Tufts Medical Center Experience With Long-Term Follow-Up of Vestibular Schwannoma Treated With Gamma Knife Stereotactic Radiosurgery: Novel Finding of Delayed Pseudoprogression

PURPOSE: Our purpose was to evaluate the long-term outcomes of patients with vestibular schwannoma (VS) treated with Gamma Knife stereotactic radiosurgery (GKSRS) with modern techniques, with attention to posttreatment tumor growth dynamics, dosimetric predictors, and late toxicities. METHODS AND MATERIALS: One hundred twelve patients with VS were treated with GKSRS with a median dose of 12.5 Gy to the 50% isodose line treated between 2004 and 2015, with patients followed up to 15 years. Target and organ-at-risk doses were recorded, and tumor diameter/volume, audiologic decline, and trigeminal/facial nerve preservation were tracked from treatment onward. RESULTS: GKSRS yielded local control of 5, 10, and 15 years at 96.9%, 90.0%, and 87.1% respectively. Pseudoprogression was found in 45%, with a novel pattern detected with peak swelling at 31 months. Pseudoprogression was associated with smaller tumor diameter at treatment and fewer treatment isocenters, but not with the development of any toxicity, nor was it predicted by any dosimetric factor. Median time to hearing loss was 3.4 years with actuarial hearing preservation at 2, 5, and 10 years of 66.5%, 43.1%, and 37.6%, with rate of hearing loss correlating with maximum cochlea and modiolus doses. Trigeminal and facial nerve preservation rates were 92.7% and 97.6%, respectively. Increasing maximum tumor dose was associated with facial paresthesia. CONCLUSIONS: Modern GKSRS is a safe and effective treatment for VS on long-term follow-up, with high levels of facial and trigeminal nerve preservation. A novel pattern of pseudoprogression has been identified suggesting longer imaging follow-up may be needed before initiating salvage in those without symptomatic progression. Several tumor and dosimetric predictors have been suggested for the development of different toxicities, requiring further evaluation.