Is a redefinition of the growth criteria of vestibular schwannomas needed?

PURPOSE: The natural history of vestibular schwannomas is poorly understood. Knowledge of growth rate and growth pattern is essential because the treatment strategy is based upon these. The purpose of this study was to determine the inter- and intraobserver variability in measuring VS size. MATERIALS AND METHODS: Two consultant neuroradiologists independently made three linear measurements (d1, d2, d3) using digital MRI scans. MRI scans from 72 patients diagnosed between 2002 and 2010 with VS were obtained. These patients had a total of 223 MRI scans. d1 (medio-lateral diameter) was made perpendicular to d2. d2 was made parallel to the posterior border of the petrous ridge, and d3 was a measure of the cranio-caudal height of the tumor. RESULTS: Limits of Agreement ranges are larger for interobserver reliability compared to intraobserver reliability. Measurement error for all diameters (except d1, intraobserver) is greater than 2mm. d1 measurements had the least variability and d3 measurements the highest variability, both for intra and interobserver measurements. CONCLUSIONS: The optimal method of estimating VS size needs further investigation, and measurements need to be standardized and clearly defined. d3 seems to be the most difficult diameter to measure reliably. Interobserver measurement error for all diameters is greater than 2mm. The current VS growth criterion of more than 1-2mm, used to triage patients to surgery, lies within this error range, and thus is problematic as a guide for clinical practice. We therefore suggest that the growth criterion for VS be redefined.