Preservation of hearing in acoustic neuromas treated by gamma knife surgery.

138 acoustic schwannomas were treated by Gamma Knife surgery from July 1992 to May 1994. Cases with neurofibromatosis were excluded because of differences in the patterns of growth and development of tumors in these cases. Hearing was evaluated by tonal and vocal audiometry and classified using the Gardner and Robertson score. 104 patients were observed at 3 years after treatment. Hearing studies, the relation of tumor volume to hearing, central and marginal dose, number of shots and preoperative brain-stem-evoked responses (BER) were all recorded. 70% of patients with normal hearing maintained a useful hearing, and 50% of patients with useful hearing maintained serviceable hearing. No correlation was found between hearing preservation and tumor volume, central and marginal dose and number of shots. Gamma Knife surgery seems to be superior to microsurgery with regard to preservation of useful hearing.