MRI follow-up of vestibular schwannoma after surgery

MRI follow-up of vestibular schwannoma after surgery

MRI surveillance protocol for vestibular schwannoma in South Africa

South Africa, like the rest of the world, lacks a standardised surveillance protocol for monitoring sporadic (non-NF2) vestibular schwannoma after surgery (1,2,10). Patients need to be informed about the frequency of follow-up MRI appointments to avoid confusion, misunderstanding, and anxiety and to ensure the timely detection of any tumour growth that could impact their quality of life. The timing of MRI scans may also affect the placement of cochlear implants in some patients, as the magnets in the implants can interfere with tumour evaluation (3).  For example, a patient showing linear enhancement on MRI at six months post-surgery may not require a cochlear implant for another seven years. Patients may benefit from early implantation in such cases but should be aware that the magnet may need to be temporarily removed for a more accurate MRI after seven years.

Patient safety and cost factors should be considered when determining the frequency of MRI scans, as the use of gadolinium contrast agents in MRI scans carries potential long-term health risks. Given the unique economic and healthcare challenges in South Africa, it may be necessary to tailor surveillance protocols to the local context, possibly by selecting from the five internationally published protocols. After careful consideration, we recommend adopting a specific MRI surveillance protocol for vestibular schwannoma in South Africa.

We have decided on the protocol suggested by Carlson and coworkers at the Mayo Clinic School of Medicine. Rochester, Minnesota, US, is the most appropriate for sporadic vestibular schwannoma in South Africa (4)

The site of the vestibular schwannoma

An intrameatal (intracannalicular) tumour is confined to the internal auditory canal, while an extrameatal (extracannalicular) tumour extends beyond the internal auditory meatus or is located purely outside of it (5).

The six-month post-operative MRI for vestibular schwannoma

The MRI taken six months after surgery serves as a baseline for future measurements and is rarely followed by additional surgery or radiotherapy. It is assumed that any post-operative changes have stabilized by this time, and significant growth has not occurred.

The use of gadolinium is crucial for accurate assessment. Residual contrast enhancement is common after surgery. Carlson and colleagues observed that over 98% of patients still showed enhancement in the surgical area on their first MRI about three months post-surgery, with only 3.5% showing complete resolution on subsequent scans.

Contrast enhancement can persist for years after surgery, and enhancement at the six-month mark does not necessarily indicate a failed surgery or tumour recurrence. Patients need to understand this information.

The measurement of the residual vestibular schwannoma

There are two methods of measuring residual vestibular schwannomas. Planimetric measurements, which involve measuring the length, width, and height of the tumour in millimetres, can provide a basic understanding of the size of the tumour. This information can be expressed as a set of dimensions, such as 15mm x 15mm x 12mm, to give an idea of the tumour’s shape and size.

On the other hand, volumetric measurements provide a more comprehensive view of the tumour by measuring the tumour in cubic centimeters (cm3) or millilitres.(6). This method involves using specialised software to measure the tumour at different levels, allowing for a more accurate assessment of the tumour’s size and shape (3). Volumetric measurements are particularly useful for tumours with irregular shapes, such as those that may develop after partial removal of the tumour.

While volumetric measurements may be more time-consuming compared to planometric measurements, they offer a more precise assessment of the tumour’s growth and changes over time. This can be especially important in monitoring residual vestibular schwannomas for potential recurrence or progression.

The definition of growth

Growth in a vestibular schwannoma is defined as the increase in the length, width or height by three or more mm per year (>2mm/y) (7).

In volumetric terms, growth is defined as more than 20% per year.

Volumetric measurement
Linear (planimetric) measurement

The difference between gross, near-total and subtotal removal of vestibular schwannoma.

For the surveillance protocol, the surgeon must record the extent of tumour removal during the surgery (8,9). There are three levels of completeness of tumour removal, namely:

  • Gross-total removal (GTR)- the tumour is completely removed with no visual remnants left
  • Near-total removal (NTR)- the tumour is removed but with a residual piece of no more than 5mm x 5mm x 2 mm intentionally left to preserve neural or vascular function
  • Sub-total removal (STR)- any situation where less than NTR is performed

The ideal MRI surveillance protocol for vestibular schwannoma in the future

One advantage of surgical removal is that tissue is obtained on which histological confirmation of the tumour can be done. The vestibular tumour tissue can be used to look for specific tumour markers not readily available in the blood. A tumour marker is a substance found in the blood, urine, or body tissues that can indicate the presence of a specific tumour. By analyzing the vestibular schwannoma post-surgery for specific tumour markers, researchers and medical professionals can obtain information regarding the tumour’s possible growth pattern and potential (10,11).

Some common tumour markers associated with vestibular schwannoma include Ki-67, cyclooxygenase 2 (COX2), vascular endothelial growth factor (VEGF) and macrophage colony-stimulating factor (M-CSF)(12).

If specific tumour markers are present at higher levels, it may indicate rapid growth or a higher likelihood of recurrence. This information can guide surgeons on when to perform follow-up MRI scans and influence current surveillance protocols for vestibular schwannoma patients.

GTR-Gross-Total Removal
NTR- Near-Total Removal
STR-Sub-Total Removal
*This protocol is not applicable when you have any suggestive symptoms, a cystic nodule and any nodule that is ≥15mm or ≥0,4ml/0,4cm3

A few point on MRI requests

  • Follow the surveillance protocol meticulously.
  • The first base-line MRI is performed six months after surgery.
  • Your surgeon should inform you after the surgery in which group, GTR, NTR or STR, you fall.
  • Inform the rooms of Dr Hofmeyr if there is a problem with obtaining an MRI.
  • If any new or altered symptoms arise in the follow-up period, contact your surgeon or the general practitioner.
  • If possible, utilise the same imaging facility.
  • Ask the facility to transfer the images to DR Hofmeyr’s facility or the facility of the referring surgeon and supply a disc with the images to the patient.
  • Gadolinium must be administered unless otherwise specified.
  • The radiologist should report possible growth by examining the previous MRI images.
  • Measurements in mm should be given for length, width, and height as well as the volume of the tumour in ml/cm3.
  • The radiologist should measure the intrameatal (intracannalicular) and extrameatal (extracannalicular) components separately and document them as such.
  • According to international standards, growth is defined as an increase in size of > 2 mm or more in any of the three measurements  or  20% of the volume of  the extrameatal component.
  • Comparative measurements for previous MRIs should be documented, and a comment on how the latest MRI compares with the first MRI.
  • The radiologist should comment on any cystic changes in the tumour and compare it with the previous MRI.

*Carlson ML, Van Abel KM, Driscoll CL, Neff BA, Beatty CW, Lane JI, Castner ML, Lohse CM, Link MJ. Magnetic resonance imaging surveillance following vestibular schwannoma resection. Laryngoscope. 2012 Feb;122(2):378-88.


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