A detailed description of the mass is needed to guide clinical management. Temporal information about the mass is also important: (1) When was the mass identified? (2) Has the mass increased in size or is it stable? (3) What is the rate of growth of the mass? (4) Does the mass fluctuate in size? The presence or absence of additional masses is
helpful in narrowing the differential diagnosis. Local and regional symptoms including pain associated with the mass, inflammatory changes in the overlying skin, and regional swelling should be noted. The presence of systemic symptoms such as fever, chills, and night sweats should also be investigated. Other important questions to answer include whether there is a history of trauma, anticoagulant use, prior malignancy, and travel to regions of the world where specific infections are prevalent.

Some clinical assumptions can be made with a carefully acquired history. A soft-tissue mass that is small, soft, superficial, and stable in size for many years is unlikely to be malignant. Conversely, a large, firm, deep mass that was recently noticed and has rapidly increased in size is more likely to be malignant. The history of trauma increases the likelihood of a hematoma or myositis ossificans. Similarly, hematomas can be seen in patients who have sustained minimal trauma and are being treated with anticoagulant agents. However, clinical vigilance is key because a hematoma could actually mask an underlying neoplastic process. Fluctuation in size is rarely seen in a malignant mass, and a ganglion cyst or vascular malformation should be considered in this clinical setting. Overlying inflammatory changes and systemic symptoms can be seen with an infectious process. The presence of multiple masses, atypical for most soft-tissue tumors, should prompt the consideration of multiple lipomas, aggressive fibromatosis, desmoid tumor, and neurofibromatosis.³ Finally, the history of prior malignancy requires the exclusion of residual, recurrent, or metastatic disease.

A detailed physical examination can help to narrow the differential diagnosis of the mass and aid in determining the need for further testing. The location, size, firmness, and depth of the mass must be determined. Deep tumors are not mobile during muscle contraction. The presence of overlying skin changes should be investigated. Palpation of the mass to determine tenderness, warmth, erythema, purulent drainage, or fluctuance also should be performed. Evaluation of the motor and sensory nerves, including the presence or absence of a Tinel sign (paresthesia along a nerve dermatome with palpation), should be performed to determine if the mass is causing nerve compression. The regional lymph nodes should be evaluated for any evidence of abnormal enlargement. Finally, the patient’s gait should be assessed for limping, pain, or weakness associated with the mass.

Following the completion of the history and physical examination, a differential diagnosis list for the mass can be developed, along with a treatment plan. An imaging algorithm can be used to guide clinical management but should always be tailored to the individual patient and clinical scenario. If the concern for malignancy is low, clinical surveillance can be considered. However, if there are indeterminant or concerning findings in the history and physical examination, further evaluation using imaging and potential tissue sampling is recommended. Specifically, masses that are small, soft, superficial, and stable in size are less likely to represent an aggressive, malignant lesion, and ongoing clinical observation can be considered. However, these features do not exclude the presence of a sarcoma and clinical surveillance should be performed on a regular and frequent basis. The patient should also be educated on red flag symptoms (pain and increased growth) that would warrant imaging evaluation. Generally, masses that are larger than 3 to 5 cm, firm, deep, and/or painful require imaging evaluation.

The imaging evaluation of a soft-tissue mass, either superficial or deep, begins with radiographs.⁹ Disruption of soft-tissue planes can be detected radiographically, denoting the presence of a soft-tissue mass. The detection and characterization of soft-tissue mineralization by radiographs can aid in narrowing the differential diagnosis for the mass. Ossification, denoted by regular, ordered trabeculae within a mass, can have characteristic patterns of formation. Peripheral ossification in the setting of prior trauma can be seen in myositis ossificans, whereas central ossification should raise the concern for a soft-tissue osteosarcoma. Calcifications, which typically appear more disorganized than areas of ossification, can be seen in phleboliths associated with a vascular malformation and as areas of dystrophic calcification as can be seen in lipomas. Dystrophic calcifications in a more aggressive-appearing
soft-tissue mass should raise concerns for a synovial sarcoma. Multiple, uniform calcifications adjacent to a joint can be seen with synovial osteochondromatosis.

