Imaging of Benign Soft-Tissue Tumors

Abstract

Soft-tissue tumors account for less than 4% of all tumors in adult patients and 7–10% of all tumors in pediatric age group. The majority of these tumors are benign in nature (more than 99%). Different imaging modalities play a significant role in the diagnosis, treatment, and follow-up of these tumors. In this chapter, we will try to cover the imaging appearances of different benign soft-tissue tumors and to demonstrate the differentiation features. In addition, we will demonstrate a systematic approach for the characterization of soft-tissue masses based on different imaging appearances.

1. Introduction

Soft-tissue tumors (STTs) include both benign and malignant processes. Benign lesions can be reactive in nature or clearly neoplastic [1]. They account for less than 4% of all tumors in adult patients and 7–10% of all tumors in pediatric patients. More than 99% of STTs are benign [2].

Recently, there have been significant changes in the diagnosis and treatment of STTs. Several developments in the field of radiology have significantly changed the way STTs are currently diagnosed and treated. The radiologic evaluation of soft-tissue masses showed dramatic advancement in the recent era. Before the development of computer-assisted imaging, the radiologic assessment of the soft-tissue masses was generally limited to conventional radiography, which gives very limited diagnostic information [1, 2, 3, 4].

The introduction of advanced imaging into the evaluation of soft-tissue masses resulted in the development of multiple assessment tools with multiple options. The introduction of magnetic resonance imaging (MRI), multidetector computed tomography (CT), dual-energy CT, and positron emission tomography (PET) has significantly improved our ability to detect and characterize musculoskeletal soft-tissue masses [3, 4].

When facing a soft-tissue lesion, the radiologist has to decide whether the lesion is benign or malignant; in some cases, this might be impossible. In these cases, biopsy is indicated. All kinds of imaging modalities play a significant role in the radiological evaluation (including conventional X-ray, ultrasound, CT scan, MRI, and radionuclide imaging) [5, 6, 7].

Figure 1 shows a practical approach to imaging a soft-tissue lesion.

Figure 1.

A practical approach to imaging a soft-tissue lesion.

2. Magnetic resonance imaging in the evaluation of soft-tissue masses

MRI is now one of the best available imaging modalities in the evaluation of soft-tissue masses as it provides detailed information of the localization and type of the lesion [8, 9]. The appropriate protocol of imaging a patient should depend on the clinical history, physical evaluation, and initial imaging, in addition to the age and location of the lesion, in order to decide what sequences and planes to use and whether there is a need for contrast material and additional imaging [8, 10, 11].

2.1 Field of view (FOV)

It is one of the most important parameters. The referring physician usually orders the examination by the anatomic area; for example, in a lesion at the groin, the MRI examination may be ordered as hip MRI, which requires a large field of view, thus decreasing the resolution and increasing the time of acquisition [12].

As a general rule, the FOV is determined by the size and location of the mass and must be of adequate size to demonstrate the entire lesion. In general, a small FOV is preferred [12, 13].

2.2 Imaging planes

In most institutions, axial plane is considered the primary plane of imaging. The axial plane together with the sagittal and coronal planes is used to provide the best assessment of the entire lesion in “profile” and to demonstrate its relation to the neurovascular bundle and the surrounding structures [14, 15].

2.3 MRI sequences

The choice of an magnetic resonance (MR) sequence is largely dependent on the type of lesion and the radiologist personal preferences; however, most musculoskeletal masses are well evaluated with conventional T1-weighted and fat-suppressed fluid-sensitive images. Additional images are used accordingly; for example, myxoid lesions generally show fluid signal intensity (SI), whereas collagenous/fibrous lesions generally have low-to-intermediate signal intensity [14, 15, 16].

Gradient echo (GRE) imaging is important in demonstrating prior intralesional hemorrhage, revealing hemosiderin deposition as a result of its greater magnetic susceptibility [16].

2.4 Contrast material

Contrast material is important in demonstrating the vascularity of a lesion, the pattern of enhancement, and the delineation of the margins of an enhancing lesion. In addition, it is useful in the assessment of the vascular anatomy; however, it has limited value in the differentiation of benign and malignant lesions since both might show increased or decreased contrast enhancement [10, 11].

Subtraction imaging is a relatively recent innovation; it is a very important technique since it eliminates the possibility of misinterpreting the T1 shortening associated with hemorrhagic change as vascular enhancement and also useful in patients with metal fixation because it eliminates the need for fat suppression at enhanced imaging [10, 11, 12].

2.5 Patient motion

The long imaging time in most of the MRI techniques makes them difficult to employ in parts of the body that are susceptible to respiratory motion. Recent advances in software and imaging applications now allow a variety of techniques that can capture a complete set of images in a single breath hold [14, 15].

The gradient version of single-shot imaging (true FISP [true fast imaging with steady-state precession], FIESTA [fast imaging employing steady-state acquisition], balanced FFE [fast field echo]) are generally preferred because they provide high signal intensity of flowing blood and increased signal-to-noise ratio [16].

Rapid fluid-sensitive images can also be acquired using a spin-echo single-shot technique (HASTE [half-Fourier acquisition single-shot turbo spin-echo], turbo spin-echo, and single-shot FSE). These sequences are motion insensitive, ideal for breath-hold imaging, and exquisitely fluid sensitive [16, 17, 18, 19].

2.6 Quantitative MRI evaluation

This term can be used in any imaging method in which there is a measurable value. Those that are most applicable to clinical practice are chemical shift and diffusion-weighted imaging, both of which allow qualitative (visual) assessment as well as a quantitative (measurable) result. In the recent years, MR spectroscopy has been added to this list; however, the inherent technical challenges have limited its widespread adoption [20].

