Acoustic schwannomas: Awareness of radiologic error will reduce unnecessary treatment. Otol Neurotol 2005;26:512-515

Objective: To measure the intra- and interobserver error in size estimation of acoustic schwannomas from magnetic resonance imaging (MRI) scans by experienced radiologists to determine whether small amounts of tumor growth that may affect management (2 mm) could be reliably measured in clinical practice.

Design: Duplicated, blinded size estimation of acoustic neuromas (according to American Academy of Otolaryngology-Head and Neck Surgery guidelines, 1995) from MRI scans of patients with acoustic neuromas.

Setting: Tertiary referral teaching hospital and DGH.

Participants: Four radiologists (including 2 dedicated neuroradiologists) measuring positive MRI scans of 26 patients with an acoustic neuroma.

Main Outcome Measure: Intraradiologist and inter-radiologist repeatability coefficients in millimeters for the maximal tumor diameter in the anteroposterior (AP) axis, medial-longitudinal (ML) axis, and the square-root of the product of these two measurements. Repeatability coefficients give the 95% range within which the differences in repeated measurements lie.

Results: The intraradiologist repeatability for AP and ML measurements ranged from 1.51 to 6.03 mm and 2.01 to 3.83 mm, respectively. The repeatability of the square-root of the product ranged from 1.43 to 4.94 mm. The interradiologist repeatability was 6.48 mm and 7.46 mm for the AP and ML measurements, respectively, giving a repeatability of 3.65 mm for the square-root of the product.

Conclusion: The study indicates that, in routine clinical practice, differences in tumor size of the order of 2 mm cannot be reliably measured, even by the same radiologist. Thus, reported growth of acoustic tumors should be interpreted with caution, especially if this is the criterion for recommending treatment.