Radiographs also allow for the evaluation of the osseous structures adjacent to the soft-tissue mass. Bony remodeling (pressure erosions), periosteal reaction, and cortical destruction can be detected radiographically. The presence of aggressive periosteal reaction or cortical destruction is highly suggestive of a malignant process. In addition, bony findings, such as a large osteochondroma or exuberant callus formation from a prior fracture, can be detected that may have mimicked a soft-tissue mass clinically. In one study, 62% of patients (281 of 454) being evaluated for a soft-tissue mass had abnormal radiographic findings.¹¹ The most common radiographic findings were a visible soft-tissue mass, calcifications, and involvement of the adjacent bone. Figure 1 demonstrates radiographic findings of a high-grade myxofibrosarcoma, including multiple lobulated areas of increased soft-tissue density and cortical destruction of the anterior distal femoral metadiaphysis.

Ultrasonography is also recommended as an initial imaging modality for the evaluation of superficial soft-tissue masses by the ACR-AC.⁹ Ultrasonography has a number of advantages, including cost, availability, portability, examination time, lack of ionizing radiation, patient interaction, vascular and dynamic imaging, and the ability to easily compare findings with those of the assumed normal contralateral body part. However, these advantages need to be balanced with the challenges of the modality, with the most important being the operator’s experience and skill level. Other challenges include patient body habitus, patient positioning, and limitation in tissue characterization by ultrasonography. Ultrasonographic evaluation of soft-tissue masses can accurately characterize location, size, echogenicity, margins, vascularity, solid versus cystic structures, and compressibility. However, many findings are nonspecific, and there is significant overlap between benign and malignant lesions. Combining the ultrasonographic imaging features with the clinical history, location of the lesion, and the relationship to surrounding structures can potentially provide a definitive diagnosis.¹² Several examples include ganglion, paralabral, or parameniscal cyst, Baker cyst, peripheral nerve sheath tumor, giant cell tumor of the tendon sheath, Morel-Lavallée lesion, plantar or palmar fibromatosis, and Morton neuroma.

Ultrasonographic studies have demonstrated a high sensitivity for the detection of a malignant soft-tissue mass, but low specificity.¹³ Although additional studies
have investigated the diagnostic accuracy of ultrasonography when assessing soft-tissue masses, with reported accuracy between 77% and 93%, care should be taken when interpreting these results.^14,15 Specifically, the prevalence of benign soft-tissue masses should be considered when interpreting the results of these studies, given the potential morbidity of misdiagnosing a malignant soft-tissue mass. Although the malignant masses made up a very small percentage of the masses included in a study in which the overall accuracy of ultrasonography was high, of the four sarcomas in the study, two were incorrectly diagnosed by ultrasonography.¹⁶ Therefore, caution should be used when interpreting ultrasonography results, and any concerning features, such as pain and a recent increase in size, should trigger the use of advanced cross-sectional imaging modalities.

The ultrasonographic evaluation of lipomas deserves specific attention. Lipomas represent one of the most common soft-tissue tumors; however, controversy exists in the literature regarding the ability of ultrasonography to accurately diagnose a mass as a lipoma and to differentiate between a lipoma and a liposarcoma. One study investigated diagnostic performance for the evaluation of lipomas. Sensitivity and accuracy for three radiologists was low (52%, 40%, and 52% and 64%, 49%, and 64%, respectively).¹⁷ Subsequent studies and systematic reviews have demonstrated much higher sensitivity and specificity, ranging from 86% to 96% and 94% to 100%, respectively.^14,18 Again, caution should be used when interpreting ultrasonographic examinations, and the presence of concerning clinical features warrants the use of advanced cross-sectional imaging.

Following the initial imaging evaluation of a soft-tissue mass, if a clear diagnosis has not been established, MRI is the next imaging modality of choice.⁹ The superior contrast resolution of MRI and the ability to perform multiplanar evaluation of lesions make MRI the ideal modality for further lesion characterization and local staging. However, the capability to provide specific diagnoses based solely on MRI findings is limited.^{6,8,19,20,21,22,23,24,25} Characterization of the soft-tissue mass includes location (superficial or deep), size, relationship with surrounding structures (fascial planes, muscles, bones, and neurovascular structures), intrinsic signal intensity on all pulse sequences, and the presence of perilesional edema. Postcontrast imaging is an important component of the evaluation of a soft-tissue mass and will be discussed later.