopsy yielded a diagnosis of benign schwannoma.

Localized or solitary neurofibromas are also slow-growing lesions with a centrally entering and exiting nerve, giving a fusiform shape to the tumors. Neurofibromas can occur anywhere in the body, including the skin, subcutaneous tissues, and viscera. The growth of neurofibromas is usually slow, but faster growth may be seen during pregnancy and puberty or in cases of malignant transformation. The three types of neurofibromas are localized, diffuse, and plexiform. All three are often associated with neurofibromatosis type 1 (NF1). Localized neurofibroma is most frequently associated with NF1, but plexiform neurofibroma is essentially pathognomonic of NF1 ( Fig. 3.14 ). Plexiform neurofibromas usually develop during childhood and adolescence and can precede the appearance of cutaneous neurofibromas. Because of their large size, plexiform neurofibromas typically extend beyond the epineurium into the surrounding tissue.

A 12-year-old girl with NF1 and a plexiform neurofibroma. Sagittal T1-weighted ( A ), T2-weighted, fat-saturated ( B ), and postcontrast fat-saturated ( C ) MRI demonstrates a well-circumscribed oval T1 intermediate, T2 hyperintense, and heterogeneously enhancing mass arising from the ulnar nerve in the distal forearm ( arrows ). Note the central T2 hypointense signal ( open arrowhead , B ) consistent with a “target” sign. Proximally, the ulnar nerve is diffusely enlarged, with multifocal nodularity ( arrowheads ) and similar signal characteristics to the dominant distal mass. The features are consistent with a plexiform neurofibroma.

Schwannomas and neurofibromas share many MRI features. Both present as well-defined, often elongated masses that rarely exceed 5 cm in diameter. Paraspinal lesions typically have a dumbbell shape, which may enlarge the surrounding neural foramen through pressure erosion over time. Continuity with a nerve, the most helpful diagnostic MRI feature, is usually evident with lesions that arise from larger nerves ( Fig. 3.12 ). Differentiation between schwannomas and neurofibromas based on the position of the tumor relative to the nerve (eccentric vs. central) is often difficult, especially when they involve smaller nerves. Intramuscular neurogenic tumors may be surrounded by a thin rim of fat; this creates the “split fat sign” on T1-weighted MRI images, especially along the long axis of the extremity, because of slow growth over an extended period ( Fig. 3.12 ). Most benign neurogenic tumors are isointense or slightly hyperintense to muscle on T1-weighted images and are markedly hyperintense with a variable degree of heterogeneity on T2-weighted images. On fluid-sensitive sequences, these tumors may exhibit high signal intensity in the periphery and low to intermediate signal intensity in the center. This appearance, the “target sign,” has been shown to correspond histologically to a central area of dense collagenous stroma, surrounded by peripheral myxomatous tissue. Although initially thought to be pathognomonic of neurofibromas, the target sign has also been observed in schwannomas ( Fig. 3.15 ), and even malignant peripheral nerve sheath tumors, and as such this sign is not sensitive or specific for differentiating between peripheral nerve lesions. MRI of neurogenic tumors may also demonstrate the presence of multiple small, ringlike structures within the tumor which has been described as the “fascicular sign” and represents the fascicular bundles.

A 56-year-old male with a schwannoma. Axial ( A ) and sagittal ( B ) T2-weighted, fat-saturated MRI demonstrates a fusiform mass arising from the left sural nerve ( arrows ) with central T2 hypointense signal ( arrowheads ) and peripheral T2 hyperintensity consistent with the “target sign.”

On contrast-enhanced images, small neurogenic tumors often show intense and relatively homogeneous enhancement ( Fig. 3.12 ), whereas large lesions may demonstrate predominantly peripheral, central, or heterogeneous nodular enhancement ( Fig. 3.13 ). Atrophy of innervated muscles distal to the tumor is another imaging finding that may be seen with neurogenic tumors.

