How to radiologically identify a spontaneous regression of sporadic vestibular schwannoma?

Abstract

Background: The natural history of sporadic vestibular schwannoma is unpredictable, with tumors growing, non-growing and even showing spontaneous regression in some rare cases.

Objective: This retrospective study aims to describe the radiologic signs characterizing and identifying the shrinking vestibular schwannoma.

Methods: Involution was considered to have occurred if tumor size had decreased by 2 mm or more on its largest diameter. All magnetic resonance imaging scans were reviewed for tumor size, internal auditory meatus size, and tumor characteristics. Volumetric measurements were performed on the first and last scan. Audiometric data were collected at the first and last visit.

Results: Fourteen patients with a confirmed spontaneous regression were included, with a mean follow-up of 5 ± 2.6 years. The mean shrinkage rate was 0.9 ± 0.59 mm/year on 2D measurements, and 0.2 ± 0.17 cm3/year on volumetric measurements, with a relative shrinkage of 40 ± 16.9%. Two remarkable radiologic features were observed: First, a festooned aspect, defined by multiple curves in the tumor outline, noticed in 12 cases (86%); second, the appearance of cerebrospinal fluid filling the internal auditory meatus, associated with an enlargement of the internal auditory meatus compared to the contralateral side, and observed in 10 out of 13 cases with internal auditory meatus invasion (77%). Those two aspects were associated in 64% of cases.

Conclusion: These two newly reported radiologic features could help neurosurgeons, oto-neurosurgeons and neuroradiologists to identify a spontaneous vestibular schwannoma involution at first visit. This could allow any treatment to be postponed, monitoring to be more widely spaced, and patients to be reassured.

Fig 1. MRI measurements.

A and B) The largest anteroposterior diameter (arrowhead) and mediolateral diameter (large arrows) were measured on axial section (A). The largest superoinferior diameter (thin arrows) was measured on coronal section. Anteroposterior and superoinferior diameters concerned cerebellopontine angle (CPA) extent only. Mediolateral diameter included both CPA and internal auditory meatus (IAM) extent. C) Volumetric measurements were performed as shown by contouring the tumor in every axial section on HRT2-weighted images. Volume was then calculated automatically using OsiriX software.

Fig 2. Evolution of each tumor on the largest diameter.

Fig 3. Volume/Diameter correlation.

Fig 4. MRI aspect of shrinking tumors (HRT2-weighted imaging).

A) First and last MRI of patient 5 that shows the festooned appearance (arrowheads). B) First and last MRI of patient 7 that shows the appearance of CSF infilling the IAM and the asymmetrical size of IAM (large arrows). Measurements are in mm and did not change between the first and last MRI. C) First and last MRI of patient 13: arrows indicate the festooned appearance and large arrows indicate CSF infilling the IAM.

Fig 5. Diagram showing the radiological aspect of spontaneously shrinking vestibular schwannoma.

The festooned aspect is easily noticeable in the medial part of the tumor. In an axial view, the CSF infilling the IAM appears as a triangle placed at its anterior part at the front of the intrameatal part of the tumor. CSF = cerebrospinal fluid; IAM = internal auditory meatus.