Unique Natural History of Very Small Vestibular Schwannoma Substantiates Size Threshold Surveillance

OBJECTIVE: Management of vestibular schwannoma (VS) remains controversial. Very small VSs represent a pivotal group: on one hand, they pose the least risk to the patient from the perspective of being small; yet, not recommending treatment may result in missing a critical window for achieving the best outcome. This debate fundamentally hinges on whether treatment improves upon the natural history of the disease. There is an unproven assumption that very small tumors behave similarly to the broader observed population. To address this central question, we conducted a multicenter retrospective review to compare the natural history of tumor growth and hearing for patients with very small VS (1-3 mm) with a referent cohort with larger tumors. METHODS: All patients diagnosed with sporadic VS 1-3 mm were reviewed at 3 large skull base referral centers and compared with a referent cohort of patients with tumors > 3 mm. Tumor growth-free survival rates, treatment-free survival rates, and rates of maintenance of class A, B, or C hearing were estimated using the Kaplan-Meier method. RESULTS: In total, 123 patients with very small VS and a referent cohort of 326 patients were studied. Seventeen patients with very small VS experienced tumor growth at a median of 3.2 years after diagnosis (IQR 2.6-5.1), demonstrating a median growth rate of 1.0 mm per year (IQR 0.9-1.8). In comparison, 167 of the patients in the referent cohort experienced tumor growth at a median of 1.7 years after diagnosis (IQR 0.8-3.1). Tumor growth-free survival rates (95% CI, number still at risk) at 5 years after diagnosis for the 2 cohorts were 84% (75-93, 39) and 45% (40-52, 100), respectively (p < 0.001). Six patients with very small VS underwent treatment after tumor growth at a median of 3.2 years after diagnosis (IQR 3.2-3.8), compared with 138 patients in the referent cohort at a median of 2.2 years after diagnosis (IQR 1.4-3.6). Treatment-free survival rates (95% CI, number still at risk) at 5 years after diagnosis for the 2 cohorts were 92% (86-98, 55) and 54% (49-61, 125), respectively (p < 0.001). Among the 105 patients with very small VS and class A, B, or C hearing at diagnosis, 3 progressed to class D. Comparatively, among the 225 patients in the referent cohort with class A, B, or C hearing at diagnosis, 70 progressed to class D at a median of 2.6 years after diagnosis (IQR 1.0-4.8). Rates of maintenance of class A, B, or C hearing (95% CI, number still at risk) at 5 years after diagnosis were 96% (92-100, 28) for patients with very small VS and 65% (57-73, 70) for the referent cohort (p < 0.001). CONCLUSIONS: The natural history of tumor growth and hearing for very small VS diverges favorably from the cumulative natural history data and provides a more appropriate benchmark to compare clinical outcomes of upfront surgery or radiosurgery for this population. At 5 years after diagnosis, 96% of very small VS maintained class A, B, or C hearing and only 8% received treatment for growth within this multicenter cohort. Cumulatively, these data substantiate initial wait-and-scan management via size threshold surveillance in this population.