Learning Objectives:

• • Pre-operative CT scan for cholesteatoma overestimates mastoid involvement.

• • CT-MRI Fusion imaging permits definition of cholesteatoma matrix from sequestered fluid, potentially avoiding an unnecessary mastoidectomy.

Introduction: Pre-operative assessment of cholesteatoma traditionally involves non-contrast temporal bone CT imaging. This can demonstrate opacification of the mastoid cavity but does not define the boundary between cholesteatoma and sequestered fluid. Non-EPI DWI MRI sequences identify the presence of cholesteatoma but do not allow precise anatomical localisation. Fusion of the two images permits estimation of the cholesteatoma/fluid interface. Minimally invasive ear surgery techniques benefit from accurate evaluation of mastoid penetration.

Methods: We retrospectively assessed 6 years of CT-MRI Fusion imaging for cholesteatoma. We included any primary pre-operative cholesteatoma which had both plain CT and CT-MRI Fusion imaging. Two reviewers assessed the scans independently, in a randomized blinded fashion to determine cholesteatoma extension into the mastoid for each imaging modality.

Results: 58 cases met inclusion criteria. Plain CT imaging demonstrated mastoid involvement in 42 of these cases, thus demonstrating 28% had cholesteatoma which did not reach the mastoid cavity, predicting feasibility for total endoscopic removal. CT-MRI Fusion imaging increased this to 60%. Information from CT-MRI Fusion changed pre-operative evaluation of whether open mastoidectomy was necessary in 45% of cases. Overall, the extent of cholesteatoma involvement in the mastoid was overestimated by plain CT in 47% of cases.

Conclusions: Cholesteatoma extension beyond the lateral semicircular canal into the mastoid is more accurately predicted by CT-MRI fusion imaging than plain CT. This tool has high clinical utility, especially in preoperative planning, potentially avoiding unnecessary mastoidectomy with a totally endoscopic (or permeatal) approach.