Otology & neurotology : official publication of the American Otological Society, American Neurotology Society [and] European Academy of Otology and Neurotology | 2019
Current Volumetric Models Overestimate Vestibular Schwannoma Size Following Stereotactic Radiosurgery.
Abstract
OBJECTIVE\nAccurate volume assessment is essential for the management of vestibular schwannoma 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 post-SRS surveillance.\n\n\nSTUDY DESIGN\nRetrospective cohort study.\n\n\nSETTING\nTertiary referral center.\n\n\nPATIENTS\nWe evaluated 54 patients with vestibular schwannoma before and after SRS.\n\n\nINTERVENTION(S)\nGold-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.\n\n\nMAIN OUTCOME MEASURE(S)\nPercent 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.\n\n\nRESULTS\nAll models overestimated volume compared with the gold standard. The cuboidal model was not reproducible before SRS (p\u200a<\u200a0.001), and no model was reproducible after SRS (cuboidal p\u200a<\u200a0.001; ellipsoidal p\u200a=\u200a0.02; spherical p\u200a=\u200a0.02). Significant bias was present before SRS for the cuboidal model (p\u200a<\u200a0.001), and post-SRS for all models [cuboidal (p\u200a<\u200a0.001), ellipsoidal (p\u200a<\u200a0.02), and spherical (p\u200a=\u200a0.005)]. Model error was negatively associated with pretreatment volume for the cuboidal (PE p\u200a=\u200a0.03; APE p\u200a=\u200a0.03), ellipsoidal (PE p\u200a=\u200a0.03; APE p\u200a=\u200a0.04), and spherical (PE p\u200a=\u200a0.02; APE p\u200a=\u200a0.03) methods and lost linearity post-SRS.\n\n\nCONCLUSIONS\nThe standard cuboidal practice for following vestibular schwannoma 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.