STEREOTAXIC RADIOSURGERY USING THE GAMMA-KNIFE FOR ACOUSTIC NEUROMAS

Purpose: To assess the efficacy and toxicity of stereotactic radiosurg ery using the gamma knife for acoustic neuromas. Methods and Materials : Between January 1990 and January 1993, 36 patients with acoustic neu romas were treated with stereotactic radiosurgery using the gamma knif e. The median maximum tumor diameter was 21 mm (range: 6-32 mm). Tumor volumes encompassed within the prescribed isodose line varied from 26 6 to 8,667 mm(3) (median: 3,135 mm(3)). Tumors less than or equal to 2 0 mm in maximum diameter received a dose of 20 Gy to the margin, tumor s between 21 and 30 mm received 18 Gy, and tumors > 30 mm received 16 Gy. The dose was prescribed to the 50% isodose line in 31 patients and to the 45%, 55%, 60%, 70%, and 80% isodose line in one patient each. The median number of isocenters per tumor was 5 (range: 1-12). Results : At a median follow-up of 16 months (range: 2.5-36 months), all patie nts were alive. Thirty-five patients had follow-up imaging studies. Ni ne tumors (26%) were smaller, and 26 tumors (74%) were unchanged. No t umor had progressed. The 1- and 2-year actuarial incidences of facial neuropathy were 52.2% and 66.5%, respectively. The 1- and 2-year actua rial incidences of trigeminal neuropathy were 33.7% and 58.9%, respect ively. The 1- and 2-year actuarial incidence of facial or trigeminal n europathy (or both) was 60.8% and 81.7%, respectively. Multivariate an alysis revealed that the following were associated with the time of on set or worsening of facial weakness or trigeminal neuropathy: (a) pati ents < age 65 years, (b) dose to the tumor margin, (c) maximum tumor d iameter greater than or equal to 21 mm, (d) use of the 18 mm collimato r, and (e) use of > five isocenters. The 1- and a-year actuarial rates of preservation of useful hearing (Gardner-Robertson class I or II) w ere 100% and 41.7% +/- 17.3, respectively. Conclusion: Stereotactic ra diosurgery using the gamma knife provides short-term control of acoust ic neuromas when a dose of 16 to 20 Gy to the tumor margin is used. Pr eservation of useful hearing can be accomplished in a significant prop ortion of patients.