Predictors of trigeminal neuropathy in patients receiving Gamma Knife stereotactic radiosurgery for vestibular schwannoma.

Objective: The authors' objective was to validate dosimetric and clinical predictors of the development of trigeminal neuropathy (Tn) in patients treated with stereotactic radiosurgery (SRS) for a diagnosis of vestibular schwannoma (VS).

Methods: In total, 301 patients were treated with SRS for VS at the authors' center between April 2013 and June 2020, with a median prescription dose of 12.5 Gy. Ninety-seven patients were excluded: 78 had pre-existing symptoms of Tn, and 19 had < 2 years of follow-up. At follow-up consultations, trigeminal nerve function was prospectively documented in an institutional database. The median follow-up was 4 years. Data from treatment plans were extracted for factors previously reported as predictors of Tn: volume of cranial nerve (CN) V that received at least 11 Gy, maximum dose to CN V, volume of the cisternal portion of CN V, maximum dose to the brainstem, volume of the brainstem that received at least 12 Gy, and tumor volume. Tumor compression of CN V at baseline was also evaluated. Univariate and multivariate analyses of results were performed to identify significant factors.

Results: In total, 23 (11.3%) patients developed symptoms of Tn after SRS; these symptoms were transitory in 7 (30%) cases. Of the 16 patients with permanent Tn, 13 had objective paresthesia (9 had grade II and 4 grade III) and 5 had pain (2 grade II and 3 grade III); included in this are 2 patients who had both paresthesia and pain. In addition, 44% developed symptoms by 1 year after SRS and 100% by 3 years after SRS. On univariate analysis of patients with permanent symptoms, maximum dose to CN V (p = 0.016) was a significant factor. This was not maintained on multivariate analysis when the volume of CN V that received ≥ 11 Gy became the only significant factor (p = 0.029).

Conclusions: The only significant factor in the risk of development of Tn after SRS for VS was the volume of CN V that received ≥ 11 Gy. This should be routinely incorporated into dosimetric planning constraints and patients should be counseled about the risk of adverse effects if it cannot be met. For those with growing VS and a gap to the trigeminal nerve, it may be prudent to provide earlier treatment with SRS to enable application of this dosimetric constraint and reduced risk of Tn.