2.7 Chemical shift imaging

Chemical shift imaging is a gradient echo technique based on the fact that signals from similar quantities of fat and water in a single voxel will cancel each other out. It is designated as in-phase and opposed-phase images; the signals from fat and water reinforce each other when in phase and cancel each other when out of phase [19]. This allows one to qualitatively and quantitatively (measured as percentage change in signal intensity on opposed-phase relative to in-phase images) assess the amount of microscopic fat in any lesion. It is useful in distinguishing reactive marrow changes from tumor and microscopic fat in higher-grade liposarcoma [19, 20].

2.8 Diffusion-weighted imaging

Diffusion-weighted imaging allows the qualitative and quantitative assessment of the movement of water molecules through tissue. Thus, showing areas where normal random motion is restricted, the restricted diffusion will be showing increased signal intensity and measured as the apparent diffusion coefficient (ADC), with restricted diffusion appearing dark on ADC maps [21, 22].

Although the value of diffusion-weighted imaging in distinguishing benign from malignant lesions is still under evaluation, it can identify restricted diffusion, which is also quite useful in identifying small pelvic lymph nodes [21, 22].

2.9 Spectroscopy

The most recent quantitative technique for the assessment of soft-tissue tumors in musculoskeletal spectroscopy, choline, which is a marker for cell membrane turnover, is measured. It is elevated in malignancies. It is a technically challenging and time-consuming technique; thus, it is not usually used in routine practice [23].

3. Computed tomography

CT scan is a useful adjunct to MRI in regard to soft-tissue imaging. It is fast, cheap, and patient friendly in comparison to MRI; also, it is a very useful tool for detecting systemic metastases in the case of malignant lesions [6, 7].

3.1 Mineralization characterization

CT scan is very useful in identifying calcification and to characterize soft-tissue mineralization. It is superior to radiography in detecting the zonal pattern of mineralization, which is essential to the radiologic diagnosis of early myositis ossificans (MO).

The ability of multiplanar reconstruction of CT images is very important to depict the character of the interface between a soft-tissue mass and the adjacent osseous cortex (to detect cortical remodeling or invasion) [9].

3.2 Dual-energy CT

Being relatively new technology, it is useful adjunct in the evaluation of soft-tissue masses. Basically, it employs the differences in the energy attenuation of soft tissue at 80 and 140 kVp, thus allowing distinction of urate crystal deposits from other soft-tissue calcifications [9].

3.3 CT angiography

The modern CT scanners are superior in their ability of rapid image acquisition; this allows an accurate assessment of lesion vascularity. CT angiography with three-dimensional reconstruction was found equivalent to MRI regarding the ability to demonstrate neurovascular involvement and, not surprisingly, superior to MRI in its ability to identify calcification/ossification and cortical/marrow involvement [8, 9].

4. Ultrasonography (US)

US is the primary imaging method to guide the biopsy of soft-tissue masses. It is available, cheap, and noninvasive, making it the preferred method of the initial evaluation of the size and consistency of a soft-tissue mass. It is helpful in differentiating a localized mass from surrounding edema and differentiating the solid from the cystic lesions [9].

Therefore, in general, conventional radiography has an important role as an initial step for the evaluation of STTs. It is cheap, fast, and delivers low radiation. It helps to evaluate the presence of calcification within the lesion and its effect on the adjacent bones and possible fracture risk. The development of new powerful ultrasound machines has helped the evaluation of these masses, their echo pattern, vascularity (using Doppler imaging), the extension through different anatomic compartments, and guiding biopsy. However, it remains operator dependent and irreproducible. CT scan is very important as a method for evaluating the pattern of contrast enhancement of the lesion, whether it contains calcifications or invading adjacent tissues, and its relation to the adjacent vessels; it can be used to guide biopsy. The best modality by far in the matter of STT evaluation is MRI; it has superior contrast capabilities, multiplanar imaging, and can accurately detect whether the lesion contains hemorrhagic foci, melanin, or calcific foci. It can also detect the extension of the lesion through different facial planes and its relation to the neurovascular bundle. Modern MRI sequences (such as spectroscopy, diffusion, prefusion, whole-body MRI, etc.) have remarkably facilitated the accurate diagnosis and treatment and improved patient’s prognosis. PET scanning (using all kinds of positron emitting radiopharmaceuticals) usually combined with CT scan is an important method of evaluating the metabolic activity of the lesion and (in cases of malignant tumors) detecting subtle metastatic deposits [7, 8, 9, 10, 11].

5. Benign lesions of soft tissues

In general, soft-tissue lesions can be classified according to their origin as follows:

1. Muscle and synovial origin

   • Myositis ossificans

   • Pigmented villonodular synovitis (PVNS) and giant cell tumor (GCT) of the tendon sheath (tenosynovial giant cell tumor [TGCT])

   • Synovial chondromatosis (SC)

2. Fibrous origin

   1. Superficial

      1. palmar (Dupuytren’s contracture)

      2. plantar (Ledderhose’s disease)

      3. penile (Peyronie’s disease)

      4. knuckle pads

   2. Deep (desmoid-type fibromatosis)

      1. extra-abdominal

      2. abdominal

3. Fat origin

   • Lipomas

4. Neurogenic origin

   • Neurofibromas

   • Schwannoma

5. Other rare conditions or pseudotumeral and benign lesions of the soft tissues

   • Hemangioma

   • Ganglia

   • Hematoma

   • Myxoma

   • Angioleiomyoma

   • Glomus tumor

   • Nodular fasciitis

   • Proliferative Fasciitis and proliferative myositis

   • Pseudo tumoral calcinosis

   • Hibernoma.

5.1 Muscle and synovial origin

5.1.1 Myositis ossificans

Myositis ossificans is a benign process that can involve any type of soft tissues (mostly muscles) resulting in fibrolamellar bone deposition following some kind of trauma such as injury, burns, and surgery; it is usually self-limiting [24].

There is a rare type of MO that is hereditary (fibrodysplasia ossificans progressiva also called Munchmeyer’s disease) that affects mainly young males and is characterized by the progressive ossification of the skeletal muscles and when respiratory muscles are affected; it can be fatal [25].

