Key points

Introduction

Soft tissue masses in children are common and comprise a heterogeneous group of lesions, including inflammatory processes, reactive processes, hamartomas, and benign and malignant tumors ( Box 1 ). Reactive processes and hamartomas are fairly frequent, as are benign soft tissue tumors. Soft tissue sarcomas, especially those involving the extremities, are fairly uncommon in children and therefore not always placed high on the list of differential diagnoses. Also, unlike bone tumors, the imaging of soft tissue tumors can be nonspecific and can make the diagnosis challenging. The histopathologic appearance of some of the soft tissue masses can be fairly similar as well, and it is essential to put together the clinical picture along with the imaging and histologic appearance to make the appropriate diagnosis of soft tissue tumors.

Introduction

Soft tissue masses in children are common and comprise a heterogeneous group of lesions, including inflammatory processes, reactive processes, hamartomas, and benign and malignant tumors ( Box 1 ). Reactive processes and hamartomas are fairly frequent, as are benign soft tissue tumors. Soft tissue sarcomas, especially those involving the extremities, are fairly uncommon in children and therefore not always placed high on the list of differential diagnoses. Also, unlike bone tumors, the imaging of soft tissue tumors can be nonspecific and can make the diagnosis challenging. The histopathologic appearance of some of the soft tissue masses can be fairly similar as well, and it is essential to put together the clinical picture along with the imaging and histologic appearance to make the appropriate diagnosis of soft tissue tumors.

Clinical presentation

A good history and physical examination are critical toward developing the appropriate differential diagnosis of soft tissue masses. Physical examination may occasionally be diagnostic, for example transillumination in a dorsal wrist mass suggests a ganglion, and a palpable or auscultable bruit suggests a high-flow vascular malformation. Imaging and pathology findings in isolation may result in an erroneous diagnosis in patients with soft tissue tumors, unless placed in the appropriate clinical setting. The age of the child, location of the tumor, preexisting conditions, duration, rate of growth, consistency, transillumination, and associated symptoms will aid not only in formulation of an appropriate differential diagnosis but also to determine the best next step.

The role of transillumination of superficial masses cannot be overemphasized. This is a simple clinical test, which appropriately done for superficial lesions, helps differentiate a cystic lesion from a solid one, a key factor in determining further workup and eventually treatment. A thorough knowledge of these clinical entities and appropriate diagnostic workup in solid lesions help avoid performing an unplanned resection (“whoops procedure”) in a patient with a malignant soft tissue tumor.

Imaging

A thorough clinical evaluation guides the best imaging strategy. The goals of imaging should be to potentially diagnose the lesion, evaluate the extent of the tumor, help determine the need and location of biopsy, and, in malignant tumors, help in the staging. Imaging can also guide preoperative planning, by illuminating proximity of neurovascular structures, underlying bone involvement, vascularity of the lesion, determining the plane of resection, and so forth. Imaging before biopsy is mandatory.

The evaluation of most soft tissue masses should include radiographs. Synovial sarcomas occasionally demonstrate amorphous calcification; the presence of phleboliths is diagnostic of a venous malformation. Pressure changes in the bone underlying a soft tissue mass implies slow growth and may give us a clue about the biology of the mass, whereas invasion of the underlying bone is strongly suggestive of an aggressive lesion. Bone involvement may also be seen in infants with infantile myofibromatosis and this finding on radiography can help narrow the differential.

Ultrasonography is frequently used in evaluation of superficial soft tissue masses in children. Although operator dependent, it has the advantage of not requiring sedation and is relatively quick. It is extremely useful in evaluation of vascular lesions, as it helps distinguish tumor versus malformation and the classification of malformations (high flow vs low flow). Ultrasonography is also useful in distinguishing cystic versus solid lesions and thus adds confidence to the clinical diagnosis of popliteal cysts. For me, it is the imaging modality of choice for superficial lesions. Ultrasound-guided biopsy of soft tissue lesions is an attractive alternative to computed tomography (CT)-guided biopsy, as it involves no radiation.

Magnetic resonance imaging (MRI) is the gold standard for evaluation of most soft tissue lesions. It has the advantage of not having any radiation, but younger children often need to be sedated for this procedure and the relative high cost is also a factor in considering its use. MRI is especially useful for evaluating deep (subfascial) soft tissue masses, which may be difficult to evaluate on clinical or ultrasonographic examination. Even though imaging appearance of most soft tissue tumors is relatively nonspecific, MRI may occasionally be diagnostic, for example, a lipoma has the same signal as subcutaneous fat on T1 and T2 pulse sequences and shows uniform suppression on fat-suppression sequences, as compared with a low-grade liposarcoma, which will demonstrate some high-signal areas on the fat-suppression sequences.

Soft tissue sarcomas grow in a centrifugal fashion, often “push” the surrounding tissues, and are usually well circumscribed. Infiltrative growth is often seen in fibromatosis (desmoid tumors), malignant peripheral nerve sheath tumors, epithelioid sarcomas, and granuloma annulare. The use of contrast helps delineate the degree of vascularity of a lesion and to differentiate cystic from solid areas. MR angiography is useful in evaluation of some vascular malformations and also to evaluate the vascularity of tumors in select situations.

MRI provides excellent detail of the soft tissue anatomy, location, proximity to vital structures, and is an excellent preoperative planning tool. MR-guided interventions (needle biopsies, injections, aspiration, drainage) are feasible and may become more common as technology improves.

CT is rarely used for diagnostic purposes secondary to its high radiation doses. It is useful in patients with pacemakers and implants that are sensitive to magnetic force (such as the noninvasive expandable prostheses used in extremity limb salvage). The main role for CT is for guided biopsies of deep soft tissue masses.

Biopsy: Imaging by itself may be inadequate in establishing the diagnosis of soft tissue tumors. For more superficial lesions that are small (less than 3 cm), an excisional biopsy may be considered. For large or deep (subfascial) lesions a needle or incisional biopsy may be performed. Precision, especially for deep lesions, can be improved by using image guidance (US, CT, MRI). The use of immunohistochemistry has made it possible to more accurately diagnose as well as subclassify tumors. ( Table 1 ) Principles of biopsy are outlined elsewhere.

Some of the common soft tissue tumors seen in children are discussed in the following sections.

Vascular Lesions and Tumors

Vascular lesions are the most frequent soft tissue masses in children, especially in infants. The International Society for the Study of Vascular anomalies emphasizes the distinction between vascular malformations, which result from localized defects in development and grow commensurate with the child’s growth, with vascular tumors ( Tables 1 and 2 ). Vascular tumors may be benign, such as hemangiomas (eg, infantile, epithelioid), or malignant, such as epithelioid hemangioendothelioma and angiosarcoma (very rare in children). Vascular malformations may be simple (single type) or combined (eg, capillary–venous malformation) ( Fig. 1 ).

Table 1

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