Soft-Tissue Tumours

Radiographs

Radiographic assessment of a soft-tissue tumour has a low diagnostic yield but may still be useful.

In a large study of 454 patients referred to a tumour centre with a soft-tissue mass, there was a radiographic abnormality in 62%. The mass itself was visible in 31% of cases, typically showing soft-tissue density and displacement of adjacent fat planes. In the presence of a visible mass, the diagnosis was more likely to be malignant, although this depended on the location of the lesion, with tumours in the hands and feet more likely to be benign or non-neoplastic and tumours in the thigh, calf and arm more likely to be malignant.

If the mass is large and contains fat in sufficient quantity, its density will be lower than that of adjacent soft tissues. Fat was seen in 7% of the masses: 50% were well-differentiated liposarcomas (WDLs)/atypical lipomatous tumours (ALTs; see below), with benign fat-containing lesions including lipoma, haemangioma, hibernoma and lipoblastoma.

Foreign bodies may be visualised on radiographs, with the mass due to a surrounding inflammatory reaction. Typically, glass and metal are visible but wood and plastic may not have sufficient density to be seen.

Mineralisation was present in 17% of the soft-tissue masses in the study. The pattern of calcification may give an indication of the underlying histological nature of the mass. Phleboliths, due to the dystrophic mineralisation of thrombus in abnormal vessels, are the radiographic hallmark of a venous malformation (VM). Chondral calcification (e.g. in soft-tissue chondral tumours and synovial chondromatosis) may take the form of punctuate foci in comma shapes, rings and arcs, suggesting cartilage. Ossification (e.g. in myositis ossificans), when mature, forms trabeculae and a cortex, with density at the periphery greater than at the centre. When immature, it forms amorphous ‘cloud-like’ density.

The pattern of calcification does not always help with the diagnosis. Approximately 30% of synovial sarcomas (SSs) are mineralised, with calcification ranging from subtle to extensive, often resembling chondroid or osteoid matrix.

Abnormality of a bone due to an adjacent soft-tissue mass may take several forms. This study showed cortical erosion, periosteal reaction, scalloping, intramedullary extension and pathological fracture, affecting approximately 14% of subjects, 39% of which were malignant. Changes were in general non-specific, with benign, non-neoplastic and malignant lesions capable of a wide range of interactions with the adjacent bone. Periosteal reaction when present (4.4% of cases) was seen in in 70% of non-malignant lesions.

Computed Tomography

Evaluation of the soft-tissue mass with computed tomography (CT) is useful in some cases but, as with radiographs, it is often not tissue-specific, as many masses show similar attenuation to muscle. The radiographic features of a soft-tissue mass are demonstrated at least as well using CT and it is more sensitive for the identification of small amounts of fat and calcium. As an adjunct to magnetic resonance imaging (MRI) and radiographs, CT is useful for showing the peripheral ossification in myositis ossificans, confirming what may be a difficult MRI diagnosis ( Fig. 42.1 ). CT angiography (and specifically arteriography) is helpful for the preoperative assessment of tumours close to vessels.

Ultrasound

US has been shown to be a useful triage tool for masses referred from primary or secondary care. It can play an important role in confirming a mass is present and not due to a normal anatomical structure or variant (such as an accessory muscle). It can also differentiate solid from cystic lesions. However, once a solid mass has been confirmed, further characterisation with US may be limited and assessment with MRI and biopsy is often required. As an adjunct to MRI, US can further characterise masses that show fluid signal intensity (SI; cysts and solid myxoid masses), assess vascularity and compressibility (typically of VMs), ( Fig. 42.2 ) and allows valuable clinical correlation by the examining radiologist.

Diagnostic features may be present in some lesions and these are more frequently benign or non-neoplastic. Examples include traumatic (muscle/tendon tear, haematoma), infective (abscess, pyomyositis), lipomatous (superficial and deep fatty masses—the latter usually need further assessment with MRI), vascular (thrombophlebitis, VM, pseudoaneurysm) and neurogenic lesions such as nerve sheath tumours and nerve pseudotumours.

In the differentiation of a malignant from a benign solid mass, the grey-scale appearances are usually non-specific. The assessment of tumour vascularity using colour/power and pulsed Doppler is possible with US and greater success has been claimed for this tool. Tumour neovascularity is an important feature of malignancy—tumour vessels are anatomically abnormal, lacking a muscular layer and showing irregular contour. This results in a heterogeneous network of abnormal vessels, often chaotically distributed within the tumour, with abnormal flow characteristics and morphology. A study of benign and malignant masses with high vascularity showed that the pattern of vessels within a mass was of limited value in the distinction of benign and malignant, but if they appeared predominantly organised, a benign diagnosis was more likely. An organised vascular pattern has been shown in 77% of benign and a disorganised pattern in 80% of malignant soft-tissue tumours. The atypical morphology of vessels in malignant masses may result in vessel trifurcations (rather than bifurcations), stenoses, occlusions and an anarchic rather than an hierarchical branching pattern. Identification of any two of these ‘major’ criteria enabled malignant and benign masses to be distinguished.