The classic myositis ossificans usually presented with swelling and pain following a trauma that usually subsides within few days, usually affecting the extensor muscles of the thigh, flexor muscles of the arm, and then adductor or gluteal muscles [26].

The X-ray appearance in the early stage is usually negative. After about 2 weeks, a periosteal reaction can be detected. After 3–4 weeks, the soft-tissue calcifications become evident. Ossification starts to be evident at the periphery with a radiolucent center after 6–8 weeks. After 6–12 months, an ossified mass is evident [27, 28].

Ultrasound examination in the early stage shows a peripheral hypoechoic area, an intermediate hyperechoic area with calcification, and an inner hypoechoic area corresponding to immature zone. CT scan in the early stage shows a hypo/isodense soft-tissue lesion, so it is not specific in the early stage. It usually shows the classic pattern of peripheral calcification and radiolucent central area only after few weeks, so it is most useful in the intermediate stage [27, 28, 29].

MRI can be confusing in the early stages showing heterogeneous T1 and hyperintense T2 signals, which may cause a diagnosis of sarcoma. After about 4–6 weeks, the central part of the lesion becomes iso-to-hypointense in T1 and slightly hyperintense in T2-weighted image (WI) to surrounding muscles, and a low T2 signal at the periphery is seen corresponding to peripheral calcification; then, in the mature phase, a low signal in all the sequences is seen corresponding to ossification (Figure 2) [27, 28, 29, 30].

Figure 2.

a.: X-ray image of the right thigh AP view showing an ill-defined ossification within the soft tissues on the medial aspect of the thigh (empty black arrows in a.) 11 months following trauma consistent with the mature ossified stage of myositis ossificans. b. and c.: MRI images: b. T2 WI coronal and c. T1 FS post-contrast axial image of the left thigh (different patient than a.) showing a mass with heterogeneous T2 signal intensity (thin white arrows in b.) within the vastus intermedius muscle with T2 central hyperintense signal and peripheral post-contrast enhancement (black arrows in c.) corresponding to myositis ossificans (4–6 week stage).

5.1.2 Pigmented villonodular synovitis (PVNS) and giant cell tumor (GCT) of the tendon sheath (tenosynovial giant cell tumor)

PVNS is a benign progressive inflammatory process in the joint, tendon sheaths, or bursae. It has no sex predilection usually presenting at 20–40 years of age; it can be either paratendinous or in the joint, diffuse or localized, the paratendinous type usually affecting the flexor tendon of the fingers and in the palm of the hand near the metacarpophalangeal joint and rarely in the foot, while the joint type mostly affects the knee and less commonly the hip, wrist, ankle, and shoulder; in rare cases, it might affect the bursae [31, 32].

Clinically, the diffuse variety presents with multiple nodules involving the whole joint causing pain, swelling, and stiffness ending up with secondary osteoarthritis, while the localized type presents with a single nodule causing locking, clicking, and swelling of a joint, usually mild pain, and it can be almost asymptomatic. The course is unpredictable [33, 34, 35, 36, 37]. TGCT is usually slowly growing and may remain unchanged for many years. However, recurrence is very frequent in the diffuse type [38, 39, 40, 41].

On conventional radiography, joint effusion can be seen; as the synovial tissue gets thickened, we can see skeletal erosions with well-defined sclerotic margins. CT shows a lobulated enhancing tissue in the affected joint, bone scan may show increased uptake, MRI can demonstrate a characteristic finding of heterogeneous, mostly low signal both in T1- and T2-weighted images, enhancing, intra and peritumoral enhancing curvilinear regions. PET scan usually shows markedly increased SUVmax values in TGCT, mimicking a malignant process (Figure 3) [33, 34, 35, 36, 39, 40, 41].

Figure 3.

Knee MRI of three different patient a. sagittal T1 WI, b. axial T2 WI, and c. sagittal gradient image; note diffuse synovial thickening with no significant bony erosions (empty black arrows in a., thin black arrows in b.); areas of signal drop (blooming) are evident in the gradient image (empty white arrows in c.) consistent with pigmented villonodular synovitis.

5.1.3 Synovial chondromatosis

Synovial chondromatosis (SC) is a benign neoplasm of the synovial membrane of the joint, the tendinous sheath, or the bursae mucosae; it is a rare entity usually affecting 30–50-year-old males. More than half of the cases are seen affecting knee, then elbow, shoulder, wrist, hip, and ankle. Extra-articular form usually seen affecting the fingers; clinically, it usually presents with pain, limitation of movement, and joint crackling; rarely, it can present with locking of the joint and effusion. Solid nodules may be palpable. The symptoms are usually slowly progressive. In aggressive type, a multilobulated hard mass expanding around a joint can be felt [42].

Surgery is usually curative; however, late recurrences are reported. Very rarely can transform into chondrosarcoma [43].

Conventional radiology may demonstrate intra-articular calcified nodules. Long standing lesions can result in degenerative changes and bone erosion or cortical scalloping in the aggressive variety. CT scan demonstrates intra-articular nodules, bone lesions, and invasion of the periarticular tissues. MRI shows joint effusion, lobulated intra-articular mass of intermediate intensity if uncalcified or with white punctuated appearance if ossified on T1-weighted images, round, ring-like, dark signal voids with strong enhancement of the synovial tissue on contrast-enhanced T1-weighted images, and hyperintense signal of the joint fluid on T2-weighted images (Figure 4) [8, 9, 42, 43].

Figure 4.

a. X-ray of the knee AP view showing multiple intra-articular calcified nodules on the superolateral aspect of the knee joint (thick white arrows in a.) with marked degenerative changes (periarticular osteopenia and osteophytes), b. T2 FS sagittal and c. axial images of the shoulder (different patient) showing innumerable intra articular intermedial signal intensity nodules (thin black arrows in b. and empty white arrows in c.) with some joint effusion consistent with synovial chondromatosis.

