(SST) Lunsford & Leksell Radiosurgery Award (2023 Award Winner): Treatment Response and Malignant Transformation Risk After Stereotactic Radiosurgery for Neurofibromatosis Type 2-associated Vestibular Schwannomas: An International Multicenter Study

Introduction: The management of neurofibromatosis type 2 (NF2)-associated vestibular schwannomas (VSs) is challenging. The increasing utilization of stereotactic radiosurgery (SRS) in these tumors necessitates investigating treatment failure and late radiation-related risks. The aim of this study was to evaluate tumor control, freedom from additional treatment (FFAT), serviceable hearing preservation, and radiation-related risks of NF2 patients who had undergone SRS for VS.

Methods: A retrospective study of 267 NF2 patients who underwent SRS at 12 centers participating in the International Radiosurgery Research Foundation was performed. SRS was utilized as a primary treatment in 232 tumors while, 96 tumors underwent adjuvant SRS after resection. The median patient age was 31 years (IQR, 21-45 years), and 52% were males.

Results: A total of 328 VSs underwent SRS. During the study period (median: 59 months [IQR, 23-112 months]) with 10- and 15-year tumor control rates of 77% (95% CI: 69-84%) and 52% (95% CI: 40-64%), respectively, with 60 tumors (18.3%) showing volumetric enlargement. Forty-two (12.8%) tumors required additional surgical or SRS treatment because of tumor progression. FFAT rates were 85% (95% CI: 79-90%) and 75% (95% CI: 65-86%) at 10 and 15 years, respectively. The serviceable hearing preservation rates were 64% (95% CI: 55-75%) at 5 years and 35% (95% CI: 25-54%) at 10 years, with a median serviceable hearing preservation of 90 months (95% CI: 85-NA months). In the multivariate analysis, age (HR: 1.03 [95% CI: 1.01-1.05]; P = 0.02) and bilateral VSs (HR: 4.56 [95% CI: 1.05-19.78]; P = 0.04) were significant predictors for serviceable hearing loss. Neither radiation-induced malignancy nor malignant transformation were encountered.

Conclusion : Although the absolute volumetric tumor progression rate was 48% at 15 years, the rate of FFAT related to VS was 75% at 15 years. None of the NF2-related VS patients developed a new radiation-related neoplasm or malignant transformation after SRS.