Objectives: Accurate volume assessment is essential for management of vestibular schwannoma (VS) after stereotactic radiosurgery (SRS). A cuboidal approximation for volume is the standard surveillance method, however this may overestimate tumor volume. We sought to evaluate several volumetric models and their suitability for postSRS surveillance.

Methods: We evaluated 54 patients with vestibular schwannoma before and after SRS. Gold-standard volumes were obtained by a radiation oncologist using contouring software. Volume was also calculated by cuboidal, ellipsoidal, and spherical formulae using tumor diameters obtained by a neuroradiologist. Main Outcome Measure(s): Percent error (PE) and absolute percent error (APE) were calculated. Paired t-test evaluated bias, and the Bland-Altman method evaluated reproducibility. Linear regression evaluated predictors of model error.

Results: All models overestimated volume compared to the gold standard. The cuboidal model was not reproducible before SRS (p<0.001), and no model was reproducible after SRS (cuboidal p<0.001; ellipsoidal p=0.02; spherical p=0.02).

Significant bias was present before SRS for the cuboidal model (p<0.001), and postSRS for all models [cuboidal (p<0.001), ellipsoidal (p<0.02), and spherical (p=0.005)]. Model error was negatively associated with pre-treatment volume for the cuboidal (PEp=0.03; APE p=0.03), ellipsoidal (PE p=0.03; APE p=0.04), and spherical (PE p=0.02; APE p=0.03) methods and lost linearity post-SRS.

Conclusions: The standard cuboidal practice for following VS tumor volume after SRS overestimates size. Ellipsoidal and spherical estimations have improved performance but also overestimate volume and lack reliability post-SRS. The development of other volumetric models or application of contouring software should be investigated.