Neuroimaging of acoustic nerve sheath tumors after stereotaxic radiosurgery.

Using a strict method for measuring tumor size, we evaluated tumor response to radiosurgery in 88 patients with 89 acoustic tumors treated over 3 years with a 201-source cobalt-60 gamma unit. Overall, tumor size was unchanged in 73% of patients and increased in 4%. In 22% of patients, tumor diameter decreased an average of 4.9 mm 3-33 months after treatment. Tumor shrinkage occurred in 36% of 50 patients who were followed for at least 1 year after treatment. Loss of tumor contrast enhancement was seen in 79% of patients 1-18 months after treatment. Delayed communicating hydrocephalus developed in four patients. In eight patients, increased signal on T2-weighted MR images developed in the adjacent cerebellar peduncle (n = 5) or the peduncle and dorsolateral pons (n = 3) 5-15 months after treatment. T1-weighted MR imaging and CT were insensitive to these adjacent brain changes. Stereotaxic radiosurgery is an important alternative treatment for selected patients with acoustic tumors. There is no mortality or major perioperative morbidity, hospitalization time and costs are smaller than for microsurgery, patient employment or functional level is maintained, and hearing preservation and facial neuropathy rates are comparable to those in published microsurgical series. Although the rate of occurrence of trigeminal neuropathy is greater than those reported in published microsurgical series, the majority of cases are mild, transient, and nondebilitating. MR imaging before and after radiosurgery is the most sensitive imaging tool to evaluate tumor response, the presence of adjacent parenchymal signal changes, and ventricular size. With a mean follow-up time of 14.6 months, the rate of complications detected by neuroimaging is low and the tumor control rate is 96%.