Several classification systems have been proposed for vascular anomalies. In 2018, the International Society for the Study of Vascular Anomalies updated its classification system. This classification system divides vascular anomalies generally into two categories: “vascular tumors,” which include proliferative changes of endothelial cells, and “vascular malformations,” which are structural abnormalities without endothelial proliferation. Vascular tumors can be subdivided based on behavior, including benign (e.g., infantile hemangioma, congenital hemangioma, and tufted angioma), locally aggressive or borderline (e.g., kaposiform hemangioendothelioma and Kaposi’s sarcoma), and malignant (e.g., angiosarcoma and epithelioid hemangioendothelioma), while vascular malformations can be subdivided based on the type of blood vessels with anomalies (capillary, lymphatic, venous, and arteriovenous).

Venous malformations are the most common type of peripheral vascular malformation and are commonly known as soft tissue hemangiomas . Although they are not true vascular tumors with endothelial proliferation, these lesions bear special mention, as they are common and may mimic benign and malignant soft tissue lesions. These lesions can be cutaneous, subcutaneous, intramuscular, or synovial. Extremities are the most common location for venous malformations, accounting for almost two-thirds of vascular malformations in these areas. Venous malformations are present at birth, but symptoms usually do not appear until late childhood or early adulthood. These lesions often present as poorly marginated masses comprised of T2 hyperintense tubular vascular structures with an intervening stroma comprised of variable quantities of fat and fibrous tissue. Internal fluid–fluid levels are rare and likely caused by hemorrhage and/or slow vascular flow. Although not sensitive, the presence of phleboliths within a lesion is specific, and can confirm the diagnosis of a venous malformation. Phleboliths can be difficult to recognize on MRI, appearing as small low-signal intensity foci on all pulse sequences, but radiographs are more sensitive for their detection ( Fig. 3.16 ). This provides another example of how radiographic correlation can be helpful when interpreting MRI of soft tissue masses. When a soft tissue mass is identified that contains tortuous vascular structures and intervening fat with calcified phleboliths, the imaging features are pathognomonic for a venous malformation ( Fig. 3.16 ). However, in some cases soft tissue hemangiomas can present with relatively nonspecific features ( Fig. 3.17 ), which may make these lesions difficult to distinguish from other benign and malignant soft tissue masses, including soft tissue sarcoma.

A 46-year-old female with a soft tissue hemangioma. Frontal radiograph ( A ) demonstrates several foci of rounded mineralization in the soft tissues along the medial left knee ( dotted oval ). The larger foci have lucent centers, typical of phleboliths. Corresponding coronal T1 ( B ), T2-weighted, fat-saturated ( C ), and postcontrast fat-saturated ( D ) MRI images demonstrate a mass in the subcutaneous fat comprised of T1 hypointense, T2 hyperintense, and enhancing tortuous vessels and soft tissue lobules with intervening fat ( dotted ovals ). There is a punctate hypointense focus within the lesion ( arrowheads , C and D ) that corresponds to a calcified phlebolith on radiograph. The combined imaging findings are specific for a soft tissue hemangioma.

A 17-year-old male with a soft tissue hemangioma. Coronal ( A ) and axial ( C ) T2-weighted, fat-saturated, and axial T1-weighted ( B ) MRI demonstrates a poorly marginated, infiltrative T2 hyperintense mass within the plantar musculature of the left foot with interspersed linear and hazy foci of fat. The imaging features were not entirely specific, and subsequent biopsy yielded a diagnosis of a soft tissue hemangioma.

A soft tissue myxoma is a benign neoplasm arising from fibroblasts that produces an excessive amount of mucopolysaccharide. The vast majority are intramuscular, and they are usually solitary. If multiple myxomas exist, they are almost always associated with fibrous dysplasia, also known as Mazabraud syndrome .