US is frequently used to guide biopsy of soft-tissue masses.

Magnetic Resonance Imaging

MRI is the technique of choice for local staging of a soft-tissue mass, especially if it is in a deep location where US may be less able to assess tumour extent and relations. It is sensitive, can be tissue-specific and allows multiplanar assessment of the mass and adjacent vital structures. Despite this, MRI is often not sufficiently tissue-specific to allow confident characterisation of some deep, solid masses. Distinction of myxoid from cystic masses can be difficult without contrast medium and small calcific foci may not be seen. MRI overlaps with radiography in its ability to identify fat (more sensitive for small foci), foreign bodies and bone involvement in a soft-tissue mass: differentiation of mineralisation and gas may require radiographs or CT, as both show hypointensity on T ₂ weighted images and also on other fluid-sensitive sequences (see below).

Most tumours, and certainly most soft-tissue sarcomas (STSs), are isointense to muscle on T ₁ weighted images, of intermediate SI on T ₂ images (similar to fat) and hyperintense on short-T ₁ inversion recovery (STIR) or T ₂ weighted images with fat saturation. As with US, specific diagnoses can sometimes be suggested based on the MRI appearances, more often with benign and non-neoplastic lesions, which may exhibit specific signal characteristics. Examples of MRI signal that allow characterisation and occasionally an MRI diagnosis (in combination with lesion morphology) include T ₁ hyperintensity, T ₂ hypointensity and T ₂ hyperintensity (fluid signal).

Hyperintensity on T ₁ weighted imaging is commonly due to fat or subacute haemorrhage (methaemoglobin), but may also be due to melanin.

Hypointensity on T ₂ weighted imaging may be due to calcification, fibrous tissue, chronic haemorrhage (haemosiderin); a signal void may be seen with rapidly flowing blood (e.g. in an arteriovenous malformation [AVM] or aneurysm) or gas. A radiograph is useful for excluding calcification as a cause.

Hyperintensity on T ₂ weighted imaging may be seen in cystic masses, abscesses, myxoid masses (which include intramuscular myxomas, sarcomas and some nerve sheath tumours) and low-flow vascular malformations.

The presence of fluid-fluid levels is a non-specific sign in soft-tissue masses: they are rare but occur with equal frequency in benign and malignant lesions. The presence of haemorrhage can be useful—subacute haemorrhage is seen in up to 47% of SSs; tumour necrosis, which often contains blood, suggests a high-grade sarcoma radiologically and histologically. High-flow vessels within the tumour are associated with certain diagnoses and may be an indication for preoperative embolisation.

Staging of STS is best undertaken with MRI. This requires documentation of the size and local extent of the tumour, including any extracompartmental spread and the relationship of the tumour to adjacent structures such as bones, joints and neurovascular bundles. Initial staging will also usually require a chest radiograph and CT to exclude pulmonary metastases.

Lipoma

This is a benign adipocytic tumour and the commonest mesenchymal neoplasm in adults. Lipomas typically occur in adults in the fifth to seventh decades with no sex predilection. They are rare in children and may be multiple. Superficial (subcutaneous) tumours are common but lipomas may also occur in deep locations (intramuscular or intermuscular), or even adjacent to bone (parosteal), where they may stimulate a periosteal response. Subcutaneous lipomas are often diagnosed with great accuracy on clinical grounds alone, presenting as small (usually <5 cm), painless, firm, mobile masses. Deep lipomas are often larger, but again usually painless. They cannot be diagnosed clinically, and imaging is needed to confirm the diagnosis in deeper lesions. Atypical clinical presentation includes pain due to nerve compression in restricted spaces such as the carpal, tarsal and cubital tunnels.

Lipomas can contain other non-lipomatous elements, contributing to a heterogeneous imaging appearance: vessels (angiolipoma), muscle (myolipoma), cartilage (chondrolipoma), bone (osteolipoma—following trauma or ischaemia) or extensive myxoid change (myxolipoma). Fibrous tissue and foci of fat necrosis can also cause a heterogeneous appearance.

Radiographs

Large lipomas are visible as areas of relative lucency (compared with soft tissue) ( Fig. 42.3 ), but small lesions are not visible. Foci of mineralisation (calcification, ossification) are visible in 11%.

