Object: The aim of this study was to assess long-term clinical outcomes in patients who underwent Gamma Knife surgery (GKS) with a low margin dose-14 Gy or less-to treat vestibular schwannoma (VS) unrelated to neurofibromatosis Type II.
Methods: Between December 1994 and December 2001, 200 patients with VSs underwent GKS, which was performed using the Leksell Gamma Knife model B. More than 10 years of follow-up is available in these patients. One hundred ninety patients (88 male and 102 female patients) were followed up using MRI (follow-up rate 95%). The mean age of these patients was 50.6 years (range 10-77 years). Gamma Knife surgery was the primary treatment for VS in 134 cases (70.5%) and was an adjunctive management approach in 56 cases (29.5%). The median tumor margin dose was 13.0 Gy (range 6.0-14.4 Gy), and the median maximum tumor dose was 28.0 Gy (range 15.0-60.0 Gy). The median tumor volume was 3.6 cm(3) (range 0.3-27.3 cm(3)). The median duration of follow-up in these patients was 109 months (range 8-195 months).
Results: In the 190 patients, the latest follow-up MRI studies demonstrated tumor regression in 122 patients (64.2%), stable tumor in 48 patients (25.3%), and tumor enlargement in 20 patients (10.5%). The total rate of tumor control was 89.5%. Using the Kaplan-Meier method, the authors found the estimated 3-, 5-, 10-, and 15-year tumor control rates to be 95%, 93%, 86%, and 79%, respectively; and the estimated 3-, 5-, and 10-year hearing preservation rates to be 96%, 92%, and 70%, respectively. Twenty-six patients (13.7%) exhibited transient mild facial palsy or facial spasm, and 2 patients (1.1%) suffered persistent mild facial palsy. Thirty-nine patients (20.5%) had transient trigeminal neuropathy, and 5 patients (2.6%) suffered from persistent mild facial numbness. The incidence of persistent severe facial and trigeminal neuropathy was 0.0%.
Conclusions: With a low prescribed margin dose of 14 Gy or less, GKS was confirmed to provide long-term tumor control for small to medium-sized VSs and largely to prevent cranial nerves from iatrogenic injury. Based on the findings of this study, GKS is also a reasonable option for the treatment of large, heterogeneously enhancing tumors without symptomatic brainstem compression. Gamma Knife surgery can preserve a high quality of life for most patients with VS who do not have symptomatic brainstem compression. Long-term follow-up is required because of the risk of delayed recurrence of VS.