Pretreatment Growth Rate as a Predictor of Tumor Control Following Gamma Knife Radiosurgery for Sporadic Vestibular Schwannoma

Introduction: Over the past 30 years stereotactic radiosurgery (SRS) has become an established non-invasive treatment alternative to microsurgery for small to medium-sized vestibular schwannoma (VS). Natural history studies have demonstrated a significant portion of untreated sporadic VS do not grow, making the true efficacy of SRS difficult to estimate. This study aims to further define long-term SRS tumor control in a cohort of patients with documented pre-treatment tumor growth who failed initial conservative observation.

Methods: A prospective clinical database was queried and all patients with sporadic VS who elected initial conservative observation and subsequently underwent SRS after documented tumor growth between 2004 and 2014 were identified. Study inclusion required a minimum initial observation period of 6 months, documented pre-SRS growth rate of ≥1 mm/year and a minimum post-SRS follow-up of 9 months. Post-treatment tumor growth or shrinkage was determined by a ≥2 mm increase or decrease in maximum linear dimension, respectively.

Results: Seventy patients met study inclusion criteria and were analyzed. The median pre-treatment observation period was 16.4 months and the median post-treatment follow-up period was 36.8 months. At time of treatment, 61 tumors exhibited extracanalicular (EC) extension while 9 were purely intracanalicular (IC). Of the 61 EC VSs, 53 (87%) remained stable or decreased in size following treatment and 8 (13%) enlarged by greater than 2 mm. No IC tumors demonstrated post-SRS growth. Among EC tumors, the median pre-treatment tumor growth rate was 2.09 mm/year for tumors that decreased or were stable in size, compared with 3.33 mm/year for tumors that grew following SRS (p = 0.005). Patients who demonstrated a pretreatment growth rate of <3mm/year exhibited a 95% tumor control rate, compared with 70% for those demonstrating ≥3 mm/year of growth prior to SRS (p = 0.01). The odds ratio for the association of pre-treatment growth rate with post-SRS tumor growth was 1.51 mm/year (95% CI 1.06–2.15; p = 0.021), indicating that each 1-mm increase in pre-treatment growth rate was associated with a 51% increased likelihood of tumor progression, as compared with a stable or decrease in tumor size, following radiosurgery. At time of last follow-up, 4 of 70 (5.7%) patients ultimately underwent repeat radiosurgery or salvage microsurgical resection for persistent tumor growth.

Conclusion: Overall, SRS is highly effective in treating VSs that fail initial observation. Tumor control is achieved in 95% of VS that exhibit slow (<3 mm/year) pretreatment growth. However, SRS is less successful in treating tumors exhibiting rapid growth (≥3mm/year). These data may help guide patient counseling regarding treatment choice following failed observation.