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Guide to pseudotumours and soft tissue tumour mimics
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current WHO Classification of Tumours of Soft Tissue and Bone.3 As myositis ossificans (MO) and pigmented villonodular synovitis (PVNS) were traditionally described as tumours mimics, we have included them in this review, however, it must be emphasized they are classified as tumours.
Guide to pseudotumours and soft tissue tumour mimics
Infection Kethesparan Paramesparan
Infection can manifest in a number of ways but it is often clinically apparent that an underlying infective process is occurring. Infection can arise from direct inoculation, haematogenous spread or due to adjacent osteomyelitis. However, there are a number of situations when patients may be asymptomatic or present with a focal soft tissue mass that is not typical of infection.
Amit Shah Winston J Rennie
Abstract There are many pathological entities that can mimic a soft tissue tumour. Even with appropriate clinico-radiological assessment, some of these lesions can be misinterpreted as soft tissue sarcomas. Also known as pseudotumours, this can lead to unnecessary further investigations or intervention. With the relevant clinical history, clinical examination and imaging characteristics, one can narrow the differential diagnosis of these problematic lesions. Occasionally the diagnosis of a pseudotumour may only be made after histopathological analysis. We present common tumour mimics using an anatomical sieve that should be considered when presented with a palpable soft tissue mass.
Tuberculosis infection Tuberculosis (TB) is aptly named the great mimicker, simulating numerous disease entities. Extra-pulmonary TB can affect bones, joints and soft tissue. In adults extra-pulmonary TB occurs between the ages of 20e50 years. Musculoskeletal manifestations of TB are very rare and account for between 1% and 3% of TB infections. Of those patients affected with musculoskeletal TB, 50% have spinal manifestations, 30% are related to the knee and the hip and 20% have involvement of other sites.4 HIV remains the biggest risk factor for the activation of TB infection but other contributory factors include the emergence of multi-drug-resistant strains of Mycobacterium, immigration and the increasingly ageing population. Acutely, patients classically present with a slow-growing painful swelling around a joint (Figure 1), and in the late stage may progress to joint stiffness. The usual symptoms seen in pulmonary TB of fever, weight loss and loss of appetite may not be seen. A non-specific complex cystic mass is generally seen, which can be either a solid or a peripherally enhancing cystic collection.
Keywords musculoskeletal system; pseudotumour; soft tissue mass; soft tissue sarcoma; tumour mimic
Introduction Soft tissue sarcomas (STS) are rare malignant mesenchymal or connective tissue tumours accounting for approximately 1% of all adult primary tumours.1 Most soft tissue lesions are benign with lipoma being the commonest pathology.2 It is often difficult to distinguish between malignant and benign lesions on clinical judgement alone and therefore imaging plays a central role. Many pathological entities can present clinically as a soft tissue mass. These tumour-mimicking lesions, also known as pseudotumours, can be defined as clinically palpable masses or lesions that have tumour-like appearances on imaging. Even with appropriate clinico-radiological assessment, these lesions can be misinterpreted as STS, leading to intervention with the diagnosis of a pseudotumour made only after histopathological analysis. We aim to provide a review of common pathologies that mimic STS and present them using an anatomical sieve. For the purposes of this article, we have excluded lesions that are included in the
Abscess An abscess is a focal collection of necrotic material and pus, often communicating with sinus tracts extending into joints, soft tissue or skin ulcerations. They are common and have been shown to account for 7% of soft tissue pseudotumours.5 Patients that are post-surgical, immunocompromised, diabetic and those that have a co-existing infection (e.g. osteomyelitis) are at increased risk of developing an abscess. Staphylococcus aureus species are the commonest causative agent followed by streptococcal species. Patients usually present with fever, pain, and a painful mass with an infective blood profile. The soft tissue swelling may be erythematous, fluctuant, warm and can have a slow onset of growth. Ultrasound is most useful in localizing the abscess and demonstrates surrounding soft tissue vascularity (Figure 2a). MRI will demonstrate a thick, irregular-walled low T1-weighted (T1W) signal intensity (SI) and high T2-weighted (T2W) SI collection (Figure 2bed).
Kethesparan Paramesparan MBBS BSc(Hons) Radiology Specialist Trainee, Dr, Department of Radiology, University Hospitals of Leicester, Leicester Royal Infirmary, Leicester, UK. Conflicts of interest: none declared. Amit Shah FRCR Consultant Radiologist, Dr, Department of Radiology, University Hospitals of Leicester, Leicester Royal Infirmary, Leicester, UK. Conflicts of interest: none declared.
Inflammatory Epidermal inclusion cyst Epidermal inclusion cysts are benign cysts filled with lamellated keratin/granular debris and lined with stratified squamous epithelium. They commonly arise from an obstructed hair follicle
Winston J Rennie FRCR Consultant Radiologist, Dr, Department of Radiology, University Hospitals of Leicester, Leicester Royal Infirmary, Leicester, UK. Conflicts of interest: none declared.
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Figure 1 Tuberculosis (a) Ultrasound image of a left chest wall mass demonstrating a complex mixed echogenic lesion. (b) Axial CT demonstrates a low attenuation lesion (white arrow) in the left chest wall eroding the sternum and arising from the sternocostal joint. (c) Axial fluid-sensitive MRI sequence demonstrates a fluid collection with surrounding soft tissue oedema (white arrow heads). (d) Axial T1 fat-saturated post-gadolinium MRI demonstrates peripheral enhancement in keeping with an abscess, which was histologically proven secondary to tuberculosis.
Figure 2 Abscess (a) Doppler ultrasound demonstrates a complex semisolid collection around the medial elbow with surrounding vascularity. (b) Axial fluid-sensitive MRI demonstrates a cystic collection (white arrow) with marked surrounding soft tissue oedema. (c) T1 fat-saturated pre-contrast MRI and (d) T1 fat-saturated post-contrast MRI demonstrates a thick walled rim-enhancing collection, which grew Staphylococcus aureus after aspiration. ORTHOPAEDICS AND TRAUMA 31:3
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Figure 3 Epidermal inclusion cyst (EIC) (a and b) Ultrasound of two cases of EIC demonstrating characteristic imaging appearances of a wellcircumscribed heterogeneous filiform lesion with posterior acoustic enhancement (white arrow heads) and no colour Doppler signal as seen in (b). (c) Axial T1 MRI demonstrates a classic subdermal low signal intensity lesion (black arrow) with no enhancement post-contrast (d).
or deep implantation of epidermis triggering an inflammatory response. Frequently found in the neck, face, scalp and trunk, less than 10% are said to occur in the extremities.6 Ultrasound demonstrates a well-circumscribed, heterogeneous filiform lesion that often demonstrates posterior acoustic enhancement and no colour Doppler signal (Figure 3a and b). MRI appearances are of low T1W SI/high T2W SI subdermal mass that may demonstrate capsular enhancement post contrast (Figure 3c and d). A ruptured epidermal cyst on the other hand may have a lobulated contour with a positive colour Doppler signal thus mimicking an aggressive lesion and biopsy may be required to exclude malignancy.
from hyperintense to hypointense depending on the levels of methaemoglobin and haemosiderin. There should be no enhancement on post-contrast imaging. Because of the complex appearances and repeated haemorrhages that can occur with anticoagulation use, a haematoma may be indistinguishable from an aggressive tumour. If haematoma is considered as the diagnosis, it is prudent to follow up these patients clinically or with imaging to ensure gradual resolution over time, which can take at least 3 months. If there is no interval reduction, a biopsy should be considered. Of note, one has to be wary of dismissing unexplained spontaneous haematomas as benign and an underlying neoplasm must be excluded. Non-specific radiological features of a malignant haemorrhagic lesion include a haemorrhagic mass demonstrating internal vascularity, associated enhancing solid components, infiltrative pattern and non-resolution over time.
