Long-term outcomes of Gamma Knife surgery for vestibular schwannomas: tumor stability supports reduced surveillance after 10 years

OBJECTIVE: The aim of this study was to evaluate the long-term efficacy and safety of Gamma Knife surgery (GKS) for sporadic vestibular schwannoma (VS), focusing on the durability of tumor control and the necessity of extended posttreatment imaging surveillance. METHODS: This retrospective study included patients with sporadic VS treated with single-session GKS between May 1991 and January 2020 at a single center. Tumors were classified into the following anatomical types using a modified Koos-based system: type A (intracanalicular), type B (cerebellopontine angle), type C (mild brainstem compression), and type D (severe brainstem compression with fourth ventricle deviation). A subgroup analysis in patients without salvage treatment within the first 5 years after GKS and with ≥ 5 years of follow-up was performed to assess long-term stability. Salvage treatment rates, functional outcomes, and adverse events were evaluated. RESULTS: Overall, 878 patients (488 female, median age 57 years) with sporadic VS treated with single-session GKS were included in the analysis. A subgroup of 793 patients (438 female, median age 58 years) remained free of salvage treatment during the first 5 years after GKS and had ≥ 5 years of follow-up. The median clinical follow-up duration was 154.5 months. Salvage treatment was required in 7.5% of the patients, with a significantly higher incidence in patients with type D tumors (24.7% at 5 years) compared with type A-C tumors (3.6%) (subdistribution hazard ratio 2.319, p = 0.036). In the subgroup of patients with stable disease, the cumulative incidence of salvage treatment at 15 years was 2.1%, and true tumor progression was identified in only 2 patients (0.3%). Notably, tumor type at the time of GKS did not significantly influence the risk of salvage treatment beyond 5 years in the subgroup. Serviceable hearing preservation improved in more recent treatment periods: patients with pre-GKS Gardner-Robertson class 1 hearing had a 10-year preservation rate of 63% in the late treatment period (2005-2023) compared with 48% in the early period (1991-2004). Among late adverse events, cyst-related complications were the most common (3.1%), whereas trigeminal neuralgia (1.6%), persistent facial palsy (0.1%), and malignant transformation (0.2%) were infrequent. CONCLUSIONS: GKS provided durable long-term tumor control and functional preservation in patients with small- to medium-sized VS. True tumor progression beyond 10 years was not observed in this cohort, suggesting that the intensity or frequency of routine imaging surveillance can be reduced or individualized after 10 years in patients with stable disease during the first 5 years after GKS. However, because delayed adverse events (e.g., cyst formation, trigeminal neuralgia, and malignant transformation) can still occur, follow-up strategies should be tailored according to initial tumor characteristics and the posttreatment clinical course.