5.2 Fibrous origin

Fibromatosis consists in a wide group of benign mesenchymal proliferation.

They can be either superficial (including palmar (Dupuytren’s contracture), plantar (Ledderhose’s disease), penile (Peyronie’s disease), and knuckle pads) or deep desmoid-type fibromatosis (including extra-abdominal and abdominal types) [2].

5.2.1 Desmoid-type fibromatosis

It is a locally aggressive proliferation of bundles of spindle cells in an abundant fibrous stroma with an infiltrative pattern of growth. It does not metastasize, but high rate of local recurrence after surgical excision is seen. It usually affects young adults and females (25–35 years), seen more often in patients with familial adenomatous polyposis, and usually seen affecting the scapula, pelvic girdle, lower limbs, and upper limbs [44, 45].

Depending on the involved parts, the clinical features will vary; the tumor is usually a slowly progressive, painless, and hard mass; it appears adherent to surrounding structures [46].

The infiltrative pattern of these tumors results in the involvement of the muscles and facial planes by multiple nodules, which can arise proximally and distally and even in different compartment of the same limb [45, 47].

If it arises close to a joint, a functional impairment may result. Neurological symptoms are only seen when nerves are involved [48].

Conventional radiography may not show any abnormality; sometimes, a calcific mass in soft tissue or a bony erosion can be seen when the tumor is located close to a cortical bone [45]. Ultrasound usually shows a hypoechoic and heterogeneous echo pattern lesion. CT scan shows an isodense lesion, showing post-contrast enhancement with lobulated outline; it is very useful in demonstrating the bony erosion (Figure 5) [45].

Figure 5.

CT scan of the abdomen with oral and IV contrast sagittal a. and axial b. images showing a slightly lobulated isodense lesion within the left rectus abdominus muscle with mild post-contrast enhancement (thick white arrows in a. and empty white arrows in b.); biopsy showed desmoid tumor.

MRI is very helpful. The active desmoid fibromatosis is usually heterogeneously isointense on T1-weighted images and heterogeneously hyperintense on T2-weighted images with heterogeneous post-contrast enhancement with the bands of low signal on all sequences [48, 49].

Changes in MRI imaging features are used as evaluation for follow-up and for the evaluation of treatment response, an increased hypointensity in T2, and a decreased contrast enhancement indicating tumor response [48, 49, 50].

5.3 Fat origin

5.3.1 Lipomas

A benign tumor is composed of well-differentiated adipocytes. It is the most common soft-tissue tumor and is more frequently observed between 40 and 60 years of age more in females if it is superficial, whereas in males if it is deep and multiple. Most commonly, it appears superficial in location mainly in the subcutaneous tissue of the back, shoulder, neck, and proximal extremities, while, rarely, it is deep between muscles or within muscles; it can be adherent to bone, tendons, joints, or nerves. In about 5% of cases, lipomas are multiple [51, 52, 53, 54].

Lipomas usually present as a solitary painless lump with slow or no growth; the superficial lipomas very rarely grow to a large size (average 4 cm) and are mobile and pliable, while deep lipomas are usually large in size (average 10 cm) and are rounded, immobile, and firm. There is a possible association with hereditary familial multiple lipomatosis (FML) [53, 54, 55, 56, 57].

On conventional radiography, it appears as a lucent lesion rarely with calcification. If it is in contact with the cortex of a bone, it might cause mild cortical thickening, CT scan shows a lobulated, sharply marginated homogenously hypodense lesion (around −50 to −60 HU). MRI shows an encapsulated mass that is hyperintense on T1 and less hyperintense on T2-weighted images with the loss of signal on fat-saturated (FS) images with no post-contrast enhancement. On angiography, it appears avascular and on bone scan shows no uptake [55, 56, 57, 58, 59].

Superficial lipomas are easily diagnosed and are asymptomatic. Usually, no treatment is required; however, deep lipomas need histopathological study to exclude liposarcoma (Figure 6) [60].

Figure 6.

MRI images of the arm axial T1 WI a. and axial T1 FS image showing a well-defined homogenously hyperintense lesion within the muscles of the anterolateral aspect of the arm (brachialis muscle) (thick white arrows in a) that shows the complete loss of signal on fat-saturated images (empty white arrows in b.) consistent with an intramuscular lipoma.

5.4 Neurogenic origin

5.4.1 Neurofibromas

It is a benign tumor of the peripheral nerve sheath. It is usually solitary; however, multiple neurofibromas are seen in neurofibromatosis type 1 (NF1) [61].

Solitary neurofibromas are more frequent (90%) than multiple neurofibromas and have no sex predilection affecting mainly 20–40-year-old patients, while multiple neurofibromas are seen more in younger male patients [62].

Solitary neurofibromas are superficial in location usually in the skin or subcutaneous tissues, while multiple neurofibromas (i.e., NF1) affect all sites and organs [63, 64, 65].

Solitary lesions present as painless nodules or with some pain and swelling, while multiple neurofibromas (NF1) present with “café-au-lait” spots, typically in the axilla, pigmented hamartomas of the iris (Lisch nodules), skeletal abnormalities, disorders of growth, and sexual maturation [62, 63, 64, 65].

MRI nicely demonstrates a nerve trapped within or obliterated by the mass; they are rarely encapsulated, usually homogeneous and isointense to muscle on T1-weighted images, and may contain the areas of high T1 signal intensity. T2-weighted images show an inhomogeneous, target appearance and hyperintense lesions, whereas contrast-enhanced images show centrally higher enhancement never with necrosis [63, 64, 65, 66].

Few variants are described; neurofibromas can be localized cutaneous, diffuse cutaneous, localized intraneural, plexiform intraneural, and massive diffuse soft-tissue plexiform. Malignant transformation is rare in solitary lesions but more frequent (5–10%) in multiple neurofibromas, particularly when in NF1 (Figure 7) [67].