Traumatic Haematoma Haematomas are localized collections of blood within a contained space, outside a vessel wall. They are often associated with traumatic muscle injury, muscle strain and contusions. Haematomas are very common and account for 5% of soft tissue masses.5 Intramuscular haematomas are often associated with patients with bleeding tendencies such as those on anticoagulant therapy and clotting disorders. Patients present with a slowly growing expanding mass as a result of recurrent haemorrhage. Bruising and discolouration of the skin may be noted at the site of the haematoma if it is superficial. Haematomas can be associated with tendon avulsions and myotendinous strain injury. The echogenicity of haematomas seen on ultrasound imaging evolves over time. (Figure 4a and b). MRI appearances can be variable and dependent on the stage of the haematoma at the time of imaging. Fluidefluid levels may be seen and the T1W SI varies
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Muscle tear Muscle tears tend to occur at the myotendinous junction or myofascial surface, which is the weakest point of the muscletendon unit. Patients may have a non-specific history of a prior traumatic event that is usually overlooked. Clinical examination may reveal an asymmetrical enlarged palpable soft tissue mass with a gap along the expected course of the tendon. Intramuscular haematomas or the retracted tendon and associated haematoma resulting from a complete tendon avulsion can mimic a soft tissue neoplasm. Common muscle ruptures include the hamstrings, quadriceps (particularly rectus femoris) and biceps brachii (Figure 4c). Imaging depends on the extent and grade of muscle strain. Muscular injuries resulting in complete myotendinous junction tears, retraction and associated haematoma
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Figure 4 Traumatic mimics (a) Patient on anticoagulation presenting with a large soft tissue mass. Ultrasound demonstrates a large lesion with layering in keeping with an acute haematoma. Haematomas can vary in appearances depending on the stage of evolution as seen in (b) which demonstrates a liquefying haematoma. (c) Extended field of view ultrasound demonstrating a complete rectus femoris tendon tear with proximal tendon retraction corresponding to the clinically palpable lump (white arrow). (d) Axial T2 MRI demonstrates a lobulated fluid collection located e lesion (black arrow). (e) between the deep subcutaneous tissue and the deep fascia of the thigh, which is a classic location for a Morel-Lavalle Coronal T1 MRI of the same patient demonstrates a low-signal rim in keeping with a fibrous pseudocapsule (black arrow heads). (f) Ultrasound of a patient presenting with a focal lump in the calf demonstrating a gap in the myofascia of the tibialis anterior muscle with herniation of the muscle (mushroom cap sign).
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or intramuscular haematomas, are injuries that can pose diagnostic challenges. In time fibrosis and or calcification can occur which can lead to the development of myositis ossificans. Biopsy of the lesion will demonstrate granulomatous chronic inflammatory cells and reactive fibrosis.
Muscle hernia Myofascial herniations of the extremities are most commonly located in the lower leg and are not routinely encountered in clinical practice. Herniations can be secondary to congenital, traumatic or predisposing conditions such as muscular hypertrophy or chronic compartment syndrome. The tibialis anterior muscle is most commonly involved with many other muscles such as the peroneus longus and brevis and gastrocnemii muscles. Muscle herniation occurs as a result of a focal fascial sheath defects typically at sites of weakness secondary to perforating nerves and vessels. Presentation may be in the form of a soft tissue mass, a subcutaneous nodule or a palpable bulge, which may or may not be reducible. Patients may be completely asymptomatic or complain of pain and weakness, which may worsen with standing or physical activity. Specific movement may pronounce the herniation, for example the tibialis anterior hernia may be more apparent on resisted dorsiflexion. Imaging evaluation with ultrasound is sufficient as there is a dynamic observation of the muscle herniation at the site of the palpable mass. The finding of a hyperechoic muscle prominence through a fascial defect (creating a ‘mushroom cap’ sign) is characteristic (Figure 4f).
e lesion Morel-Lavalle The Morel-Lavallee lesion (MLL) occurs secondary to a blunt force degloving injury typically in the proximal thigh and pelvis causing separation of the hypodermis from the underlying fascia.7 The created space between the subcutaneous fat and underlying fascia fills with the haemo-lymphatic fluid and becomes surrounded by granulation tissue. This forms a fibrous capsule further preventing reabsorption of the collection leading to formation of a pseudotumour. MLLs can be diagnosed on clinical examination in patients presenting with a history of trauma and symptoms of skin mobility, local contusion, soft fluctuant area, decreased sensation, friction burns and variable ecchymosis. Imaging can be particularly useful in demonstrating a collection between the subcutaneous fat and deep fascia to help diagnose MLLs (Figure 4d). MRI appearances depend on timing of imaging and complications. MLLs follow fluid SI internally except for fat nodules and blood products. The fibrous pseudocapsule is hypointense on all sequences (Figure 4e). Post-contrast nodular or peripheral enhancement may be seen with granulation tissue, inflammation, or infective complications.
Vascular Aneurysm Aneurysms can be classified as ‘true aneurysms’ when all three layers of the arterial wall are abnormally dilated or ‘false
Figure 5 Popliteal artery aneurysm (PAA) (a) Right femur radiograph demonstrating a soft tissue density with a calcific rim (white arrow). (b) Axial CT of both thighs demonstrates a right PAA (white arrow) (c) Axial T1 MRI demonstrating the PAA is largely thrombosed (white arrow) and the high T1 signal intensity is in keeping with methaemoglobin. (d) Fat-suppressed MRI demonstrates pulsation artefact (white arrow heads) which is an important clue in distinguishing a solid mass from a vascular one, as biopsy of this would have significant consequences. Images courtesy of Department of Radiology, Royal Orthopaedic Hospital, Birmingham, UK.
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aneurysms’ (pseudoaneurysms), which are due to a defect in the arterial wall related to trauma or (mycotic) infection. Popliteal artery aneurysms (PAA) are the most common peripheral aneurysm accounting for 70e80% of extremity aneurysms. They are often bilateral and can present as a focal mass in the popliteal fossa, which is often pulsatile or incidentally found on imaging. Imaging will demonstrate aneurysmal dilatation of the artery, which may contain intraluminal thrombus, seen as concentric
rings on MRI, and calcification. Thrombotic aneurysms will have a pseudosolid appearance (Figure 5c). Pulsation artefact can be seen which will help distinguish this from a solid mass (Figure 5d). Arterio-venous malformation Unlike haemangiomas, which are benign vascular tumours, arterio-venous malformations (AVM) are abnormal congenital
Figure 6 Arteriovenous malformation (AVM) (a) Ultrasound demonstrating a large echogenic subcutaneous mass with multiple tortuous channel typical of an AVM. (b) Radiograph of the forearm demonstrating two oval lamellated calcific densities (white arrows) in keeping with phleboliths within an underlying AVM. (c) Axial CT of another patient demonstrating a partially calcified serpiginous paraspinal mass (black arrow), which demonstrates fat hypertrophy as seen on the axial T1 MRI (d). This lesion was histologically proven as an AVM. (e) AVM in another patient demonstrates the typical appearance of a serpiginous fat-containing AVM.