The intrinsic CT and MRI characteristics of soft tissue myxomas are similar to those of a cyst, as myxomas demonstrate high mucin content, a large amount of water, and a low amount of collagen. The large amount of water in myxomas accounts for the hypoechoic appearance of these tumors on ultrasound, low attenuation similar to simple fluid on CT; low signal intensity on T1-weighted MRI images relative to skeletal muscle; and markedly high signal intensity on T2-weighted images ( Fig. 3.18 ). Most cases demonstrate a thin rim of fat around the lesion, most often at the superior and inferior margins. On fluid-sensitive sequences, many lesions also exhibit feathery increased signal in the surrounding soft tissues, due to leakage of myxomatous content. Given how closely these lesions may mimic ganglion cysts, bursae, or other nonneoplastic cystic lesions, contrast administration is especially useful to demonstrate internal enhancement and reflect the solid but hypocellular nature of myxomas. Enhancement can be variable, which can range from mild and diffuse to a thick peripheral and septal pattern ( Fig. 3.18 ). Myxomas, like other myxoid soft tissue lesions, most often display only low-level uptake on FDG PET/CT, though moderate uptake may occasionally be seen.

A 62-year-old male with an intramuscular myxoma. Coronal T1 ( A ), T2-weighted, fat-saturated ( B ), and postcontrast fat-saturated ( C ) MRI demonstrates an intramuscular mass ( arrows ) in the lateral left thigh that is T1 hypointense relative to skeletal muscle and T2 hyperintense similar to fluid, with mild heterogeneous enhancement. Note the prominent perilesional edema ( arrowheads , B ), thought to be due to leakage of myxomatous content into the surrounding soft tissues.

Tenosynovial giant cell tumor (TSGCT) is a benign tumor that arises from the synovium, and therefore, may occur in any synovial-lined structure, including joints, tendon sheaths, and bursae. There are both localized (L-TSGCT) and diffuse (D-TSGCT) forms. The localized type is characterized by a typically small, well-circumscribed mass that may be intra- or extraarticular. The extraarticular form has been referred to as giant cell tumor of the tendon sheath in the past. The diffuse form, formerly called pigmented villonodular synovitis , most commonly presents as an intraarticular mass with an infiltrative and multifocal growth pattern.

Intraarticular D-TSGCT is usually a monoarticular process of the large joints, affecting the knee in 80% of cases and also known to affect the hip, ankle, shoulder, and elbow joints. ^, MRI is the preferred modality for diagnosing TSGCT because of its specific imaging features that aid in distinguishing it from other synovial processes. Intraarticular D-TSGCT presents as areas of plaque-like and nodular synovial thickening with low signal intensity on T1- and T2-weighted sequences ( Fig. 3.19 ), and an associated joint effusion. Magnetic susceptibility artifact (“blooming”) within the affected joint space on gradient echo-recalled sequences is also characteristic of TSGCT, a result of hemosiderin in the lesion ( Fig. 3.19 ). ^, Areas of variable signal intensity, with foci of higher T1 and T2 signal that indicate relatively low concentrations of hemosiderin, may represent more recent hemorrhage. While variable, moderate to marked enhancement of the masses and synovium is common because of the hypervascular nature of the disease. The presence of erosions depends on the disease extent and size of the joint affected; but when erosions are present, they present as areas of extrinsic scalloping, typically with sclerotic margins. L-TSGCT may be intra- or extraarticular and appears as a discrete soft tissue nodule or mass ( Fig. 3.20 ). While L-TSGCT may present with similar signal characteristics as D-TSGCT (low T1 and low T2 signal with blooming), the localized forms often demonstrate intermediate, and even bright T2 signal, and less prominent blooming on gradient echo sequences.

A 54-year-old female with intraarticular D-TSGCT of the knee. Sagittal proton density ( A ), T2-weighted, fat-saturated ( B ), postcontrast fat-saturated ( C ), and gradient echo ( D ) MRI demonstrates a proliferative synovial process characterized by multifocal nodularity and diffuse synovial thickening with hypointense signal on all sequences ( arrows ), and gradient echo blooming ( dotted ovals , D ).

A 24-year-old female with intraarticular L-TSGCT of the knee. Sagittal T1 ( A ), T2-weighted, fat-saturated ( B ), and postcontrast fat-saturated ( C ) MRI demonstrates a hypointense nodule with mild enhancement along Hoffa’s fat pad in the anterior knee joint ( arrows ).