Figure 7.

a. Sagittal T2 FS image of the lower femur showing a well-defined nodule in the posterior aspect of the lower thigh with a nerve entering and exiting the nodule (thin white arrows in a) consistent with a nerve sheath tumor, biopsy reviled a schwannoma, b. sagittal T2 FS images and c. axial T1 WI of the upper leg (different patient than a.) showing multiple T2 heterogeneous masses along the course of the posterior tibial nerve (empty white arrows in b.); these masses appear isointense to muscle on T1 WI with few areas of hyperintensity (thick black arrows in c.) consistent with multiple neurofibromas in this patient with known NF1.

5.4.2 Schwannoma

It is a benign nerve sheath tumor composed of differentiated neoplastic Schwann cells, usually affecting 20–50-year-old patients with no sex predilection. Rarely associated with neurofibromatosis of the mediastinum or of the retroperitoneum, peroneal, and ulnar nerve [67, 68]. It is solitary in more than 90% of cases. Schwannoma is usually asymptomatic or can present with mild and progressive pain; increase in pain at night, stiffness, and even spinal contractures can be seen [68, 69].

Conventional radiography demonstrates scalloping of bone, while CT scan shows a well-defined, homogeneous lesion isodense to muscle. Post-contrast images usually show a non-enhancing necrotic lesion with cystic areas that cause an inhomogeneous hyperdense lesion. MRI nicely demonstrates a nerve along the site of the mass that is encapsulated, homogeneous, and isointense to muscle with frequent areas of low signal on T1-weighted images and heterogeneous; sometimes, target appearance, hyperintense mass on T2-weighted images, and post-contrast images show diffused or peripheral enhancement [70].

Very rarely, it can show malignant transformation in epithelioid malignant peripheral nerve sheath tumor (PNST), primitive neuroectodermal cells, epithelioid angiosarcoma, or rhabdomyosarcoma [68, 69].

Melanotic schwannoma is a type of schwannoma that has a low-malignant potential and rarely show late metastases (Figure 7) [69].

5.5 Other rare conditions or tumor-like and benign soft-tissue lesions

5.5.1 Hemangiomas

Benign vascular lesions are composed of various vessels and are common in infancy and childhood; however, all age groups might be affected. Clinically, they present with bluish skin discoloration with changing size; sometimes, pain may occur following exercise [71, 72].

Hemangiomas can contain serpentine vessels, fat, smooth muscle, hemosiderin, and phleboliths, so the identification of phleboliths on X-ray or CT scan images is helpful in the diagnosis [73, 74]. MRI shows a well-defined or poorly defined margins; periosteal reaction, cortical and medullary changes, and overgrowth can be seen, with varying amount of hyperintense T1 signal owing to either reactive fat overgrowth or hemorrhage (Figure 8) [74].

Figure 8.

a. X-ray AP view of the knee and leg showing a small mass at the medial aspect of the upper leg containing areas of fat density and few tiny calcified phleboliths (thin black arrows in a.), b. axial T2 FS, c. axial T1 WI, and d. T1 FS post-contrast coronal images showing a lesion that is heterogeneously hyperintense on T2 FS images (thin white arrows in b. and thick white arrows in c.) with some flow voids and is showing post-contrast enhancement (empty white arrows in d.) indicating soft-tissue hemangioma.

5.5.2 Ganglia

It is not a true tumor. However, since it is a common lesion, it should be considered in the work-up of a soft-tissue lesion [75]. Most common locations include the hand, wrist, and feet; it can arise from joint capsules, bursae, ligaments, tendons, and even subchondral bone [75, 76, 77].

At arthrography and MRI, ganglia do not always show communication with the joint [76]. They are usually asymptomatic; nerve compression can cause pain [78].

Typically, X-rays are normal; sometimes, the nonaggressive remodeling of the bone is seen. On MRI, they appear as round masses, uni- or multiloculated, with smooth surface, almost always in proximity to a joint or tendon (rarely far from a joint). They are usually isointense or slightly hypointense to muscle on T1-weighted images and hyperintense on T2-weighted images. They show a rim of contrast enhancement, with or without thin low-SI enhancing septae; sometimes, a track extending toward the joint can be seen (Figure 9) [77, 78, 79, 80, 81, 82].

Figure 9.

a. Axial T2 FS image of the knee showing a cystic lesion with few septae on the medial aspect of the knee (thick white arrows in a.), and it appears to be related to the joint consistent with a ganglion. b. Coronal T2 FS image of the wrist joint (different patient than a) showing same features (empty white arrows in b.) of a ganglion of the wrist joint.

5.5.3 Hematomas

Hematomas occur after trauma, as well as in a patient who is using anticoagulant treatment or who has a clotting problem. Clinically, ecchymosis may be present; the appearance of a hematoma varies with its age. Acute-stage (a few days old) hematomas are typically iso- or hypointense to muscle on T1- and T2-weighted MR images. Subacute (1–3 months old) hematomas are usually T1 and T2 hyperintense. The high T1 signal intensity, caused by methemoglobin, may initially be seen in the periphery [83]. Chronic hematomas are T1 and T2 hyperintense but can have a prominent hypointense rim representing a wall of fibrous tissue and/or hemosiderin. Hematomas can be seen in association with underlying tumors, so any hematoma with nodular areas of soft-tissue enhancement should be followed up especially if there is no history of trauma. Hematomas that are persistent may calcify or may continue to bleed, resulting in a chronic expanding hematoma [84].

5.5.4 Myxoma

It represents a group of relatively common, benign unrelated lesions. These lesions usually involve large muscles (intramuscular myxoma) and may occur around large joints (juxta-articular myxoma) or in the skin (cutaneous myxoma). All types are characterized by abundant myxoid matrix, bland stellate to spindled cells, and are hypovascular. Local excision is sufficient treatment, but juxta-articular myxoma may show local recurrence in 30% of cases, particularly if incompletely excised [85].