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vascular lesions consisting of direct arterial and venous connections with an associated (minimal) soft tissue component. AVMs are differentiated from vascular tumours by the lack of mitotic activity. AVMs can be subdivided into high- or low-flow lesions and occur commonly in children and early adulthood. In the paediatric population, vascular malformations represent the most common cause of soft tissue masses.8 AVMs can occur in various anatomical locations such as the face, trunk, extremities and skeletal muscle. They can present as a red pulsatile mass with a thrill growing out of proportion to the growth of the child/ adult. Commonest AVMs are venous vascular malformations (VVM) and seen on ultrasound as multiple anechoic, tubular vascular channels that can infiltrate subcutaneous fat, muscles, fascia, and tendons in the diffuse type or appear as a focal mass (Figure 6a). Phleboliths are hallmarks of VVMs and can be seen as shadowing echogenic foci and can be detected on radiographs (Figure 6b). MRI is the most valuable imaging modality for identification and classification of AVMs as it can define the extension of the lesions to the adjacent structures. Flow voids, enlarged feeding arteries and tortuous draining veins are seen with surrounding subcutaneous fat hypertrophy, muscle atrophy, and fatty replacement (Figure 6d and e).
Metabolic Tumoural calcinosis Tumoural calcinosis (TC) is a benign disorder characterized by the non-osseous calcification of peri-articular soft tissues and mainly presents in large joints (hips, elbows, shoulders). It is classified into primary hyperphosphatemic (PHP) and secondary TC categories. There is some evidence of an autosomal dominant inheritance in PHP TC. Associations with end-stage renal disease, vitamin D toxicity and hyperparathyroidism have also been established.9 TC can be found in any age group but is mainly prevalent in the first and second decades of life and has a strong predominance with the African American population.9 Patients will present with a firm, non-tender mobile mass in the periarticular soft tissue with a focal functional deficit of the joint if it is large. Superficial skin ulceration with drainage of a chalky milk-type consistency is also a common finding. Plain film radiographs typically reveal well-defined, large, multi-lobulated, amorphous, cloud-like densities within the soft tissue with radiolucent septae (Figure 7). MRI fluid sensitive imaging will demonstrate high SI in the cystic areas, peripheries and adjacent oedema with lower SI representing solid calcium. Fluid-calcium levels may be seen in keeping with due to different states of calcium, known as the ‘sedimentation sign’. There is ‘chicken wire’ enhancement post-contrast. Scintigraphy will demonstrate uptake at sites of calcium deposition. Differentials are myriad but include synovial osteochondromatosis, myositis ossificans, calcific myonecrosis, synovial sarcoma, and osteosarcoma.
Figure 7 Tumoural calcinosis (TC) (a) Radiograph of the pelvis demonstrating a right groin periarticular cloud-like dense calcific mass (black arrow) and a peritoneal catheter (white arrow) in a patient on dialysis with end-stage renal disease. TC in patients on dialysis are typically multiple and occur in those with a longer duration of uncontrolled calcium. (b) Axial CT demonstrates the dense amorphous calcific mass as soft tissue in origin with areas of ‘sedimentations’ (white arrows).
extremities and are named according to the muscle with which they are structurally associated. Patients are usually young adolescent adults and present with lower leg soft tissue fullness with intermittent pain and swelling after exercise or due to neurovascular compromise symptoms due to mass effects.
Degenerative Bursa A bursa is a synovial-membrane-lined fluid-filled sac located between bones and tendons and/or muscles and around joints. These can become inflamed and distended secondary to trauma, autoimmune disorders, infection and iatrogenic causes, although they can become distended and inflamed in any anatomical location. A frequently encountered bursal cyst presenting as palpable mass is the acromioclavicular joint (ACJ) cyst (Figure 9a). Fluid from a degenerate glenohumeral joint (GHJ) with underlying chronic full-thickness rotator cuff tear distends the subacromial-subdeltoid bursa and extends through a degenerative ACJ. Imaging characteristically shows fluid communicating from the GHJ through the ACJ forming a collection superficial to the ACJ known as a ‘geyser sign’ (Figure 9f). The cyst can vary from simple fluid collection to complex pseudosolid structures (Figure 9d and e).
Congenital Accessory muscles Accessory muscles are morphologically normal muscles and may be uni- or bilateral. Accessory muscles can be misinterpreted as a tumour on imaging. An accessory soleus muscle is a frequent finding especially on MRI (Figure 8). Numerous accessory muscles have been described in the literature throughout the
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Figure 8 Accessory muscles mimicking a palpable lump (a and b) Axial and sagittal T1 MRI demonstrates a mass (white arrow) within Kager’s fat pad, which is isointense to muscle in keeping with an accessory soleus muscle.
approximately 1.8 patients per million population. Malignant variants, malignant transformation, and metastases have been rarely reported. Adults between the ages of 20 and 50 years are most commonly affected with no specific gender representation. Approximately 80% occur in the knee, followed by the hip, ankle, shoulder and elbow. The typical presentation is of knee joint swelling, which is mildly painful on movement. The onset of swelling is often insidious, recurrent and haemorrhagic with the size of effusion out of proportion to the degree of mild discomfort. Plain radiographs may be normal with only a dense effusion seen or alternatively well-defined subchondral cysts and erosions with sclerotic edges (Figure 11a). Joint space is relatively preserved. MRI will demonstrate the classical appearance of low signal structures that demonstrate ‘blooming artefact’ on gradient-echo sequences due to haemorrhagic synovium and haemosiderin deposits (Figure 11c). As PVNS is locally aggressive, a high rate of recurrence has been reported. Recurrence, often multiple, can occur in 33e50% of cases.
Ganglion Ganglions are fluid-filled cysts found in peri-articular soft tissues arising from joint capsules or tendons in areas under repetitive stress. It is a pseudocyst, lined by a thick fibrous capsule composed of flat spindle cells and do not contain a synovial lining. Ganglion cysts represents one of the most common benign soft tissue masses encountered in the foot and ankle and occur more commonly in women10 (Figure 10a). Most ganglion cysts are uni-locular and contain clear viscous gelatinous fluid, however some may be lobulated in appearance and have thin septations. Plain radiographs are non-specific but may show some findings of non-aggressive bony remodelling. Ultrasound is the modality of choice demonstrating a hypoechoic, well-defined, non-compressible cystic mass with a neck arising from a joint or related to a tendon (Figure 10b). MRI typically shows a welldefined, lesion with smooth borders which is hypo to intermediate SI on T1W images and high SI on T2W images, with a smooth rim of peripheral enhancement after contrast administration. Ultrasound-guided aspiration can be performed if the patient is symptomatic but like surgical excision, recurrence rates are high.
Myositis ossificans MO is the formation of benign heterotopic ossification in soft tissue. The WHO Classification of Tumours of Soft Tissue and Bone includes MO because of its hypercellularity, cytological atypia, and mitotic activity. It has a reported incidence of between 9% and 17% and is a common trauma-related tumour mimic.5 Patients classically present in their teen years but have an age range between 6 months and 84 years.11 MO may develop anywhere in the body including the extremities, trunk, and head and neck with the most common locations being those most susceptible to trauma such as the elbow, thigh, buttock, and shoulder. MO is usually related to repeated trauma, which initiates the proliferation of mesenchymal stem cells that produce activated fibroblasts and
Miscellaneous Pigmented villonodular synovitis PVNS is characterized by proliferation of intra-articular synovial tissue and tendon sheaths. PVNS is a locally aggressive synovial neoplasm with a potential for local recurrence and is grouped within the same family of disorders as the giant cell tumour of tendon sheath (GCTTS). GCTTS is histologically similar to PVNS but are extra-articular and affect only tendon sheaths. PVNS accounts for approximately 0.9% of all benign soft tissue tumours within the lower limbs and the annual incidence is
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Figure 9 Degenerative bursa (a) Patient with a large firm lump over the acromioclavicular joint (ACJ) (b) Aspiration of the lump reveals typical straw-coloured thick synovial fluid. (c) Radiograph of the right shoulder demonstrates a well-defined dense soft tissue mass (white arrow) overlying a degenerative ACJ with complete loss of the subacromial space in keeping with underlying rotator cuff tear. (d) Ultrasound demonstrates a simple fluid collection with internal echoes overlying the ACJ. (e) Ultrasound of a different patient demonstrates a more complex appearing cyst. The location and underlying rotator cuff tear helps differentiate these from more sinister pathologies. (f) Axial fat-saturated MRI demonstrates full thickness cuff tear with a degenerate glenohumeral joint and ACJ. There is fluid in the subacromial-subdeltoid bursa with fluid escaping through the ACJ (Geyser’s sign) (white arrow head) forming a palpable ACJ cyst.
