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Neuroradiology Case of the Week

Case 213

Jerry Lee, MD and Per-Lennart Westesson, MD, PhD, DDS

Clinical Presentation: A 51-year-old male with progressive hearing loss in his right ear.

Radiological Findings: There is a bilobed enhancing mass in the right cerebellar pontine angle extending into the right internal auditory canal. The mass is intermediate to low signal on T1 weighted images high signal on T2 weighted images, and enhances homogeneously after administration of intravenous Gadolinium (Figs. 1 & 2).
     Post radiation therapy MRI performed one year later demonstrates low signal on T1 and peripheral enhancement with central non-enhancing region consistent with necrosis (Figs. 3 & 4).

Diagnosis:  Acoustic schwannoma with post radiation therapy central necrosis

Discussion: Acoustic schwannoma, also called vestibular schwannoma or acoustic neuroma, arises in the superior vestibular division of the 8th cranial nerve.  It accounts for 60 to 90% of all cerebellopontine angle (CPA) masses.  The most common clinical findings are sensorineural hearing loss and tinnitus.  Vestibular symptoms also occur but are less frequent.  Facial nerve paralysis is uncommon.  These tumors are benign, well encapsulated, and slow growing.  Acoustic schwannomas are present in 5% of individuals with neurofibromatosis I, and occur bilaterally in 96% of neurofibromatosis II patients. 
     MRI is the procedure of choice for evaluating vestibular schwannomas or any other CPA mass.  Small acoustic schwannomas may be entirely intracanalicular, and large ones are usually intracanalicular with CPA cistern extension, as in this case.  On T1WI they usually are intermediate signal and demonstrate higher signal on T2WI.  They enhance strongly and homogenously with gadolinium.   Differential includes:  Epidermoid cyst, Arachnoid cyst, Meningioma, Facial nerve schwannoma, metastasis and lymphoma, and aneurysm.
     Treatment options include surgical resection, stereotactic radiation therapy such as with a gamma knife, and/or careful serial observation.  Surgical resection is usually the treatment of choice and the method depends on the tumor size and patients hearing status.  Patients with small and medium sized tumors (less than 3 cm) in older patients (over 65) or any patient who has medical problems (such as heart disease, etc) that prevent undergoing surgery are the best candidates for radiation. The radiation is given by one of several methods: Linear accelerator and Gamma Knife. Radiation therapy is effective in stopping the growth or reducing the tumor size of 90% of patients, however is not a cure. At least half of the patients whose tumors grow after radiation will need surgery. Those patients who need surgery after radiation generally have poor facial nerve function because radiation leads to scarring of the tumor. The long-term consequences of stereotactic radiation to acoustic neuromas are not known.

References:

1. Osborn, AG. Diagnostic Neuroradiology. St. Louis: Mosby, 1994
2. Zimmerman RA, Bilaniuk L. Neuroimaging: Clinical and Physical Principles. Eds. Robert A. Zimmerman, Wendell A. Gibby, Raymond F. Carmody. New York: Springer, 2000.