The Medially Migrating Intracanalicular Vestibular Schwannoma: Can We Count on MR Imaging to Give Us an Accurate Estimate of the Lateral Extension of a Tumor?
Background/Objective: The goals of early proactive treatment of small vestibular schwannomas (VS) are tumor control, maintenance of normal facial nerve function, and durable preservation of useful hearing. Favorable prognostic factors include good pretreatment hearing, small tumor size, and limited fundal extension with the tumor being more medially based within the internal auditory canal (IAC). In particular, limited fundal extension has been suggested to be important in preserving hearing for both microsurgery to limit manipulation of the very delicate distal cochlear nerve fibers and stereotactic radiosurgery (SRS) to minimize dose to the cochlea. Herein, the authors present a unique case where a 5mm intracanalicular VS seemingly migrated 5mm, from the fundus to the porus acousticus, over a period of 1 year. Clinical implications and possible explanations will be discussed.
Design: Case report
Setting: Tertiary referral center
Results: A 46-year-old female was diagnosed with a 2mm incidental left-sided VS in 2001, located at the fundus of the IAC. Serial imaging demonstrated 3mm of growth between 2005 and 2009 and the patient elected to undergo SRS (13Gy marginal dose, 26Gy maximum dose). Within a year following treatment, routine follow-up MRI including axial and coronal post-gadolinium T1-weighted and fluid inversion steady state acquisition (FIESTA) heavily T2-weighted images revealed that the tumor had migrated medially to the porus acousticus, 5mm from its original position. Subsequent imaging confirmed a persistent change and notably, there has been no detectable tumor growth following treatment and the patient continues to maintain symmetric hearing and normal facial nerve function 4 years later.
Conclusion: While rare, the current case demonstrates the potential for tumor migration within the IAC. Imaging artifact may potentially explain small variations between MRI studies; however after careful review of this index case the authors conclude that MRI artifact alone cannot explain the significant change seen on serial imaging. To the best of our knowledge, no study to date has specifically evaluated longitudinal change in fundal extension and tumor location of small intracanalicular VS. Further studies are required to establish the prevalence of this phenomenon and to determine whether it can also occur in previously untreated VS.