Abstract
Forty-three patients with the unilateral type of acoustic neurinoma who were treated with gamma knife radiosurgery were analyzed from the viewpoint of tumor control. The follow-up period ranged from 22 to 55 months (mean 36 months). The tumors, which ranged in volume from 0.1 to 18.7 ml (mean 4.0 ml), were treated with marginal radiation doses of 9-15 Gy (mean 13.4 Gy) with multiple isocenters (mean 8.7). The Actuarial tumor reduction rates were 42% at one year, 75% at 2 years, and 92% at 3 years after gamma knife radiosurgery. Transient tumor expansion (mean 2.1 mm) was seen in 33% of patients, which correlated with previous surgical cases (p=0.002 by multiple regression analysis). The present control rate was 91%. Single-photon emission computed tomography (SPECT) was performed on 15 selected patients before and 1 year and 2 years after gamma knife radiosurgery ^<201>TICl SPECT was used to determine tumor viabillty and the early and delayed ^<99m>Tc-DTPA-human serum albumin (HSA-D) SPECT images were used to assess tumor vascularity and permeability, respectively. The T1 index and HSA-D index of the delayed images were not significantly different from the respective preoperative values. However, there was a statistically significant decrease in the HSA-D index of the early images 1 year after treatment (p=0.02). A statistically significant reduction in tumor volume was seen 2 years after treatment in these 15 patients, meaning that a reduction in tumor vascularity was followed by a reduction in tumor size. One patient underwent srugical excision of the tumor 18 months after gamma knife radiosurgery because the tumor had expanded and resulted in cerebellar ataxia. Histopathologic investigation revealed the presence of some tumor cells with irregularly shaped nuclei and marked intimal thickening or obliteration of the tumor vessels. These findings suggest that the reduction of tumor vascularity may be one of the effects of gamma knife radiosurgery, which gradually decreases the size of acoustic neurinomas.
