Nerve sheath tumors: evaluation with CT and MR imaging.

Tumors of nerves are classified into benign (schwannoma and neurofibroma) and malignant nerve sheath tumors. Schwannomas almost always occur as solitary lesions, whereas neurofibromas may occur alone or in a greater number, especially in patients with the peripheral form of von Recklinghausen's disease. Benign nerve sheath tumors often present as asymptomatic, slowly growing soft tissue masses. Although malignant nerve sheath tumors are relatively rare, a sudden increase in the size of a lesion, in particular in a patient with neurofibromatosis, should raise the suspicion of malignant change. On computed tomography (CT) and magnetic resonance imaging (MR) a benign nerve sheath tumor usually appears as a well-defined, oval, spherical or fusiform mass with smooth borders and distinct outlines, located in the subcutaneous tissue or centered at the expected anatomic location of a nerve, with displacement of adjacent soft tissues. Generally nerve sheath tumors have a low density on unenhanced CT scans. On MR they are isointense to muscle on T1-weighted images, whereas on T2-weighted images the signal intensity is high. Both on CT and MR the degree of contrast enhancement is moderate to marked and may be homogeneous or inhomogeneous. MR has become the method of choice for evaluating the anatomic location, contour, and relation of a nerve sheath tumor to adjacent neural, vascular, and muscular structures. The imaging criteria for malignant nerve sheath tumors are not specific enough to distinguish them from other malignant soft tissue tumors, so that neither CT nor MR can establish a definite diagnosis.