5.5.5 Angioleiomyoma

Also known as angiomyoma or vascular leiomyoma, it is a benign dermal or subcutaneous lesion consisting of well-differentiated smooth muscle cells arranged around many vascular channels. It involves all groups (most commonly between the fourth and sixth decades) [12, 13].

Angioleiomyoma can occur anywhere in the body, mostly seen in the extremities, the head, and trunk presenting as a small, slowly growing firm nodule measuring <2 cm in diameter associated with pain in half of patients. It may show local recurrence if incompletely excised [86].

5.5.6 Glomus tumor

Also known as glomangioma and glomangiomyoma, it is a benign neoplasm composed of cells resembling cells of the normal glomus body.

It is a rare tumor seen in about 2% of soft-tissue tumor, usually in young adults, with no gender prelection. It is most commonly seen in the skin or superficial soft tissue in the distal extremities presenting with a history of pain. Very rarely, glomus tumor shows malignant changes.

Most commonly, it is a benign neoplasm that requires only simple excision. If malignant, it is highly aggressive with metastases, and death occurs in up to 40% patients [10, 12].

5.5.7 Nodular fasciitis

Is a pseudo-sarcomatous process, it is a common, benign, self-limiting lesion mainly composed of myofibroblasts and fibroblasts. It is a solitary, small (<3 cm), sometimes painful subcutaneous nodule usually presenting 20–50 years old with equal sex distribution; it develops rapidly (often less than a month). It can be seen anywhere but most commonly in the upper extremities especially in the forearm. Nodular fasciitis is a proliferative lesion and is commonly mistaken for a sarcoma [87].

Simple excision is the treatment of choice with a percentage of local recurrences of less than 2% of cases [30].

5.5.8 Proliferative fasciitis and proliferative myositis

They are morphologically similar to nodular fasciitis; the difference is that they contain ganglion-like myofibroblastic cells [87].

Proliferative fasciitis is usually seen in the subcutaneous tissue of the upper limbs of middle-aged adults (40–60 years), whereas proliferative fasciitis mainly affects the muscles of the trunk and shoulder girdle. In children, proliferative fasciitis may be cellular and mitotically active, mimicking rhabdomyosarcoma or epithelioid sarcoma [30, 87].

Both these lesions are benign, self-limiting, and reactive process. Simple excision is the treatment of choice. They very rarely show local recurrence [88].

5.5.9 Pseudotumoral calcinosis

It is also called tumoral calcinosis, calcifying bursitis, calcifying collagenolysis, tumoral lipocalcinosis, and hip stone disease. It is a term used for an extraskeletal soft-tissue calcification caused by hydroxyapatite deposition with a granulomatous response seen in patients with secondary hyperparathyroidism or hypercalcemia, usually idiopathic, or because of end-stage kidney disease [89].

5.5.10 Hibernoma

It is a benign adipocytic tumor composed of brown fat cells mixed with mature white adipose tissue. It usually presents as a painless, slow-growing mass, mostly involving the subcutaneous tissues of young adults with a predilection for the thigh, the trunk, the upper limbs, and the head and neck areas. The differential diagnosis is with atypical lipomatous tumor with hibernoma-like features. Local excision is curative [90].

6. A systematic approach for the characterization of soft-tissue masses

The utilization of the systematic approach can help in reaching a diagnosis or narrowing the differential diagnosis and, thus, help in the clinical decision making. If a lesion cannot be characterized as benign, biopsy should be done to exclude malignancy [2].

6.1 Clinical history and physical examination

The clinical history and physical examination represent the starting point of evaluation of any soft-tissue mass. Relevant information, including age, recent trauma, fluctuating mass size, history of malignant cancer and familial syndromes, and physical examination can help with lesion characterization [91].

A history of trauma definitely helps in the diagnosis of a hematoma or myositis ossificans; however, some patients do not recall a history of trauma. In addition, changes in the size of the mass are helpful indicator of the nature of the mass. Most benign masses grow slowly or show no change in size; however, if intralesional hemorrhage occurs, a rapid increase in the size can be seen. Fluctuation in lesion size can be seen with ganglia or hemangiomas [91, 92].

Physical examination is important in determining whether the mass is fixed or mobile, mobile masses are more likely to be benign, and skin changes are also helpful for narrowing the differential diagnosis; for example, ecchymosis can be seen with trauma [92].

6.2 Location

Location is of great importance to the characterization of soft-tissue masses since certain masses occur more frequently in certain parts of the body; elastofibroma, for example, is a benign tumor that occurs almost exclusively along the inferomedial border of the scapula, deep to the latissimus dorsi, and rhomboid major muscles [93, 94].

Morton neuroma is another example, which typically occurs at the plantar aspect of the second or third interspace of the foot [50].

While location can be used to favor a given diagnosis, other lesions must be considered if the imaging and, similarly, recognizing the structure from which the lesion is arising (e.g., nerves, vessels, or tendons) can help in lesion characterization. Tumors arising from nerves are most likely benign schwannomas and neurofibromas [61].

Vascular tumors typically have dilated tortuous vessels entering and/or exiting the lesion; these include hemangiomas, lymphangiomas, and angiosarcomas [71, 73].

Pseudoaneurysms usually develop following trauma, and it is important to make the correct diagnosis avoiding biopsy [2].

Tumors arising from tendons are usually GCTs of the tendon sheath; other possibilities include ganglia, lipomas, and fibromas [39].

6.3 Radiographs

The important points that radiographs can provide regarding the evaluation of soft-tissue lesions include any distortion of tissue planes, radiolucent fatty areas, the presence and type of bone remodeling (indolent or aggressive), and soft-tissue calcifications or ossification [7].

Clustered phleboliths should suggest the diagnosis of soft-tissue hemangioma, while the presence of juxta-articular calcifications or ossific foci should suggest the diagnosis of synovial sarcoma or synovial osteochondromatosis [9, 42].