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Figure 10 Ganglion cyst (a) Photograph of the foot demonstrating two palpable lumps over the base of the fifth metatarsal and fourth webspace. (b)The proximal lump corresponded to a simple anechoic lump overlying the fifth tarso-metatarsal joint in keeping with a ganglion. (c) Ultrasound and (d) fluid-sensitive MRI of the fourth webspace demonstrated marked tenosynovitis of the fourth extensor tendon which bulges into the dorsal soft tissue presenting as a lump (white arrow).
osteoblasts. Immobilized patients undergoing physical therapy are also at risk of developing MO. MO is also known to affect ligaments and can often have an early course in mimicking a soft tissue infection. Individuals present with a clinically painful mass, which can be inflamed. The classical appearance in the later stage of the disease is the formation of mature bone within soft tissues. CT and MRI findings vary depending on the time of presentation. Early imaging would demonstrate the development of a peripheral rim of calcification of the mass 6e8 weeks postinjury. After 5e6 months there are chronic appearances of diffuse ossification (Figure 12b) and mature bone formation with MR SI approximating that of marrow without associated oedema. It may be difficult to differentiate the lesion from a malignancy (such as osteosarcoma or synovial sarcoma) without the history of trauma (Figure 12c and d). Biopsy of the mass in its late stage will confirm myofibroblasts and fibroblasts. Biopsy should be avoided in the early phase of development as histology can be indistinguishable from osteosarcoma, thus making histological diagnosis of MO very challenging. Clinical and radiological appearances and follow-up are crucial to avoid this potential devastating misdiagnosis.
Conclusion Figure 11 Pigmented villonodular synovitis (a) Shoulder radiograph demonstrating a dense soft tissue swelling around the left shoulder. (b) Extended view shoulder ultrasound demonstrating a large echogenic mass (white arrow) surrounding the glenohumeral joint. (c) Susceptibility weighted MRI of the affected shoulder demonstrates the ‘blooming’ phenomena (white arrow) relating to the presence of haemosiderin and osseous erosions (white arrow heads) which are typical for pigmented villonodular synovitis.
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Patients presenting with a soft tissue mass are frequently encountered in clinical practice. Clinical characterization can be limited and imaging is often required. Despite this, lesions can often appear radiologically as indeterminate or be misinterpreted as a malignant lesion. In such cases, biopsy is required to exclude malignancy. However, with relevant clinical history, clinical
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Figure 12 Myositis ossificans (MO) (a) Radiograph of the right humerus demonstrates soft tissue calcification calcific mass within the medial aspect of the arm (white arrow) (b) CT of the same patient demonstrates an intramuscular lesion consisting of peripherally well-organized mature lamellar bone which is typical for MO (white arrow). Radiographic appearances of MO are similar to that of synovial sarcoma which is seen in (c) right femur radiograph demonstrating a soft tissue calcific mass. However, note the lesion is more densely calcified (d) Coronal lower limb fluidsensitive MRI of the same lesion in (c) which demonstrates a large associated soft tissue component. This was histologically proven as synovial sarcoma.
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examination and imaging characteristics, one can narrow the differential diagnosis of these problematic lesions. It is important to remember that if a lesion is classified as indeterminate, a biopsy to exclude malignancy should be considered. We have presented common tumour mimics using an anatomical sieve that should be considered when presented with a palpable soft tissue mass. A
2 Shah A, Botchu R, Ashford RU, Rennie WJ. Diagnostic triage for sarcoma: an effective model for reducing referrals to the sarcoma multidisciplinary team. Br J Radiol 2015; 88. http://dx.doi.org/10. 1259/bjr.20150037. 3 Fletcher CDM, Bridge JA, Hogendoorn PC, et al. WHO classification of tumours of soft tissue and bone. 4th edn. Lyon, France: IARC Press, 2013. 4 De Backer AI, Vanhoenacker FM, Sanghvi DA. Imaging features of extraaxial musculoskeletal tuberculosis. Indian J Radiol Imaging 2009; 19: 176e86. http://dx.doi.org/10.4103/0971-3026.54873. 5 Crundwell N, O’Donnell P, Saifuddin A. Non-neoplastic conditions presenting as soft-tissue tumours. Clin Radiol 2007; 62: 18e27. doi: S0009-9260(06)00316-3 [pii]. 6 Yuan WH, Hsu HC, Lai YC, Chou YH, Li AF. Differences in sonographic features of ruptured and unruptured epidermal cysts. J Ultrasound Med 2012; 31(2): 265e72. doi: 31/2/265 [pii]. 7 Scolaro JA, Chao T, Zamorano DP. The morel-lavallee lesion: diagnosis and management. J Am Acad Orthop Surg 2016; 24: 667e72. http://dx.doi.org/10.5435/JAAOS-D-15-00181. 8 Navarro OM, Laffan EE, Ngan BY. Pediatric soft-tissue tumors and pseudo-tumors: MR imaging features with pathologic correlation: part 1. Imaging approach, pseudotumors, vascular lesions, and adipocytic tumors. Radiographics 2009; 29: 887e906. http://dx. doi.org/10.1148/rg.293085168. 9 Sobhani Eraghi A, Athari B, Kheirkhah Rahimabad P. Tumoral calcinosis of the foot: an unusual differential diagnosis of calcaneal mass. Int J Surg Case Rep 2015; 10: 219e22. http://dx.doi. org/10.1016/j.ijscr.2015.04.006. 10 Hochman MG, Wu JS. MR imaging of common soft tissue masses in the foot and ankle. Magn Reson Imaging Clin N Am 2017; 25: 159e81. doi: S1064-9689(16)30069-1 [pii]. 11 Edwards DS, Kuhn KM, Potter BK, Forsberg JA. Heterotopic ossification: a review of current understanding, treatment, and future. J Orthop Trauma 2016; 30(suppl 3): S27e30. http://dx.doi. org/10.1097/BOT.0666.
Key learning points C
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Numerous non-neoplastic pathologies can mimic a soft tissue tumour and should be considered in the differential when faced with a palpable lump. A biopsy should be considered with clinically or radiologically indeterminate lesions to exclude an underlying malignancy. Infection can present in numerous ways including as pseudotumours. The diagnosis is generally evident with appropriate clinico-radiological and laboratory findings. Although these are not always present and histopathological confirmation may be required. Haematomas should be considered within the context of trauma and anticoagulation use. Where there is unexplained spontaneous haemorrhage, an underlying malignancy should always be excluded. Clinical and/or radiological follow-up and/or biopsy should be considered. Tumoural calcinosis is typically multilobulated and has welldefined cloud-like periarticular soft tissue densities. Imaging plays a vital role in differentiating from myositis ossificans and synovial sarcoma.
REFERENCES 1 Cancer Research. Soft tissue sarcoma incidence statistics. UK. 2016. Updated, Cancerresearchuk.org.uk.
