Purpose: To present our experience in treating large vestibular schwannomas abutting the brainstem with stereotactic radiosurgery (SRS) or fractionated stereotactic radiosurgery (fSRS) alone without surgery.
Methods: At our institution, we have increasingly performed Gamma Knife (GK) SRS/fSRS alone for patients clinically diagnosed as having a Koos Grade 3 or 4 acoustic neuroma. In this study, we retrospectively reviewed the treatment plans of 23 patients treated with this approach from 3/2014-1/2021, and reported the dosimetric indexes and treatment outcomes. All patients were prescribed a dose of 12Gyx1 or 6Gyx5 fractions, planned using the Leksell GammaPlan (LGP), and treated with the GK Perfexion (before 5/2018) or ICON system (after 6/2018) of our institution. Dosimetric indexes collected included gradient index (GI), Paddick Conformity Index (CI), and maximal dose and V10/23Gy (per TG101 dose constraints) of the brainstem. To measure changes in tumor volume, post-treatment imaging was imported into LGP and the index lesion was individually contoured.
Results: All treatments demonstrated excellent local control with limited toxicity. All except 2 treated targets touched the brainstem. The mean±standard deviation for the CI is 1.23±0.08 and for the GI is 3.00±0.24. For the brainstem, the mean maximal dose and V10Gy are respectively 12.20±3.77 Gy and 0.097± 0.097 cm3 for 12Gyx1 fractionation. The mean maximal dose and V23Gy are respectively 26.53±1.078 Gy and 0.096±0.130 cm3 for 6Gyx5 fractionation. The majority of the follow-up scans showed increased tumor volumes at 6 months and decreased/stabilized volume in the 18-month plan.
Conclusion: Large vestibular schwannomas can be safely treated with SRS/fSRS alone. The brainstem constraints in TG101 for single and five fractions can be met. This treatment approach is proven effective based on the follow-up scans. Further data is also needed to appropriately compare hypofractionated vs single-fractionated SRS in this population.