Comparison of two radiologic methods for measuring the size and growth rate of extracanalicular vestibular schwannomas

Objective: To compare two methods for measuring the size and growth rate of extracanalicular vestibular schwannomas: the method recommended in 1995 by the American Academy of Otolaryngology - Head Neck surgery (AAO-HNS) and t he maximum cerebellopontine angle (CPA) tumor diameter, i.e., the method of ten used in radiologic reports. Study Design: Retrospective clinical study. Setting: Tertiary referral center. Patients: Fifty-four patients with a radiologic diagnosis of unilateral ext racanalicular vestibular schwannoma whose tumors were managed conservativel y for a mean duration of 39.8 months (range 12-194 months). Intervention: The extracanalicular component was measured by use of high-re solution magnetic resonance imaging (2- to 3-mm axial slices) at 6- to 12-m onth intervals. Main Outcome Measures: Tumor diameter was calculated by two methods. In the AAO-HNS method, the axial image with the: largest extracanalicular tumor d iameter was selected, and the maximum anteroposterior (A-P) and medial-late ral (M-L) tumor diameters were calculated. The A-P diameter was calculated parallel to the posterior surface of the petrous temporal bone, and the M-L diameter was calculated perpendicular to it. The size of the tumor was cal culated as the square root of the product of these two diameters. In the ma ximum CPA method, the maximum CPA tumor diameter in any direction was also measured. Results: There was no significant difference between the two methods for me asuring the extracanalicular tumor size and growth rate, although the maxim um CPA diameter method was consistently greater than the AAO-HNS method. Th ere was a strong positive correlation between the two methods for assessing tumor size and growth. The A-P and M-L extracanalicular tumor diameters al so showed a strong positive correlation, suggesting that the extracanalicul ar portion of vestibular schwannomas tends to enlarge equally in both these directions. Conclusion: There is a strong positive correlation between these two method s for assessing both the tumor size and the growth rate of extracanalicular vestibular schwannomas. However, because vestibular schwannomas tend to gr ow in both the A-P and the M-L directions, this suggests that the AAO-HNS m ethod is preferable.