Ultrasound

Subcutaneous lipomas are usually elliptical masses, often well-defined, with both their long axis and internal septations orientated parallel to the skin surface. Their reflectivity is variable: while subcutaneous fat is usually low in reflectivity (unless previously inflamed or traumatised), lipomas are often relatively reflective ( Fig. 42.4 ). One study showed 29% had lower reflectivity than surrounding tissues and the remainder was of similar, greater or mixed reflectivity. Light pressure with the US probe shows the lesions are compressible. They may appear encapsulated, although many lipomas appear to blend with the surrounding tissues without evidence of a capsule.

Magnetic Resonance Imaging

The mass is isointense to subcutaneous fat on T ₁ weighted and T ₂ weighted imaging ( Fig. 42.5 ), showing low SI on STIR images and suppression of fat signal on T ₂ weighted imaging with fat saturation. Fine septa (often immeasurable, typically <2 mm) are often seen, but the absence of thick or nodular septa is useful for differentiating a lipoma from a WDL/ALT. Fine septa are also less likely to show gadolinium enhancement in benign lesions, which is also useful for differentiation. Focal areas of non-adipose tissue (including fibrosis and necrosis) are not infrequent in benign tumours (particularly deep lesions) and make the distinction from a WDL impossible on imaging alone. Mineralisation (calcification and ossification) may also be seen in benign tumours.

Intramuscular lipomas can be diagnosed with a high degree of confidence: an infiltrative margin and intermingled muscle fibres within and at the periphery of the lesion are features that make it possible to differentiate from other lipomatous tumours ( Fig. 42.6 ).

Although they may be seen in simple lipomas, the following MRI features are atypical and should suggest the possibility of an atypical lipomatous tumour:

• incomplete suppression of signal on fat-suppressed sequences

• thick, nodular septa (particularly if they enhance)

• focal non-lipomatous areas.

Other Benign Adipocytic Tumours

Atypical Lipomatous Tumour/Well-Differentiated Liposarcoma

These lesions are synonymous and classified as intermediate (locally aggressive) malignancies. They account for 40% to 45% of all liposarcomas and are the commonest subtype of liposarcoma. ALT/WDL are well-differentiated tumours, often predominantly fatty, and show no capacity for metastasis unless dedifferentiation occurs (see below). Although ALT and WDL are identical morphologically and genetically, the term ALT is used for tumours in surgically accessible sites, such as the limbs and trunk, where complete excision is possible and curative. WDL is usually used for tumours in inaccessible sites, such as the mediastinum and retroperitoneum, where complete excision may not be possible and the disease may eventually be fatal following uncontrolled recurrence.

These tumours occur in middle age, most commonly in the sixth decade, and there is no sex predilection. They arise in the deep soft tissues of the limbs, retroperitoneum, paratesticular area and mediastinum—rarely they occur subcutaneously. Imaging shows a deep lipomatous mass with varying degrees of heterogeneity. The following features favour a diagnosis of a malignant rather than a benign tumour: lesion size greater than 10 cm; thick, nodular septa; presence of globular or nodular non-adipose areas; and fat content less than 75% ( Fig. 42.7 ). Incomplete fat suppression suggests a liposarcoma, while septal enhancement is more likely if the lesion is malignant and may be more important than the thickness of the septa. Although on imaging the differentiation of a lipoma in a deep location from ALT/WDL is unreliable, with considerable overlap in the appearances, it is largely irrelevant to initial management. Both are well-differentiated lipomatous neoplasms and would usually be treated by primary excision, with as wide a margin as possible, without prior biopsy.

Other Adipocytic Malignancies

The other major types of liposarcoma are dedifferentiated, myxoid and pleomorphic (pleomorphic liposarcoma is a rare high-grade tumour). With higher grades of malignancy, fat becomes less conspicuous on MRI, such that there may be little suggestion of an adipocytic tumour.

Dedifferentiated Liposarcoma

Dedifferentiated liposarcoma results from the transition of ALT/WDL to a non-lipogenic sarcoma of variable grade and may occur in the primary tumour (90%) or in a recurrent lesion. The risk of dedifferentiation is greatest in retroperitoneal tumours and occurs in up to 10% of ALT/WDL—this may be time-dependent rather than location-dependent. It is usually a high-grade tumour, with the dedifferentiated component seen as a discrete non-lipomatous tumour within the fatty mass, reflected in both the histological and radiological appearances ( Fig. 42.8 ). Occasionally, there is a gradual transition from lipomatous to non-lipomatous sarcoma.