ABSTRACTThe sensitivity of gadolinium-enhanced magnetic resonance imaging (MRI) is such that inflamed areas of individual nerves can be visualized. Given this, might the lateral extent of an acoustic neuroma in the internal auditory canal be inaccurately depicted by MRI with gadolinium contrast due to variable enhancement in inflamed neural tissues? The authors studied the correlation between preoperative MRI and intraoperative observation of the lateral extent of tumor in the internal auditory canal (IAC) of 82 patients with acoustic neuromas. The surgeon made a visual determination of tumor extent when the lateral-most aspect of the tumor was exposed. The radiologist, blind to surgical findings, noted the degree of extension into the IAC based on the MRI. The majority (83%) of the tumors involved the lateral third and fundus of the IAC. Agreement between scans and surgical findings was excellent for tumors extending into the lateral third of the IAC, with a test sensitivity of 94 percent and a positive predictive value of 98.5 percent. Knowledge of the accuracy of MRI in demonstrating extent of tumor in the IAC is important in making decisions regarding surgical approach.