BACKGROUND: The Koos classification of vestibular schwannomas is designed to 
stratify tumors based on extrameatal extension and compression of the brainstem. 
While this classification system is widely reported in the literature, to date 
no study has assessed its reliability.
OBJECTIVE: To assess the intra- and inter-rater reliability of the Koos 
classification system.
METHODS: After institutional review board approval was obtained, a 
cross-sectional group of the Magnetic Resonance imagings of 40 patients with 
vestibular schwannomas varying in size comprised the study sample. Four raters 
were selected to assign a Koos grade to 50 total scans. Inter- and intrarater 
reliability were calculated and reported using Fleiss' kappa, Kendall's W, and 
Intraclass correlation coefficient (ICC).
RESULTS: Inter-rater reliability was found to be substantial when measured using 
Fleiss' kappa (.71), extremely strong using Kendall's W (.92), and excellent as 
calculated by ICC (.88).Intrarater reliability was perfect for 3 out of 4 raters 
as assessed using weighted kappa, Kendall's W and ICC, with the intrarater 
agreement for the fourth rater measured as extremely high.
CONCLUSION: We have demonstrated that the Koos classification system for 
vestibular schwannoma is a reliable method for tumor classification. This study 
lends further support to the results of current literature using Koos grading 
system. Further studies are required to evaluate its validity and utility in 
counseling patients with regard to outcomes.