All forms of TSGCT are FDG-avid on PET/CT, and may have intense uptake, mimicking a primary malignant tumor or metastatic disease. It is therefore a frequently encountered pitfall on FDG PET/CT ( Fig. 3.21 ). An important distinguishing feature is the location, as intraarticular or periarticular metastases are rare. A differing response to therapy on PET/CT can also be a critical feature. For example, if an FDG-avid intraarticular mass has stable FDG uptake, and there is improving metastatic disease at other sites, the diagnosis may be suggested on PET/CT and subsequently confirmed by MRI.

A 77-year-old male with extraarticular L-TSGCT along the popliteus tendon sheath. Sagittal T2-weighted, fat-saturated MRI ( A ) demonstrates an oval, well-circumscribed hypointense nodule along the popliteus tendon sheath ( arrow ). Sagittal fused PET/CT ( B ) and maximum intensity projection PET ( C ) images demonstrate intense FDG uptake within the mass ( arrows ).

The radiologic differential for TSGCT usually includes synovial chondromatosis, a proliferative synovial neoplasm of joints, tendon sheaths, and bursa. Synovial chondromatosis usually manifests as multiple small mineralized intraarticular bodies of fairly uniform size that are readily identifiable on radiographs ( Fig. 3.22 ). Calcification or ossification rarely occurs with TSGCT, and therefore, the presence of mineralization should lead to consideration of other diagnoses on the differential that would include synovial chondromatosis. Although approximately 75% of cases of synovial chondromatosis exhibit calcification, the extent of calcification can be variable, and when noncalcified, synovial chondromatosis may be more challenging to distinguish from TSGCT. The signal characteristics of synovial chondromatosis on MRI can also be variable, with calcified nodules demonstrating hypointense T1 and T2 signal ( Fig. 3.22 ) and uncalcified nodules showing hyperintense T2 signal reflecting less mature hyaline cartilage. These findings contrast with the low and intermediate T2 signal that is typically seen with TSGCT. In addition, the small and uniform size of the nodules with synovial chondromatosis contrasts with larger, irregular nodular and mass-like foci with TSGCT.

A 15-year-old female with synovial chondromatosis of the left hip. Axial T2-weighted, non-fat-saturated ( A ), and postcontrast fat-saturated ( B ) MRI demonstrates a left hip joint effusion containing numerous T2 hypointense nodules ( arrowhead , A ). The peripheral enhancement nicely illustrates the extent of synovial proliferation ( arrows , B ). Corresponding frontal radiograph ( C ) demonstrates innumerable foci of calcification, best seen along the inferomedial aspect of the hip ( dotted oval ).

Other differential diagnostic considerations of TSGCT include gouty tophus ( Fig. 3.23 ), amyloid deposition, and siderotic synovitis from recurrent hemarthrosis, such as hemophilic arthropathy ( Fig. 3.24 ). Gout and amyloidosis are systemic illnesses that usually demonstrate multiple sites of involvement and may have characteristic laboratory findings or clinical features. The osseous changes seen in hemophilic arthropathy ( Fig. 3.24 ) are not a feature of TSGCT. However, there can be significant overlap in appearance between TSGCT and other synovial processes, and differentiation based on imaging alone may be difficult in some cases.

A 49-year-old male with tophaceous gout involving the patellar tendon. Sagittal T1 ( A ), T2-weighted, fat-saturated ( B ), and postcontrast fat-saturated ( C ) MRI demonstrates an intermediate signal, avidly enhancing fusiform mass involving the patellar tendon ( arrows ), which was biopsied and shown to represent tophaceous gout.

A 13-year-old female with hemophilic arthropathy of the right knee. Sagittal proton density ( A ) and T2-weighted, fat-saturated ( B ) MRI demonstrates extensive destruction of the cartilage and subchondral bone of the femur and tibia, with prominent low signal thickening of the synovium ( arrows ). Corresponding frontal radiograph ( C ) demonstrates demineralization, epiphyseal enlargement, diaphyseal thinning, and articular surface deformity of the right knee ( arrow ).