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current WHO Classification of Tumours of Soft Tissue and Bone.3 As myositis ossificans (MO) and pigmented villonodular synovitis (PVNS) were traditionally described as tumours mimics, we have included them in this review, however, it must be emphasized they are classified as tumours.
Guide to pseudotumours and soft tissue tumour mimics
Infection Kethesparan Paramesparan
Infection can manifest in a number of ways but it is often clinically apparent that an underlying infective process is occurring. Infection can arise from direct inoculation, haematogenous spread or due to adjacent osteomyelitis. However, there are a number of situations when patients may be asymptomatic or present with a focal soft tissue mass that is not typical of infection.
Amit Shah Winston J Rennie
Abstract There are many pathological entities that can mimic a soft tissue tumour. Even with appropriate clinico-radiological assessment, some of these lesions can be misinterpreted as soft tissue sarcomas. Also known as pseudotumours, this can lead to unnecessary further investigations or intervention. With the relevant clinical history, clinical examination and imaging characteristics, one can narrow the differential diagnosis of these problematic lesions. Occasionally the diagnosis of a pseudotumour may only be made after histopathological analysis. We present common tumour mimics using an anatomical sieve that should be considered when presented with a palpable soft tissue mass.
Tuberculosis infection Tuberculosis (TB) is aptly named the great mimicker, simulating numerous disease entities. Extra-pulmonary TB can affect bones, joints and soft tissue. In adults extra-pulmonary TB occurs between the ages of 20e50 years. Musculoskeletal manifestations of TB are very rare and account for between 1% and 3% of TB infections. Of those patients affected with musculoskeletal TB, 50% have spinal manifestations, 30% are related to the knee and the hip and 20% have involvement of other sites.4 HIV remains the biggest risk factor for the activation of TB infection but other contributory factors include the emergence of multi-drug-resistant strains of Mycobacterium, immigration and the increasingly ageing population. Acutely, patients classically present with a slow-growing painful swelling around a joint (Figure 1), and in the late stage may progress to joint stiffness. The usual symptoms seen in pulmonary TB of fever, weight loss and loss of appetite may not be seen. A non-specific complex cystic mass is generally seen, which can be either a solid or a peripherally enhancing cystic collection.
Keywords musculoskeletal system; pseudotumour; soft tissue mass; soft tissue sarcoma; tumour mimic
Introduction Soft tissue sarcomas (STS) are rare malignant mesenchymal or connective tissue tumours accounting for approximately 1% of all adult primary tumours.1 Most soft tissue lesions are benign with lipoma being the commonest pathology.2 It is often difficult to distinguish between malignant and benign lesions on clinical judgement alone and therefore imaging plays a central role. Many pathological entities can present clinically as a soft tissue mass. These tumour-mimicking lesions, also known as pseudotumours, can be defined as clinically palpable masses or lesions that have tumour-like appearances on imaging. Even with appropriate clinico-radiological assessment, these lesions can be misinterpreted as STS, leading to intervention with the diagnosis of a pseudotumour made only after histopathological analysis. We aim to provide a review of common pathologies that mimic STS and present them using an anatomical sieve. For the purposes of this article, we have excluded lesions that are included in the
Abscess An abscess is a focal collection of necrotic material and pus, often communicating with sinus tracts extending into joints, soft tissue or skin ulcerations. They are common and have been shown to account for 7% of soft tissue pseudotumours.5 Patients that are post-surgical, immunocompromised, diabetic and those that have a co-existing infection (e.g. osteomyelitis) are at increased risk of developing an abscess. Staphylococcus aureus species are the commonest causative agent followed by streptococcal species. Patients usually present with fever, pain, and a painful mass with an infective blood profile. The soft tissue swelling may be erythematous, fluctuant, warm and can have a slow onset of growth. Ultrasound is most useful in localizing the abscess and demonstrates surrounding soft tissue vascularity (Figure 2a). MRI will demonstrate a thick, irregular-walled low T1-weighted (T1W) signal intensity (SI) and high T2-weighted (T2W) SI collection (Figure 2bed).
Kethesparan Paramesparan MBBS BSc(Hons) Radiology Specialist Trainee, Dr, Department of Radiology, University Hospitals of Leicester, Leicester Royal Infirmary, Leicester, UK. Conflicts of interest: none declared. Amit Shah FRCR Consultant Radiologist, Dr, Department of Radiology, University Hospitals of Leicester, Leicester Royal Infirmary, Leicester, UK. Conflicts of interest: none declared.
Inflammatory Epidermal inclusion cyst Epidermal inclusion cysts are benign cysts filled with lamellated keratin/granular debris and lined with stratified squamous epithelium. They commonly arise from an obstructed hair follicle
Winston J Rennie FRCR Consultant Radiologist, Dr, Department of Radiology, University Hospitals of Leicester, Leicester Royal Infirmary, Leicester, UK. Conflicts of interest: none declared.
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Figure 1 Tuberculosis (a) Ultrasound image of a left chest wall mass demonstrating a complex mixed echogenic lesion. (b) Axial CT demonstrates a low attenuation lesion (white arrow) in the left chest wall eroding the sternum and arising from the sternocostal joint. (c) Axial fluid-sensitive MRI sequence demonstrates a fluid collection with surrounding soft tissue oedema (white arrow heads). (d) Axial T1 fat-saturated post-gadolinium MRI demonstrates peripheral enhancement in keeping with an abscess, which was histologically proven secondary to tuberculosis.
Figure 2 Abscess (a) Doppler ultrasound demonstrates a complex semisolid collection around the medial elbow with surrounding vascularity. (b) Axial fluid-sensitive MRI demonstrates a cystic collection (white arrow) with marked surrounding soft tissue oedema. (c) T1 fat-saturated pre-contrast MRI and (d) T1 fat-saturated post-contrast MRI demonstrates a thick walled rim-enhancing collection, which grew Staphylococcus aureus after aspiration. ORTHOPAEDICS AND TRAUMA 31:3
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Figure 3 Epidermal inclusion cyst (EIC) (a and b) Ultrasound of two cases of EIC demonstrating characteristic imaging appearances of a wellcircumscribed heterogeneous filiform lesion with posterior acoustic enhancement (white arrow heads) and no colour Doppler signal as seen in (b). (c) Axial T1 MRI demonstrates a classic subdermal low signal intensity lesion (black arrow) with no enhancement post-contrast (d).
or deep implantation of epidermis triggering an inflammatory response. Frequently found in the neck, face, scalp and trunk, less than 10% are said to occur in the extremities.6 Ultrasound demonstrates a well-circumscribed, heterogeneous filiform lesion that often demonstrates posterior acoustic enhancement and no colour Doppler signal (Figure 3a and b). MRI appearances are of low T1W SI/high T2W SI subdermal mass that may demonstrate capsular enhancement post contrast (Figure 3c and d). A ruptured epidermal cyst on the other hand may have a lobulated contour with a positive colour Doppler signal thus mimicking an aggressive lesion and biopsy may be required to exclude malignancy.
from hyperintense to hypointense depending on the levels of methaemoglobin and haemosiderin. There should be no enhancement on post-contrast imaging. Because of the complex appearances and repeated haemorrhages that can occur with anticoagulation use, a haematoma may be indistinguishable from an aggressive tumour. If haematoma is considered as the diagnosis, it is prudent to follow up these patients clinically or with imaging to ensure gradual resolution over time, which can take at least 3 months. If there is no interval reduction, a biopsy should be considered. Of note, one has to be wary of dismissing unexplained spontaneous haematomas as benign and an underlying neoplasm must be excluded. Non-specific radiological features of a malignant haemorrhagic lesion include a haemorrhagic mass demonstrating internal vascularity, associated enhancing solid components, infiltrative pattern and non-resolution over time.
