Imaging Post Stereotactic Radiosurgery for Vestibular Schwannomas-When Should We Scan?

Objective: To identify the optimal time for imaging following stereotactic radiosurgery (SRS) in patients with vestibular schwannomas (VS). Study Design: Retrospective case series. Setting: Tertiary, university center. Patients: Patients with VS treated with SRS. Interventions: Pre- and post-SRS surveillance with magnetic resonance imaging (MRI); patients should have at least two pre- and two post-SRS scans available to be included. Main Outcome Measures: Patient demographics, tumor size, and growth per month ratio pre- and post-SRS and time interval between serial MRI. Results: Forty-two patients fulfilled the inclusion criteria. The average tumor size before the treatment was 16.9 mm (range, 10-28 mm) while 2 years posttreatment it was 16 mm (range, 7-25 mm) (p = 0.5). Average time of the first MRI post-SRS was 11 months (range, 5-14) with an average change in tumor size at that time of +0.53 mm (range, -5-8). Average time to second MRI was 22.3 months (range, 12-33) with an average change in tumor size at second scan of -1.14 (range, -5-2) mm (p = 0.117). The average growth/mo ratio before SRS was 0.26 mm/mo (range, 0-1), while post-SRS 0.05 mm/mo (range, -0.3-0.5) and -0.16 mm/mo (range, -18-0.25) at the time of the first and second scan, respectively (p < 0.001). Conclusions: Given the initial increase in size following SRS, unless clinically indicated, MRI post-SRS at less than 1 year has no clinical value. The growth per month ratio provides more meaningful values for response to treatment than tumor size measurements.