Most parts of the human body have a symmetric appearance. If there is a discrepancy in size, course, or orientation between sides (Fig. 1), there may be a subjective impression of the presence of a mass lesion.

Normal variants, such as accessory bones, can also manifest as hard, palpable lesions in the hands or feet, with or without pain (7, 8), including the os acromiale, os externum tibiale, styloid bone, and sesamoid bones. Accessory bones are usually symmetric but can be unilateral. CT and MRI are effective at differentiating tumor-mimicking lesions from other osseous neoplasms such as osteochondromas that show continuity in cortical and medullary bones. Use of markers and bilateral imaging for comparison may help in obtaining an accurate diagnosis.

Hypertrophy of a tendon or muscle may result in the presence of a palpable mass-like lesion with mild pain (7, 9). Findings such as thickening of a tendon, with or without muscle contraction in the absence of evidence of an adjacent mass lesion, allow for the correct diagnoses of these entities (Figs. 2, 3). These entities may be associated with a large region of altered marrow signal intensity, a cortical focal defect, and altered adjacent soft tissue at the tendon insertion site. Muscle hypertrophy can develop in any location and is related to overuse, excessive physical activity, cervical dystonia, nerve lesions, and idiopathic causes (Fig. 4).

Supernumerary or anomalous muscles in the forearms and hands are relatively common (1, 3, 4). These muscles can mimic pathologic tumors and may result in symptoms of nerve entrapment if the muscles are located in the carpal tunnel or in Guyon's canal (4). Upon MR imaging, supernumerary or anomalous muscles, show homogeneous signal intensity similar to that of normal skeletal muscle. Awareness of the typical locations and identification of action potentials with electromyography of the palpable lesions can be helpful (1, 4).

Localized prominent fat tissue occasionally develops from rapid changes in weight or from steroid overuse (7). In addition, as part of the aging process, fat tissue has a decrease in consistency, decreased fibrous strands interspersed between fat lobules, and increased mobility of superficial fat tissues, all of which may lead to misconception of a soft tissue tumor.

A plain radiograph may show hypodensity with a bulging configuration or displacement of the adjacent soft tissue or fascial plane. Sonography demonstrates abundant fat deposition with isoechogenicity or slightly increased echogenicity in the subcutaneous layer of the involved site compared to that of the normal adjacent area. Abundant localized fat deposition with less fibrotic strands can be easily detected with MR and CT imaging. Both a weak mass effect and signal intensity similar to that of the adjacent fat, occasionally result in these lesions being missed on imaging. In this situation, a marker or bilateral imaging may be useful (Fig. 5).

Superficial thrombophlebitis is an inflammation of a superficial vein resulting from a blood clot and may occur spontaneously or as a complication of intervention (10). Symptoms including pain, swelling, or redness can be associated with thrombophlebitis; whereas, hardening and a cord-like palpable mass can be confused with a superficial soft tissue mass lesion. Superficial thrombophlebitis can be easily diagnosed by sonography, and the typical finding is characterized by an echogenic cord-like structure that has a connection with veins, with no collapse on compression (Fig. 6).

Pseudoaneurysms are frequently encountered in arteriovenous fistula or traumatic events (10) (Fig. 7). A specific clinical history of catheterization, biopsy, or other interventional procedures at the involved site assists in the diagnosis of these non-neoplastic vascular masses.

Crystal deposition disease, such as calcific tendinitis and gouty tophi, can present as a palpable lesion with joint pain and can be misdiagnosed as a bone tumor. Calcific tendinitis is caused by the deposition of calcium hydroxyapatite crystals in tendons (11) (Fig. 8), while gouty arthritis is caused by the deposition of monosodium urate (12). Characteristic findings on radiography and US, in combination with clinical and laboratory data, are diagnostic for these disorders. The actual role of magnetic resonance imaging seems limited to the exclusion of neoplasms and better demonstration of inflammatory changes in the early stages of the disease.

Granulomas resulting from injection or foreign bodies, can occasionally mimic neoplasms upon physical examination and imaging (13, 14). Radiography shows streaky and nodular calcifications of soft tissue with or without mass formation. Upon sonography, an irregular calcific mass with posterior shadowing can be observed.

Granulomas due to a parasite infection, while less common, can also be mistaken for soft tissue tumors (10) (Fig. 9). Clinical history of migrations of palpable lesions and ingestion of related foods, along with sonographic demonstration of the parasite, are important in their diagnosis.

Commonly encountered pseudolesions that may give the impression of true osteolytic or cystic lesions on plain radiographs include the superolateral humeral head, rhomboid fossa of the clavicle, scapular defects, the supratrochlear foramen, and an anterior lytic defect of the calcaneus (15).

Superimposition of normal structures can result in increased density on a specific radiographic view that may simulate a pathological lesion. Occasionally, a positioning, in which the scapula projects over a vertebral body, results in an increased opacity that simulates sclerosis on a lateral chest radiograph (16). Correlations with different views may be sufficient to confirm a normal finding (Fig. 10).

Tumor-mimicking lesions manifesting as osteolytic lesions on plain radiographs are mainly attributable to focal fat deposition in marrow or localized osteoporosis. Localized osteoporosis results from the immobilization and disuse followed by paralysis or a previous fracture (7) (Fig. 11). These radiographic findings may result in a misdiagnosis as osseous tumors, such as a simple bone cyst, giant cell tumor, plasma cell tumor, or metastasis. Therefore, CT or MR imaging can be used for the differential diagnosis; both a fat deposition in marrow and localized osteoporosis demonstrate the non-visualization of the soft tissue component and are unlikely to be true osseous tumors.

