Measurement of tumor volume in acoustic neurinomas

The so-called wait-and-scan policy was proposed in acoustic neurinoma (AN) patients with small tumors several years ago. A major effort was undertaken to classify spontaneous tumor progression as found in follow-up magnetic resonance imaging (MRI) scans. Although tumors progress in three dimensions, most authors rely exclusively on single linear measurements. Volumetric estimations from MRI data are possible but are not routinely performed by radiologists. Therefore, a calculation of tumor volume based on measurements in three axes and calculation of a sphere was adopted, and results were compared to intraoperative volume measurements of removed tumor tissue in 61 middle fossa AN cases. Intraoperative and calculated volumes were correlated (r(2) = 0.85). The volume model was found to be especially appropriate for medium-sized tumors. In purely intrameatal tumors, the calculated volume tended to underscore, and in larger ones to overscore, the intraoperative measurements. Although neither progress of symptoms nor intraoperative findings and outcome are well correlated to radiological findings, follow-up of tumor volume is generally proposed. Tumor volume calculated from three axes is helpful if a voxel count is not available.