Traumatic Haematoma Haematomas are localized collections of blood within a contained space, outside a vessel wall. They are often associated with traumatic muscle injury, muscle strain and contusions. Haematomas are very common and account for 5% of soft tissue masses.5 Intramuscular haematomas are often associated with patients with bleeding tendencies such as those on anticoagulant therapy and clotting disorders. Patients present with a slowly growing expanding mass as a result of recurrent haemorrhage. Bruising and discolouration of the skin may be noted at the site of the haematoma if it is superficial. Haematomas can be associated with tendon avulsions and myotendinous strain injury. The echogenicity of haematomas seen on ultrasound imaging evolves over time. (Figure 4a and b). MRI appearances can be variable and dependent on the stage of the haematoma at the time of imaging. Fluidefluid levels may be seen and the T1W SI varies
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Muscle tear Muscle tears tend to occur at the myotendinous junction or myofascial surface, which is the weakest point of the muscletendon unit. Patients may have a non-specific history of a prior traumatic event that is usually overlooked. Clinical examination may reveal an asymmetrical enlarged palpable soft tissue mass with a gap along the expected course of the tendon. Intramuscular haematomas or the retracted tendon and associated haematoma resulting from a complete tendon avulsion can mimic a soft tissue neoplasm. Common muscle ruptures include the hamstrings, quadriceps (particularly rectus femoris) and biceps brachii (Figure 4c). Imaging depends on the extent and grade of muscle strain. Muscular injuries resulting in complete myotendinous junction tears, retraction and associated haematoma
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Figure 4 Traumatic mimics (a) Patient on anticoagulation presenting with a large soft tissue mass. Ultrasound demonstrates a large lesion with layering in keeping with an acute haematoma. Haematomas can vary in appearances depending on the stage of evolution as seen in (b) which demonstrates a liquefying haematoma. (c) Extended field of view ultrasound demonstrating a complete rectus femoris tendon tear with proximal tendon retraction corresponding to the clinically palpable lump (white arrow). (d) Axial T2 MRI demonstrates a lobulated fluid collection located e lesion (black arrow). (e) between the deep subcutaneous tissue and the deep fascia of the thigh, which is a classic location for a Morel-Lavalle Coronal T1 MRI of the same patient demonstrates a low-signal rim in keeping with a fibrous pseudocapsule (black arrow heads). (f) Ultrasound of a patient presenting with a focal lump in the calf demonstrating a gap in the myofascia of the tibialis anterior muscle with herniation of the muscle (mushroom cap sign).
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or intramuscular haematomas, are injuries that can pose diagnostic challenges. In time fibrosis and or calcification can occur which can lead to the development of myositis ossificans. Biopsy of the lesion will demonstrate granulomatous chronic inflammatory cells and reactive fibrosis.
Muscle hernia Myofascial herniations of the extremities are most commonly located in the lower leg and are not routinely encountered in clinical practice. Herniations can be secondary to congenital, traumatic or predisposing conditions such as muscular hypertrophy or chronic compartment syndrome. The tibialis anterior muscle is most commonly involved with many other muscles such as the peroneus longus and brevis and gastrocnemii muscles. Muscle herniation occurs as a result of a focal fascial sheath defects typically at sites of weakness secondary to perforating nerves and vessels. Presentation may be in the form of a soft tissue mass, a subcutaneous nodule or a palpable bulge, which may or may not be reducible. Patients may be completely asymptomatic or complain of pain and weakness, which may worsen with standing or physical activity. Specific movement may pronounce the herniation, for example the tibialis anterior hernia may be more apparent on resisted dorsiflexion. Imaging evaluation with ultrasound is sufficient as there is a dynamic observation of the muscle herniation at the site of the palpable mass. The finding of a hyperechoic muscle prominence through a fascial defect (creating a ‘mushroom cap’ sign) is characteristic (Figure 4f).
e lesion Morel-Lavalle The Morel-Lavallee lesion (MLL) occurs secondary to a blunt force degloving injury typically in the proximal thigh and pelvis causing separation of the hypodermis from the underlying fascia.7 The created space between the subcutaneous fat and underlying fascia fills with the haemo-lymphatic fluid and becomes surrounded by granulation tissue. This forms a fibrous capsule further preventing reabsorption of the collection leading to formation of a pseudotumour. MLLs can be diagnosed on clinical examination in patients presenting with a history of trauma and symptoms of skin mobility, local contusion, soft fluctuant area, decreased sensation, friction burns and variable ecchymosis. Imaging can be particularly useful in demonstrating a collection between the subcutaneous fat and deep fascia to help diagnose MLLs (Figure 4d). MRI appearances depend on timing of imaging and complications. MLLs follow fluid SI internally except for fat nodules and blood products. The fibrous pseudocapsule is hypointense on all sequences (Figure 4e). Post-contrast nodular or peripheral enhancement may be seen with granulation tissue, inflammation, or infective complications.
Vascular Aneurysm Aneurysms can be classified as ‘true aneurysms’ when all three layers of the arterial wall are abnormally dilated or ‘false
Figure 5 Popliteal artery aneurysm (PAA) (a) Right femur radiograph demonstrating a soft tissue density with a calcific rim (white arrow). (b) Axial CT of both thighs demonstrates a right PAA (white arrow) (c) Axial T1 MRI demonstrating the PAA is largely thrombosed (white arrow) and the high T1 signal intensity is in keeping with methaemoglobin. (d) Fat-suppressed MRI demonstrates pulsation artefact (white arrow heads) which is an important clue in distinguishing a solid mass from a vascular one, as biopsy of this would have significant consequences. Images courtesy of Department of Radiology, Royal Orthopaedic Hospital, Birmingham, UK.
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aneurysms’ (pseudoaneurysms), which are due to a defect in the arterial wall related to trauma or (mycotic) infection. Popliteal artery aneurysms (PAA) are the most common peripheral aneurysm accounting for 70e80% of extremity aneurysms. They are often bilateral and can present as a focal mass in the popliteal fossa, which is often pulsatile or incidentally found on imaging. Imaging will demonstrate aneurysmal dilatation of the artery, which may contain intraluminal thrombus, seen as concentric
rings on MRI, and calcification. Thrombotic aneurysms will have a pseudosolid appearance (Figure 5c). Pulsation artefact can be seen which will help distinguish this from a solid mass (Figure 5d). Arterio-venous malformation Unlike haemangiomas, which are benign vascular tumours, arterio-venous malformations (AVM) are abnormal congenital
Figure 6 Arteriovenous malformation (AVM) (a) Ultrasound demonstrating a large echogenic subcutaneous mass with multiple tortuous channel typical of an AVM. (b) Radiograph of the forearm demonstrating two oval lamellated calcific densities (white arrows) in keeping with phleboliths within an underlying AVM. (c) Axial CT of another patient demonstrating a partially calcified serpiginous paraspinal mass (black arrow), which demonstrates fat hypertrophy as seen on the axial T1 MRI (d). This lesion was histologically proven as an AVM. (e) AVM in another patient demonstrates the typical appearance of a serpiginous fat-containing AVM.
