A spheroid weighted-axis converter of vestibular schwannoma size: maximum diameter and cisternal volume.

OBJECTIVES To evaluate spheroid models of vestibular schwannoma (VS) size for bidirectional conversion of maximum diameter in the cistern and 3-dimensional volume. STUDY DESIGN Methodological study. SETTING Academic tertiary referral center. SUBJECTS AND METHODS Magnetic resonance imaging studies from 91 patients with VS from 2003 to 2011 were analyzed. Linear measurements defining meatal and cisternal components were extracted. Geometrically based conformal models of tumor volume were compared with measured tumor volume using a semiautomated computerized tracing method. Models were inverted to predict maximum axial cisternal length at the level of the internal auditory canal (IAC). RESULTS A spheroid-weighted axis converter (SWC) of VS size was identified by minimizing input measurement parameters while maximizing output prediction performance. Computation steps of tumor volumes were (1) meatal-measure tumor lengths along the IAC and at the porus acusticus and take the average of a cone and cylinder and (2) cisternal-measure maximum tumor length in the axial IAC plane, use the median major:minor axis ratio of 1.26:1 to estimate minor axis length, take the geometric mean of axial dimensions to estimate axis length in the coronal plane, and apply input lengths to a spheroid. Performance error of this SWC had interquartile ranges of 33% for volume and 2 to 3 mm for maximum cisternal length. CONCLUSION Reporting variability of VS tumor size has made it difficult to reconcile outcomes studies. We propose an accessible tool for bidirectional conversion of volumetric and linear indices of tumor size to unlock potential for meta-analyses of disparate data sets.