Ossification in soft tissues is indicative of heterotopic ossification or myositis ossificans, which may simulate an aggressive sarcoma in MRI [26, 27].

6.4 MR images

MRI allows local tumor staging, detection of tumor relation to the neurovascular bundle, detection of foci of tumor necrosis, and preoperative planning [1].

6.5 Newer techniques

The use of recent techniques, such as MR spectroscopy and diffusion imaging, is being considered in the evaluation of soft-tissue masses and mainly in monitoring response to treatment; however, these techniques are not a part of the routine clinical practice till now [23].

7. MRI of soft-tissue masses: technical considerations

A number of general principles apply to the MRI of soft-tissue masses; for example, the lesion should be well demarcated before imaging, effort should be made not to compress or distort the mass, and T1- and T2-weighted images are routinely obtained for tissue characterization. The axial plane is most important for the compartmentalization of the lesion, and accordingly, the relevant longitudinal planes are acquired [10, 11].

7.1 Imaging strategy

In cases where the required target is to detect the presence of a mass, a large field of view (including the contralateral side) should be used to make it easier in detecting asymmetry by comparing the abnormal side to the normal one; however, a large FOV results in decreased special resolution, while in cases where detailed assessment of a mass is required, a small field of view is employed to increase the resolution of the image [13, 14].

7.2 Imaging sequences

The standard basic sequences used to evaluate any soft-tissue mass are T1- and T2-weighted sequences. Depending on the protocol used, a fat-suppressed T1-weighted sequence is used in lesions that have a high T1 signal intensity to show the loss of the bright signal indicating the presence of fat within the lesion; a fat-suppressed T2-weighted sequence is also important to demonstrate the areas of edema within and around the mass. All of these anatomic imaging sequences should all be obtained prior to the administration of contrast agent [13, 14, 16].

7.3 Additional sequences

A T2*-weighted gradient echo sequence is important in detecting the presence of hemosiderin as hemosiderin with result in chemical shift artifact, which is an accentuated loss of signal on T2*- and T2-weighted images (referred to as blooming). This is observed in pigmented villonodular synovitis, some hemangiomas, and late-phase hematomas [16].

7.4 Imaging plane

Axial images are the primary images in the evaluation of any soft-tissue lesion; they are used for demonstrating the relevant anatomy of the lesion whether it is confined to one compartment or is extending to the adjacent compartments.

Images obtained in other planes—coronal, sagittal, or oblique—are important to demonstrate the extent of the mass and its relationship to anatomic landmarks [16].

7.5 Intravenous gadolinium-based contrast agents

An intravenous contrast agent is helpful in the evaluation of the soft-tissue masses and to differentiate cystic from solid components, to assess the vascularity of the tumor, and may help in evaluating the involvement of the vessels and other structures by the mass.

Contrast enhancement can also help in defining the target tissue for biopsy [95, 96, 97].

7.6 Lesion characterization on the basis of MR images T1 hypo- or isointense lesions

The vast majority of soft-tissue masses are iso- or hypointense to muscle on T1-weighted images; therefore, low T1 signal intensity provides a limited benefit in the ability to characterize a lesion. The differential diagnosis of a lesion with T1 low or isointense signal is wide; for example, ganglia, fibrosarcomas, and pleomorphic sarcomas can all demonstrate T1 hypo- or isointensity [2, 3].

7.7 T1 hyperintense lesions

The signal intensity should be determined on nonfat-saturated images. The substances that show increased T1 signal intensity include fat, methemoglobin, proteinaceous fluid, and melanin.

T1 hyperintense lesions that do not show post-contrast enhancement include fat containing mass, a hemorrhagic mass containing methemoglobin, various collections that contain an appropriate concentration of proteinaceous fluid, and lesions that contain melanin.

Fat suppression should be done for any lesion that shows hyperintense T1 signal. If the T1 hyperintensity is suppressed on fat-saturated images, then this hyperintensity is due to fat, and the most likely diagnoses include lipoma, well-differentiated liposarcoma, hemangioma, and mature ossification [2, 3, 4, 5].

If the mass is a lipoma, it will show minimal thin septations, whereas if the lesion is greater than 10 cm in diameter, contains septa greater than 2 mm thick and/or globular or nodular nonfatty components, or contains less than 75% fat, then a diagnosis of well-differentiated liposarcoma is likely [10].

If a lipomatous mass contains benign soft-tissue component, it might show a more complex features; thus, it might be difficult to differentiate from the well-differentiated liposarcoma [10].

Hemangiomas, since they usually contain fatty component, show the suppression of signal on fat-saturated images, but they tend to show certain features helping to distinguish it from a lipoma. They tend to be lobulated and to have high-SI vascular channels on T2-weighted images (due to slow flow) and may contain rounded low-SI phleboliths on T1- and T2-weighted images (which are more apparent on X-ray images than MRI images) [74].

Ossification (as seen with mature myositis ossificans or heterotopic ossification) can show T1 hyperintensity due to fatty marrow; correlation with radiographs is important to demonstrate the ossification (which may not be present in the early stage of myositis ossificans; CT scan can help in these cases) [27, 29].

If the lesion does not lose signal intensity on the fat-suppressed T1-weighted MR images, then it contains other T1 hyperintense substances such as methemoglobin, proteinaceous fluid, or melanin.

Hematomas whether secondary to trauma or bleeding from a tumor show methemoglobin, so it should be followed up till it resolves (to exclude tumor as a source of bleeding) [4].

Mass that contains an appropriate concentration of protein can have high T1 SI; these incude ganglia, abscesses, and epidermoid inclusion cysts with high protein content [12].

If the patient has a history of melanoma, any T1 hyperintense lesion should be considered metastatic; however, not all melanotic lesions show T1 hyperintense signal [10].

