MR Imaging of Soft Tissue Masses in Children

Abstract

In approaching a child who has a soft tissue mass, clinical history and physical examination (including lesion location) play a critical role in diagnosis, and patient age also can narrow the differential. The typical presenting complaint is a palpable mass; larger masses are more likely to present with pain. Reactive processes and benign neoplasms are the most common lesions, whereas malignant tumors are rare. Radiography and CT offer limited assistance, being most useful for characterization of calcification or ossification, as in myositis ossificans. Ultrasonography can be helpful in the evaluation of small, superficial masses and in determining the cystic nature of some lesions. In general, however, MR imaging provides the most information, determining lesion extent and, in some cases, specific diagnoses. MR imaging sometimes provides definitive diagnosis of common benign lesions. The most common benign lesions are hemangioma/lymphangioma, lipoma, periarticular cyst, inflammatory masses, fat necrosis, neurofibroma, and giant cell tumor of the tendon sheath. 1 Coincidentally, this list correlates well with those lesions for which definitive diagnosis at MR imaging is possible: hemangioma, lymphangioma, and other vascular tumors; lipoma; periarticular cysts; hematoma; giant cell tumor of the tendon sheath; benign neural tumors; and fat necrosis. Many benign and malignant soft tissue masses have common imaging characteristics. 2–5 Many lesions are iso- or hypointense to muscle on T1weighted (T1-W) imaging and hyperintense on T2-weighted (T2-W) imaging. Although some investigators have found MR imaging capable of determining the benignity of specific lesions (discussed previously), accuracy often is uncertain, necessitating biopsy. When MR imaging appearance is nondiagnostic, benign origin may be more likely if a patient is less than 20 years old, the mass measures less than 10 cm, its position is subcutaneous or fascial, and it appears well circumscribed, homogeneous on T2-W imaging, and with no surrounding edema. 6 Enhancement characteristics may facilitate diagnosis of malignant lesions, which may show less rapid enhancement and more rapid washout. 7 MR imaging also defines the cystic nature of some lesions, provided T1 and T2 characteristics are evaluated carefully and contrast is administered in equivocal cases. 8 Only 1% to 6% of soft tissue masses are malignant. 1 Of the malignant tumors in the 0- to 5-year age group, fibrosarcoma is most common, followed by rhabdomyosarcoma. In the older age group, 6 to 15 years old, malignant fibrous histiocytoma is most common, followed by synovial sarcoma and rhabdomyosarcoma. 9 Even when MR imaging fails to differentiate between benignity and malignancy or provide a tissue diagnosis of a malignant tumor, it does provide essential information, delineating the extent of the lesion, extension beyond facial planes, and involvement of adjacent structures, such as the neurovascular bundle, joints, and bone. It also is useful in assessing response to therapy, although differentiation of residual or recurrent tumor from postoperative edema,