Soft tissue hematomas may be associated with a traumatic event or minimal trauma in patients with bleeding tendencies such as hemophilia (Fig. 12). In hemophiliacs, it is not rare to present with an intraosseous hematoma mimicking a bone tumor.

In soft tissue hematomas, it may sometimes be challenging to distinguish a simple hematoma from a hemorrhage into a preexisting malignant mass or associated secondary infection. Various patterns of sonographic, CT, and MR images represent different stages of the hemorrhagic event (Fig. 12). The presence of hemoglobin and associated degradation products influence the MRI signal intensities and evolve in a somewhat predictable fashion over time (7).

Myositis ossificans is a common tumor-mimicking lesion that occasionally occurs in immobilized patients after physical therapy or in patients with a history of trauma. A sharply circumscribed bony mass is usually apparent by 6-8 weeks, becomes smaller and mature by 5-6 months, and may show diffuse ossification (17, 18) (Fig. 13B). Early lesions demonstrate an inhomogeneous mass with a high signal intensity on MR T2-weighted images (Fig. 13A). The lesions are seen as isointense relative to muscle on T1-weighted images, and may be accompanied by soft tissue edema (18). Intermediate or older lesions typically demonstrate a rim of curvilinear decreased signal intensity corresponding to peripheral ossification of the lesions. Mature lesions are well-defined homogeneous masses with a signal intensity approximating that of fat on both T1-weighted and T2-weighted images and are present without associated edema. Active lesions demonstrate enhancement as seen on enhanced MR images (18, 19). Without a history of trauma, it may be difficult to differentiate a lesion from a malignant tumor such as an osteosarcoma.

Focal myositis can manifest as a localized intramuscular soft tissue mass that may rapidly increase in size over a period of weeks (20, 21). CT imaging features are nonspecific, with the lesions demonstrating attenuation less than or similar to that of the adjacent muscle. On MR imaging, focal myositis may have a heterogeneous appearance with increased signal intensity and surrounding edema on T2-weighted images (Fig. 14). Sonography shows an increase in the diameter of the affected muscle group and a diffuse increase in the echogenicity of the muscle in the early stages of myositis (10). Relative preservation of muscle fibers is suggestive of myositis rather than a tumorous lesion (20).

Osteomyelitis demonstrates moth-eaten or permeative bone destruction and can frequently mimic a malignant bone tumor, such as metastasis (22) (Fig. 15), multiple myeloma, or Ewing's sarcoma. Chronic osteoarthritis may show extensive bone destruction with extension to the adjacent soft tissue. Without a clinical history or information from previous radiographs, osteomyelitis might be mistaken as a tumor with soft tissue mass formation (Fig. 16).

Hyperparathyroidism can accompany brown tumors. Brown tumors of the bone are not true neoplasms; rather, they are unusual, diffuse, reactive lesions caused by hyperparathyroidism; with the radiological features of osteolytic bone lesions resembling benign tumorous lesions such as simple bone cysts (Fig. 17). Other typical findings of primary or secondary hyperparathyroidism, including subperiosteal bone resorption, osteosclerosis, soft tissue calcifications, and osteoporosis, are commonly associated. Appropriate clinical history and laboratory data are also helpful in the diagnosis.

Amyloidosis can present in a variety of ways at different anatomic sites. Skeletal amyloidosis can manifest as an aggressive, expansile lesion with mixed sclerotic and lytic bony lesions that often simulate neoplasms (23); these are commonly encountered in the hip and shoulder.

Insufficiency fractures can show an ill-defined lytic appearance on radiographs and may mimic bony metastasis (7) (Fig. 18). A background of decreased bone density, other insufficient fractures with the typical appearance of irregular sclerotic lines at typical locations, and evident fracture lines visualized on CT or MRI are helpful in correctly making this diagnosis.

Postoperative complications such as osteolysis may result in a tumor-mimicking lesion, which can cause a well-defined osteolytic lesion with an adjacent soft tissue mass via cortical bone disruption (Fig. 19). Radiographs may underestimate the extent and location of the osteolysis compared with MR or CT imaging. MRI and CT are effective for the evaluation of the surrounding soft tissue envelope and intracapsular synovial deposits. They are also more effective than radiographs for the detection and evaluation of osteolysis, thus aiding in clinical management (24). Differential points of these occurrences from neoplasms are multifocal involvement along the prosthesis and preservation of fat plane in deep soft tissue. Metallosis, which presents as an osteolytic lesion accompanied by faint increased densities, is often associated with significant osteolysis.

In conclusions, we described a wide variety of tumor-mimicking lesions involving the musculoskeletal system. Awareness of the various conditions responsible for tumor-mimicking lesions should help in minimizing unnecessary examinations and patient concerns, making the correct diagnosis, and carrying out the proper treatment. Correlation with clinical history, such as underlying diseases, age, trauma history, surgery, or procedures, is mandatory for an accurate diagnosis. The proper selection of an imaging modality is important in differentiating tumor-mimicking lesions from true neoplasms. Sometimes, a solitary imaging study may misdiagnose a non-tumorous lesion as a tumor. Sonography is a noninvasive method for patients with clinically palpable lesions and can be performed rapidly at a relatively low cost. Meanwhile, CT and MR imaging can provide more information about the components of a lesion in relation to adjacent structures. In addition, use of a marker attached to a suspected site and scanning of the opposite side can be very useful in corroborating the diagnosis.