Normal muscle variants

These can present as a clinically palpable mass. They have presumably been present for years but for some reason the patient has only just noticed the swelling. One of the commonest sites is at the ankle where an accessory soleus muscle may mimic a soft tissue mass. Knowledge of the normal anatomy will assist in making the diagnosis along with the observation that the area of interest has identical signal to adjacent muscle on all MR sequences. The palmar aspect of the wrist is another area where anomalous developments in the flexor tendons may sometimes result in compression of the ulnar or median nerves in their fibro-osseous tunnels.

Muscle tears

Symptomatic muscle tears are not usually a problem since there is typically a history of trauma and painful onset of swelling. Clinically there is therefore no suggestion that this is a soft tissue mass. However some patients, particularly in an older age group, present with a painless swelling with no clear history of any single incident to account for it. Characteristically this is seen in the anterior thigh. The reason is spontaneous rupture of rectus femoris muscle. The proximal muscle can contract unopposed with a resulting soft tissue mass palpable in the thigh. In cases of clinical doubt an MRI scan will confirm that the swelling is muscle and demonstrate the defect at the site of rupture (see Fig. 18.29, p. 414).

Haematoma

An acute tissue haematoma, whether from injury or post-surgery, should not pose a problem for clinical diagnosis. Difficulty can occur when a large deep haematoma does not resolve and enters a chronic phase that may mimic the swelling from a recurrent lesion. The MRI scan should aid diagnosis showing a peripheral dark rim from the presence of iron-containing haemoglobin breakdown products (Fig. 10.1).

Fig. 10.1 A and B T2 weighted axial and sagittal MR scans of a chronic haematoma in the calf. There is a well-circumscribed mass which is of high signal in the posterior compartment. The rim is very low (dark) signal indicating haemosiderin deposition seen with breakdown of blood.

Aneurysm

Occasionally an aneurysm originating from a deep vessel may mimic a soft tissue tumour, but careful clinical examination will normally identify arterial pulsation. Imaging with MR and digital arteriography can confirm the nature of the lesion and the vessel of origin.

Synovial cysts and ganglia

Ganglia are of unknown origin situated typically around joints and contain mucoid tissue. Trauma and synovial herniation are suggested as causative factors. They do not usually have a demonstrable connection with the underlying joint. They are seen most commonly around the wrist and ankle, but also occur near the knee arising from the proximal tibio fibular joint (Fig. 10.2).

Fig. 10.2 A and B T2 weighted axial and sagittal MR scans showing a large ganglion arising from the proximal tibio-fibular joint. The high signal of the well-circumscribed mass in the anterior compartment suggests a fluid content. The sagittal image demonstrates the long thin proximal extension from the joint.

Synovial cysts are a synovial lined outpouching which connects with the underlying joint space.

These therefore occur around joints most commonly and are typically seen especially in the knee. Popliteal (Baker’s) cysts (Fig. 18.32A, p. 418) and cysts associated with meniscal tears can present as palpable soft tissue masses. Ultrasound is the easiest method of confirming that these are cystic and not solid lesions.

Myositis ossificans (heterotopic ossification)

This is an uncommon but troublesome lesion because of its apparent malignant behaviour. It presents as a painful and tender lump in a limb muscle, with or without a history of recent trauma to the site. Imaging is crucial to obtain the correct diagnosis and MRI and ultrasound are the most useful in the very early stages as they can detect the presence of calcification in the lesion which is the key to diagnosis (Fig. 10.3A). Later imaging with CT scanning will reveal a diagnostic rim of ossification at the periphery of the well-circumscribed lesion. Plain radiographs after several weeks will show some calcification in the muscle which should indicate the correct diagnosis (Fig. 10.3B). However, in some patients where the early changes are not recognised these appearances may be misinterpreted and may lead to a presumed diagnosis of soft tissue osteosarcoma. This assumption may be reinforced if a biopsy is performed, as this will show a very active lesion containing immature mesenchymal cells with atypical nuclei and osteoid formation. No active treatment is required, other than careful observation, as the lesion will resolve in 2–3 months.

Fig. 10.3 A T2 weighted axial MR scan of the arm with myositis ossificans in the biceps muscle. Inside the mass of high signal there is a ring of low signal representing calcification. MR or ultrasound will detect calcification long before it is visible on plain films. B Radiograph of humerus in the same patient as Fig. 10.3A. This was obtained several weeks after the MR scan and the calcification within the muscle is now evident and a diagnosis of myositis ossificans is most likely.

Benign peripheral nerve sheath tumours

Tumours arising from nerves are of two types: schwannomas and neurofibromas.

Neurofibroma

These are a little commoner and present as a soft, circumscribed, rounded, and slightly tender swelling in the skin or deeper tissues. They are more often associated with neurological symptoms. Pathologically they are not separate from the nerve which can be seen entering and leaving the tumour. Thus resection of the tumour requires sacrifice of the involved nerve.

Histologically the tumour is composed of cellular fibrous tissue arranged in whorls. The tumour may be solitary; but in the condition known as multiple neurofibromatosis (von Recklinghausen’s disease) (p. 70) numerous tumours are associated with pigmented areas in the skin. A neurofibroma growing within the spinal canal is an important cause of compression of the spinal cord or cauda equina.

Imaging with MR scanning can identify the nature of the lesion and sometimes its neural origin (Fig. 10.4).

Fig. 10.4

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