2582 - Radiologic and Clinical Outcomes of a Frameless Non-Invasive Stereotactic Radiosurgery Instrument for Vestibular Schwannomas

Purpose/Objective(s): Though a non-invasive stereotactic radiosurgery instrument remains a well-established treatment modality for vestibular schwannomas (VS), the fixed frame approach has long been accepted as the optimal method for motion restriction of the patient’s head. We have previously reported on the long-term functional outcomes of framed non-invasive stereotactic radiosurgery for VS and described dose-rate effects on both clinical and radiographic outcomes. With the adoption of the non-invasive stereotactic radiosurgery instrument Icon system, there has been a shift toward a frameless approach using cone-beam computed tomography (CBCT) with a custom thermoplastic mask for non-invasive target immobilization. Here, we aim to assess long-term functional outcomes and radiographic tumor response in what is - to our knowledge - the largest cohort of patients with VS treated with non-invasive stereotactic radiosurgery using a frameless immobilization approach.

Materials/Methods: We conducted retrospective review of 187 patients treated at our institution between 2018 and 2023, collecting demographic data, pretreatment symptoms, non-invasive stereotactic radiosurgery instrument parameters, and the most recent radiologic and clinical follow up data. Patients who had prior surgical management or non-invasive stereotactic radiosurgery were excluded. Clinical outcomes assessed include patient-reported hearing loss, vestibular symptoms, and facial or trigeminal nerve dysfunction (FND and TND, respectively). Radiographic freedom from progression (defined as persistently increased maximal tumor diameter by at least 2 mm at 3 years following treatment completion) was determined using the reverse Kaplan-Meier method. We used Cox proportional hazards models to test for the effects of age, gender, tumor volume, dose rate, fraction number, coverage, conformity index, and gradient index on radiographic progression.

Results: The final study cohort included 151 patients. Of those, 130 patients had radiologic follow up (FU) (median FU: 25.9 months), and 117 had clinical FU (median FU: 26.8 months). Notably, 28.7% percent of patients experienced progression of hearing loss (24.0% worsened, 4.7% new-onset), and 27% of patients experienced progression of vestibular symptoms (9.0% worsened, 18.0% new-onset). Few patients experienced new onset FND (n=7) or TND (n=6) at any point following treatment. Among the 130 patients with radiologic FU, radiographic progression-free survival (PFS) was 92% (95% CI: 88-97%), 91% (95% CI: 89-97%), and 88% (95% CI: 81-96%) at 2, 3, and 4 years, respectively. On multivariate analysis, only gradient index was significantly associated with radiographic progression (HR 1.76, 95% CI 1.11-2.81, p=0.017).

Conclusion: Frameless non-invasive stereotactic radiosurgery for VS offers great tumor control and reasonable avoidance of symptom progression, with outcomes comparable to those using a frame-based approach. Therefore, frameless CBCT-based mom-invasive stereotactic radiosurgery may be a reasonable, less invasive alternative for patients undergoing treatment of VS. Further work is needed to elucidate the impact of gradient index on likelihood of tumor progression.