Local Control and Radiographic Response as a Function of Isodose Adjustment in Stereotactic Radiosurgery Treatment for Sporadic Vestibular Schwannoma

Background: Stereotactic radiosurgery (SRS) is a common, safe, and effective treatment for vestibular schwannoma (VS), which is typically prescribed at the 50% isodose line (IDL). Decreasing the IDL for a fixed prescription dose increases both the maximum dose and the fraction of tumor receiving a higher physical dose; however, the impact on clinical outcomes of such an approach to treatment planning has not been studied.

Methods: Propensity score-matched analysis of patients with sporadic VS treated with SRS planned at IDL = 40% vs. IDL = 50%, with prescription dose of 12 to 14 Gy, treatment volume ≤10 mL, and follow-up ≥2 years. Cohorts were matched on age, volume, and prescription dose.

Results: After matching 40% IDL and 50% IDL groups, mean ages were 55 versus 54 years (p = 0.891), mean volumes were 2.15 versus 2.03 cm³ (p = 0.844), and median prescription doses were 12 Gy (p = 1). Median total follow-up times were 111 versus 72 month. Tumor control was assessed actuarially at 2, 5, and 10 years, observed rates of 100, 96.4, and 96.4% for 40% IDL and 96.4, 86.7, and 86.7% for 50% IDL (p = 0.243). Follow-up MRI studies were volumetrically analyzed for 19 patients treated at 40% IDL and 21 patients treated at 50% IDL; overall volume reduction was significantly greater in the 40% IDL cohort (48.1 vs. 38.3%, p = 0.05), but annual rates-of-reduction were comparable (5.5 vs. 6%, p = 0.749). Serviceable hearing was documented at time of SRS in eight patients treated at 40% IDL and 13 patients treated at 50% IDL. Hearing preservation was significantly greater among 40% IDL patients, with 2-, 5-, and 10-year rates of 100, 83.3, and 62.5%, as compared with 76.2, 57.1, and 11.4% (p = 0.017). Differences in rare complications including facial paresthesia, facial palsy, ataxia/gait disturbance, and new shunt-dependent hydrocephalus were not significantly difference between the IDL groups.

Conclusion: Based on a small, retrospective sample, SRS treatments for VS planned at 40% IDL may achieve improved tumor control and hearing outcomes, as compared with treatments planned at 50% IDL.