GENERAL IMAGING CHARACTERISTICS (PRE-THERAPY)

INTRODUCTION (continued)

On MR imaging extremity soft-tissue sarcomas generally present as deep-seated heterogeneous masses. They do not generally involve bone or metastasize to lymph nodes and tend to remain compartmentalized. The adjacent neurovascular bundle is generally displaced but not encased. In some instances when tumor closely abuts and displaces vessels clear adventitial resection margins may be difficult to obtain surgically. Lobulation, hemorrhage and cystic or 'necrotic' degenerative changes may be seen within the tumor in a non-specific fashion.

MR signal intensity characteristics are non-specific in most instances (well differentiated liposarcoma is an exception). However, sarcomas in most instances, tend to exhibit signal intensity equal to or slightly higher than adjacent skeletal muscle on T1W images. When a lesion exhibits considerably low signal intensity relative to adjacent skeletal muscle on T1W images, a benign lesion should be considered in the MR differential diagnosis.

A benign lesion may exhibit either low (fluid, fibrosis), intermediate (solid, proteinaceous fluid) or high (hemorrhage, fat) signal intensity relative to skeletal muscle on T1W images. Areas of cystic degeneration or necrosis within soft-tissue sarcomas may be visualized as low signal intensity on T1W images and foci of hemorrhage may be visualized as high signal intensity areas on T1W images. Soft-tissue sarcomas generally exhibit heterogenoeous high signal intensity on T2W images.

Most benign lesions are also generally hyperintense relative to skeletal muscle on T2W images. Rarely, soft-tissue sarcomas present as subcutaneous or cystic masses and occasionally tumors such as myxoid liposarcoma may present as well defined cystic masses exhibiting low signal intensity relative to muscle on T1W images. Calcifications within benign or malignant tumors may be visualized as low signal intensity on T1W and T2W MR images. Neurofibrosarcomas (malignant peripheral nerve sheath tumors) may mimic other sarcomas on MR imaging. It should be noted that in approximately 50-60% of cases, a characteristic target pattern may be seen in benign extracranial nerve sheath tumors (neurofibroma or schwannoma).