7.8 T2 hypointense lesions

Any mass that shows lower signal intensity than the muscle on T2-weighted images is considered hypointense [98].

Substances that appear hypointense on T2-weighted images include fibrosis, hemosiderin, and calcification (distinct from ossification). Lesions with fibrotic components have a low number of mobile protons because they are hypocellular and appear hypointense on T2-weighted images, while hemosiderin appears hypointense due to magnetic susceptibility. When present in sufficient quantities, hemosiderin can appear more prominent (blooming) on T2*-weighted MR images [98].

Calcifications are typically T2 hypointense due to the lack of mobile protons; on the other hand, calcifications may appear as higher SI when calcium crystals are surrounded by a hydration shell [98].

Substances that have low proton density, such as air and some foreign bodies, can also appear to be T2 hypointense [99].

Masses that appear hypointense on T2-weighted images are usually composed of fibrotic material (can be benign or malignant) ranging from fibrotic scars to fibromas and some fibrosarcomas; however, not all fibrous masses have low T2 SI; hypercellular fibrous masses, such as desmoids and leiomyomas, may demonstrate higher T2 SI [98, 99].

Masses that contain large amounts of hemosiderin such as pigmented villonodular synovitis and GCT of the tendon sheath also appear hypointense on T2-weighted images [100].

Masses that are diffusely calcified may or may not have low T2 SI because this will depend on the extent and distribution of calcification, the hydration of the calcification, and any associated edema or inflammation [98].

In evaluating a mass with low T2 SI, the first step is to review the radiographs for the presence and pattern of calcifications, for example, a cloud-like para-articular calcifications seen in gout or the flocculent calcifications seen in tumoral calcinosis [89].

If the radiographs show no calcification, then the mass most likely contains substantial amount of fibrosis; in this case, the location of the lesion is of greatest importance; for example, single or multiple masses within a joint may indicate pigmented villonodular synovitis. Similarly, a mass that abuts a tendon may be a GCT of the tendon sheath. A history of prior surgery at the site of the lesion is indicative of a scar fibrosis [98, 99].

7.9 T2 hyperintense (cyst-like) lesions

A wide variety of lesions are hyperintense or heterogeneously hyperintense on T2-weighted images and are difficult to specifically characterize.

The differential diagnosis for lesions that shows hyperintensity on T2-weighted images includes fluid-filled lesions (e.g., ganglia, synovial cysts, and seromas) and some solid lesions (e.g., myxomas, myxoid sarcomas, and small synovial sarcomas). Other tissues that can mimic fluid on T2-weighted MR images are hyperemic synovium and hyaline cartilage [101].

In case of a cyst-like lesion, IV gadolinium-based contrast agent is an important to distinguish between true cysts and solid lesions. Cysts and fluid-filled components of masses will not demonstrate internal enhancement, while solid structures will [102].

Ganglia are very common and should be considered in any periarticular T2 hyperintense mass.

Postoperative seromas, posttraumatic cysts, epidermoid inclusion cysts lymphoceles, and lymphangiomas are other types of lesions that may demonstrate a thin rim of peripheral enhancement [102].

When the rim of post-contrast enhancement is thick and/or irregular, the differential diagnosis should include inflamed or infected ganglia, abscesses, hematomas, and necrotic tumor masses [102].

A T2 hyperintense mass demonstrates internal enhancement (whether homogeneous or heterogeneous); then, the differential diagnosis should include soft-tissue masses (e.g., intramuscular myxomas, myxoid sarcomas, PNSTs, and synovial sarcomas) [103, 104].

Because of its high water content, myxoid material appears hyperintense on T2-weighted MR images; it can be seen in a variety of benign and malignant masses; for example, intramuscular myxomas are benign masses that typically have a uniform hyperintense signal on T2-weighted MR images with internal enhancement on contrast-enhanced MR images. They have a thin rim of peripheral enhancement and also demonstrate nodular or heterogeneous internal enhancement. Myxoid sarcomas show the same features [104, 105, 106].

If an enhancing hyperintense lesion is in a para-articular location, synovial sarcoma should be considered. If the lesion is fusiform and is associated with a nerve, then most likely it is a PNST [103].

7.10 Contrast enhancement

Contrast agent administration is important for the purpose of differentiating between cystic and solid lesions and demonstrating tumor nodules in cystic lesions.

Only a thin rim of enhancement with no central enhancement indicates a cystic lesion of some sort. Internal enhancement is an indication that the lesion is partially or totally solid.

The degree of enhancement can relate to the vascularity of the lesion, which is important for preoperative planning.

The enhancement pattern cannot be used to reliably distinguish benign from malignant lesions [95, 96, 97].

7.11 Other MRI features

A number of additional imaging features can be used in developing a more specific diagnosis, for example, lesion size, homogeneity (versus heterogeneity) of the signal intensity, contrast enhancement, shape and margins of the lesion, necrosis or surrounding edema, presence of bone and/or neurovascular involvement, and extension of the lesion beyond compartments.

Both hemangiomas and lipomas are T1 hyperintense, but hemangiomas usually demonstrate circular, linear, or serpentine high T2 SI caused by slow flow, not a feature of lipoma. Similarly, both myxomas and synovial sarcomas are T2 hyperintense; however, perilesional edema and the presence of superior and inferior fat caps are the features of myxomas, while the presence of triple signal (areas of hyper-, iso-, and hypointensity to fat on T2-weighted MR images) is a feature of synovial sarcomas [2, 3, 4, 5].

7.12 The indeterminate lesion

If the lesion cannot be confidently characterized as a benign entity, then it is an indeterminate lesion. These types of lesions needs further evaluation; a biopsy should be strongly considered. Indeed, the World Health Organization recommends that “soft-tissue masses that do not demonstrate tumor-specific features on MR images should be considered indeterminate, and biopsy should always be obtained to exclude malignancy.” In some instances, short-term imaging follow-up may be considered [4].