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vascular lesions consisting of direct arterial and venous connections with an associated (minimal) soft tissue component. AVMs are differentiated from vascular tumours by the lack of mitotic activity. AVMs can be subdivided into high- or low-flow lesions and occur commonly in children and early adulthood. In the paediatric population, vascular malformations represent the most common cause of soft tissue masses.8 AVMs can occur in various anatomical locations such as the face, trunk, extremities and skeletal muscle. They can present as a red pulsatile mass with a thrill growing out of proportion to the growth of the child/ adult. Commonest AVMs are venous vascular malformations (VVM) and seen on ultrasound as multiple anechoic, tubular vascular channels that can infiltrate subcutaneous fat, muscles, fascia, and tendons in the diffuse type or appear as a focal mass (Figure 6a). Phleboliths are hallmarks of VVMs and can be seen as shadowing echogenic foci and can be detected on radiographs (Figure 6b). MRI is the most valuable imaging modality for identification and classification of AVMs as it can define the extension of the lesions to the adjacent structures. Flow voids, enlarged feeding arteries and tortuous draining veins are seen with surrounding subcutaneous fat hypertrophy, muscle atrophy, and fatty replacement (Figure 6d and e).
Metabolic Tumoural calcinosis Tumoural calcinosis (TC) is a benign disorder characterized by the non-osseous calcification of peri-articular soft tissues and mainly presents in large joints (hips, elbows, shoulders). It is classified into primary hyperphosphatemic (PHP) and secondary TC categories. There is some evidence of an autosomal dominant inheritance in PHP TC. Associations with end-stage renal disease, vitamin D toxicity and hyperparathyroidism have also been established.9 TC can be found in any age group but is mainly prevalent in the first and second decades of life and has a strong predominance with the African American population.9 Patients will present with a firm, non-tender mobile mass in the periarticular soft tissue with a focal functional deficit of the joint if it is large. Superficial skin ulceration with drainage of a chalky milk-type consistency is also a common finding. Plain film radiographs typically reveal well-defined, large, multi-lobulated, amorphous, cloud-like densities within the soft tissue with radiolucent septae (Figure 7). MRI fluid sensitive imaging will demonstrate high SI in the cystic areas, peripheries and adjacent oedema with lower SI representing solid calcium. Fluid-calcium levels may be seen in keeping with due to different states of calcium, known as the ‘sedimentation sign’. There is ‘chicken wire’ enhancement post-contrast. Scintigraphy will demonstrate uptake at sites of calcium deposition. Differentials are myriad but include synovial osteochondromatosis, myositis ossificans, calcific myonecrosis, synovial sarcoma, and osteosarcoma.
Figure 7 Tumoural calcinosis (TC) (a) Radiograph of the pelvis demonstrating a right groin periarticular cloud-like dense calcific mass (black arrow) and a peritoneal catheter (white arrow) in a patient on dialysis with end-stage renal disease. TC in patients on dialysis are typically multiple and occur in those with a longer duration of uncontrolled calcium. (b) Axial CT demonstrates the dense amorphous calcific mass as soft tissue in origin with areas of ‘sedimentations’ (white arrows).
extremities and are named according to the muscle with which they are structurally associated. Patients are usually young adolescent adults and present with lower leg soft tissue fullness with intermittent pain and swelling after exercise or due to neurovascular compromise symptoms due to mass effects.
Degenerative Bursa A bursa is a synovial-membrane-lined fluid-filled sac located between bones and tendons and/or muscles and around joints. These can become inflamed and distended secondary to trauma, autoimmune disorders, infection and iatrogenic causes, although they can become distended and inflamed in any anatomical location. A frequently encountered bursal cyst presenting as palpable mass is the acromioclavicular joint (ACJ) cyst (Figure 9a). Fluid from a degenerate glenohumeral joint (GHJ) with underlying chronic full-thickness rotator cuff tear distends the subacromial-subdeltoid bursa and extends through a degenerative ACJ. Imaging characteristically shows fluid communicating from the GHJ through the ACJ forming a collection superficial to the ACJ known as a ‘geyser sign’ (Figure 9f). The cyst can vary from simple fluid collection to complex pseudosolid structures (Figure 9d and e).
Congenital Accessory muscles Accessory muscles are morphologically normal muscles and may be uni- or bilateral. Accessory muscles can be misinterpreted as a tumour on imaging. An accessory soleus muscle is a frequent finding especially on MRI (Figure 8). Numerous accessory muscles have been described in the literature throughout the
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Figure 8 Accessory muscles mimicking a palpable lump (a and b) Axial and sagittal T1 MRI demonstrates a mass (white arrow) within Kager’s fat pad, which is isointense to muscle in keeping with an accessory soleus muscle.
approximately 1.8 patients per million population. Malignant variants, malignant transformation, and metastases have been rarely reported. Adults between the ages of 20 and 50 years are most commonly affected with no specific gender representation. Approximately 80% occur in the knee, followed by the hip, ankle, shoulder and elbow. The typical presentation is of knee joint swelling, which is mildly painful on movement. The onset of swelling is often insidious, recurrent and haemorrhagic with the size of effusion out of proportion to the degree of mild discomfort. Plain radiographs may be normal with only a dense effusion seen or alternatively well-defined subchondral cysts and erosions with sclerotic edges (Figure 11a). Joint space is relatively preserved. MRI will demonstrate the classical appearance of low signal structures that demonstrate ‘blooming artefact’ on gradient-echo sequences due to haemorrhagic synovium and haemosiderin deposits (Figure 11c). As PVNS is locally aggressive, a high rate of recurrence has been reported. Recurrence, often multiple, can occur in 33e50% of cases.
Ganglion Ganglions are fluid-filled cysts found in peri-articular soft tissues arising from joint capsules or tendons in areas under repetitive stress. It is a pseudocyst, lined by a thick fibrous capsule composed of flat spindle cells and do not contain a synovial lining. Ganglion cysts represents one of the most common benign soft tissue masses encountered in the foot and ankle and occur more commonly in women10 (Figure 10a). Most ganglion cysts are uni-locular and contain clear viscous gelatinous fluid, however some may be lobulated in appearance and have thin septations. Plain radiographs are non-specific but may show some findings of non-aggressive bony remodelling. Ultrasound is the modality of choice demonstrating a hypoechoic, well-defined, non-compressible cystic mass with a neck arising from a joint or related to a tendon (Figure 10b). MRI typically shows a welldefined, lesion with smooth borders which is hypo to intermediate SI on T1W images and high SI on T2W images, with a smooth rim of peripheral enhancement after contrast administration. Ultrasound-guided aspiration can be performed if the patient is symptomatic but like surgical excision, recurrence rates are high.
Myositis ossificans MO is the formation of benign heterotopic ossification in soft tissue. The WHO Classification of Tumours of Soft Tissue and Bone includes MO because of its hypercellularity, cytological atypia, and mitotic activity. It has a reported incidence of between 9% and 17% and is a common trauma-related tumour mimic.5 Patients classically present in their teen years but have an age range between 6 months and 84 years.11 MO may develop anywhere in the body including the extremities, trunk, and head and neck with the most common locations being those most susceptible to trauma such as the elbow, thigh, buttock, and shoulder. MO is usually related to repeated trauma, which initiates the proliferation of mesenchymal stem cells that produce activated fibroblasts and
Miscellaneous Pigmented villonodular synovitis PVNS is characterized by proliferation of intra-articular synovial tissue and tendon sheaths. PVNS is a locally aggressive synovial neoplasm with a potential for local recurrence and is grouped within the same family of disorders as the giant cell tumour of tendon sheath (GCTTS). GCTTS is histologically similar to PVNS but are extra-articular and affect only tendon sheaths. PVNS accounts for approximately 0.9% of all benign soft tissue tumours within the lower limbs and the annual incidence is
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Figure 9 Degenerative bursa (a) Patient with a large firm lump over the acromioclavicular joint (ACJ) (b) Aspiration of the lump reveals typical straw-coloured thick synovial fluid. (c) Radiograph of the right shoulder demonstrates a well-defined dense soft tissue mass (white arrow) overlying a degenerative ACJ with complete loss of the subacromial space in keeping with underlying rotator cuff tear. (d) Ultrasound demonstrates a simple fluid collection with internal echoes overlying the ACJ. (e) Ultrasound of a different patient demonstrates a more complex appearing cyst. The location and underlying rotator cuff tear helps differentiate these from more sinister pathologies. (f) Axial fat-saturated MRI demonstrates full thickness cuff tear with a degenerate glenohumeral joint and ACJ. There is fluid in the subacromial-subdeltoid bursa with fluid escaping through the ACJ (Geyser’s sign) (white arrow head) forming a palpable ACJ cyst.
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Figure 10 Ganglion cyst (a) Photograph of the foot demonstrating two palpable lumps over the base of the fifth metatarsal and fourth webspace. (b)The proximal lump corresponded to a simple anechoic lump overlying the fifth tarso-metatarsal joint in keeping with a ganglion. (c) Ultrasound and (d) fluid-sensitive MRI of the fourth webspace demonstrated marked tenosynovitis of the fourth extensor tendon which bulges into the dorsal soft tissue presenting as a lump (white arrow).
osteoblasts. Immobilized patients undergoing physical therapy are also at risk of developing MO. MO is also known to affect ligaments and can often have an early course in mimicking a soft tissue infection. Individuals present with a clinically painful mass, which can be inflamed. The classical appearance in the later stage of the disease is the formation of mature bone within soft tissues. CT and MRI findings vary depending on the time of presentation. Early imaging would demonstrate the development of a peripheral rim of calcification of the mass 6e8 weeks postinjury. After 5e6 months there are chronic appearances of diffuse ossification (Figure 12b) and mature bone formation with MR SI approximating that of marrow without associated oedema. It may be difficult to differentiate the lesion from a malignancy (such as osteosarcoma or synovial sarcoma) without the history of trauma (Figure 12c and d). Biopsy of the mass in its late stage will confirm myofibroblasts and fibroblasts. Biopsy should be avoided in the early phase of development as histology can be indistinguishable from osteosarcoma, thus making histological diagnosis of MO very challenging. Clinical and radiological appearances and follow-up are crucial to avoid this potential devastating misdiagnosis.
Conclusion Figure 11 Pigmented villonodular synovitis (a) Shoulder radiograph demonstrating a dense soft tissue swelling around the left shoulder. (b) Extended view shoulder ultrasound demonstrating a large echogenic mass (white arrow) surrounding the glenohumeral joint. (c) Susceptibility weighted MRI of the affected shoulder demonstrates the ‘blooming’ phenomena (white arrow) relating to the presence of haemosiderin and osseous erosions (white arrow heads) which are typical for pigmented villonodular synovitis.
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Patients presenting with a soft tissue mass are frequently encountered in clinical practice. Clinical characterization can be limited and imaging is often required. Despite this, lesions can often appear radiologically as indeterminate or be misinterpreted as a malignant lesion. In such cases, biopsy is required to exclude malignancy. However, with relevant clinical history, clinical
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Figure 12 Myositis ossificans (MO) (a) Radiograph of the right humerus demonstrates soft tissue calcification calcific mass within the medial aspect of the arm (white arrow) (b) CT of the same patient demonstrates an intramuscular lesion consisting of peripherally well-organized mature lamellar bone which is typical for MO (white arrow). Radiographic appearances of MO are similar to that of synovial sarcoma which is seen in (c) right femur radiograph demonstrating a soft tissue calcific mass. However, note the lesion is more densely calcified (d) Coronal lower limb fluidsensitive MRI of the same lesion in (c) which demonstrates a large associated soft tissue component. This was histologically proven as synovial sarcoma.
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examination and imaging characteristics, one can narrow the differential diagnosis of these problematic lesions. It is important to remember that if a lesion is classified as indeterminate, a biopsy to exclude malignancy should be considered. We have presented common tumour mimics using an anatomical sieve that should be considered when presented with a palpable soft tissue mass. A
2 Shah A, Botchu R, Ashford RU, Rennie WJ. Diagnostic triage for sarcoma: an effective model for reducing referrals to the sarcoma multidisciplinary team. Br J Radiol 2015; 88. http://dx.doi.org/10. 1259/bjr.20150037. 3 Fletcher CDM, Bridge JA, Hogendoorn PC, et al. WHO classification of tumours of soft tissue and bone. 4th edn. Lyon, France: IARC Press, 2013. 4 De Backer AI, Vanhoenacker FM, Sanghvi DA. Imaging features of extraaxial musculoskeletal tuberculosis. Indian J Radiol Imaging 2009; 19: 176e86. http://dx.doi.org/10.4103/0971-3026.54873. 5 Crundwell N, O’Donnell P, Saifuddin A. Non-neoplastic conditions presenting as soft-tissue tumours. Clin Radiol 2007; 62: 18e27. doi: S0009-9260(06)00316-3 [pii]. 6 Yuan WH, Hsu HC, Lai YC, Chou YH, Li AF. Differences in sonographic features of ruptured and unruptured epidermal cysts. J Ultrasound Med 2012; 31(2): 265e72. doi: 31/2/265 [pii]. 7 Scolaro JA, Chao T, Zamorano DP. The morel-lavallee lesion: diagnosis and management. J Am Acad Orthop Surg 2016; 24: 667e72. http://dx.doi.org/10.5435/JAAOS-D-15-00181. 8 Navarro OM, Laffan EE, Ngan BY. Pediatric soft-tissue tumors and pseudo-tumors: MR imaging features with pathologic correlation: part 1. Imaging approach, pseudotumors, vascular lesions, and adipocytic tumors. Radiographics 2009; 29: 887e906. http://dx. doi.org/10.1148/rg.293085168. 9 Sobhani Eraghi A, Athari B, Kheirkhah Rahimabad P. Tumoral calcinosis of the foot: an unusual differential diagnosis of calcaneal mass. Int J Surg Case Rep 2015; 10: 219e22. http://dx.doi. org/10.1016/j.ijscr.2015.04.006. 10 Hochman MG, Wu JS. MR imaging of common soft tissue masses in the foot and ankle. Magn Reson Imaging Clin N Am 2017; 25: 159e81. doi: S1064-9689(16)30069-1 [pii]. 11 Edwards DS, Kuhn KM, Potter BK, Forsberg JA. Heterotopic ossification: a review of current understanding, treatment, and future. J Orthop Trauma 2016; 30(suppl 3): S27e30. http://dx.doi. org/10.1097/BOT.0666.
Key learning points C
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Numerous non-neoplastic pathologies can mimic a soft tissue tumour and should be considered in the differential when faced with a palpable lump. A biopsy should be considered with clinically or radiologically indeterminate lesions to exclude an underlying malignancy. Infection can present in numerous ways including as pseudotumours. The diagnosis is generally evident with appropriate clinico-radiological and laboratory findings. Although these are not always present and histopathological confirmation may be required. Haematomas should be considered within the context of trauma and anticoagulation use. Where there is unexplained spontaneous haemorrhage, an underlying malignancy should always be excluded. Clinical and/or radiological follow-up and/or biopsy should be considered. Tumoural calcinosis is typically multilobulated and has welldefined cloud-like periarticular soft tissue densities. Imaging plays a vital role in differentiating from myositis ossificans and synovial sarcoma.
REFERENCES 1 Cancer Research. Soft tissue sarcoma incidence statistics. UK. 2016. Updated, Cancerresearchuk.org.uk.
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