Stereotactic radiosurgery and stereotactic radiotherapy are minimally invasive ways of treating VS and meningiomas with the aim of avoiding open surgical procedures. Stereotaxy means to touch or place in space and describes the use of a frame or other localising system that is fixed to the patient’s head and used as a guidance system. The patient, with the frame in place, is then scanned and this scan is used to generate a map with a grid system that precisely locates the tumour. This map, which is of course specific for each individual patient, is used with the frame to target the radiation treatment, which is concentrated on the tumour, so sparing the surrounding tissues. The radiation can be delivered as a single high dose treatment, which is called stereotactic radiosurgery:  alternatively, the treatment can be fractionated, that is given as a course of multiple lower dose treatments – called stereotactic radiotherapy. The techniques differ in a number of ways.

• A stereotactic radiosurgery treatment is completed in a single day and relies on the frame and precise targeting of the tumour to avoid damage to surrounding structures.
• A stereotactic radiotherapy treatment is delivered over several days or even weeks, hence re-locatable frames or masks which can be taken on and off are used. Whilst this will increase positional errors, the fractionation, that is the use of multiple low dose treatments, may allow for recovery of the tissues between the individual doses being given.

Importantly, single dose radiosurgery and fractionated radiotherapy may have different biological effects on the tumour being treated. This can be likened to comparing a single hard hit with multiple light blows – the total energy imparted may be the same, but the response of the tissue may be very different. Hence 12Gy given as a single radio-surgical treatment will have a much more marked effect than six fractions of 2Gy given as a course of radiotherapy (Gray, abbreviated to Gy, is the unit used to prescribe and measure the absorbed dose of radiation).

A separated but inter-related issue is the choice of machine used to deliver the radiation. This can be either gamma knife or a linear accelerator (also called a LINAC).

• The gamma knife is a specialised machine that can only be used to treat conditions within the head. Gamma knife treatments are almost invariably performed as single fraction treatments, and so the terms “gamma knife” and “stereotactic radiosurgery” have become to some extent synonymous. This technique relies on a ridged frame for stereotactic localisation.
• In the UK most LINACs are used for general radiotherapy, treating different parts of the body, but they can be adapted and set up to deliver either single fraction or multi fractionated stereotactic radiotherapy treatments, using either frame or less accurate mask based localisation systems.
• A new technology for delivering stereotactic treatment is now available in the UK, known as Cyberknife. This is a light weight LINAC capable of being positioned flexibly by a high precision robotic arm.

In practice, reflecting the clinical experience and issues related to total radiation exposure, most radiation treatments both for sporadic VS tumours in general and for NF2 tumours have been framed based gamma knife radiosurgery delivered as a single fraction

Gamma knife radiosurgery has become a mainstay of active management for VS, this being an increasing alternative to surgical resection. Reflecting this, the Sheffield Unit has been carrying out up to 300 VS treatments per year and to date in total over 4300 of these tumours have been treated.  As a result, a radiosurgery treatment for VS is a very routine standard procedure. Most of these treatments are for unilateral sporadic non-inherited (non NF2) tumours.  Whilst the treatment is the same, the results and clinical issues in treating NF2 may be very different.

In NF2, VS presents typically at a younger age and tend to grow more rapidly than sporadic tumours. As the disease is often bilateral, deafness or hearing preservation is much more of an issue. Clinical experience is that treatment of NF2 VS either with surgery or radiosurgery is less effective than for sporadic (non-NF2) tumours. For these reasons we feel that it is essential to describe material that is specific for NF2 and not to generalise from the experience with sporadic tumours as this may be falsely optimistic. The results presented here are based on our cumulative experience from 1993 to 2023 treating 339 NF2 VS in 231 patients.

Patients are generally referred from specialist services throughout the UK with details of how the individual’s condition has progressed, serial MRI scans and audiogram information. In deciding whether an individual is potentially suitable for radiosurgery, tumour size is the main consideration as the larger a tumour is the less likely it is to respond to and be controlled by radiosurgery.

If a patient is thought to be potentially suitable for radiosurgery, they are offered a clinic appointment to come and discuss this. All new patients are seen by a Consultant Neurosurgeon, who has a specialist interest in NF2 radiosurgery. It is strongly recommended that all NF2 patients attend clinic before treatment. There is also an opportunity in clinic to meet with and talk to the therapy radiographers.

The treatment is carried out has a day case. On the day of treatment, a stereotactic frame is fitted. This is the reference guide for the whole of the treatment and must be rigidly fixed to the patient. This procedure is carried out with local anaesthetic, takes around 10 minutes, and is very well tolerated. With the frame in place, an MRI scan is carried out to pinpoint the tumour and to map and plan the treatment. The treatment itself which is likened by many patients to “having another scan” is then performed. The frame is used to position the patient very precisely on the bed, which then moves into a tunnel. It is less enclosed than an MRI scanner, and there is no inherent noise (although patients can listen to music, talk over an intercom etc). At the end of the procedure, the frame is removed. Patients, who have travelled a considerable distance, are generally kept overnight, before being discharged the following day. Local patients can be discharged on the day of treatment.

Follow-up is generally performed locally by the patient’s referring specialist. It is recommended that all patients are followed up with MRI scanning but this, of course, is part of the normal NF2 follow-up.

After reviewing our NF2 practice, the most recent results suggest that eight years after the radiosurgery:

• The overall tumour control rate was 80%
• In 50% the tumour were seen radiologically to have shrunk.
• In 30% the tumours were stationary.
• In 20% there will have been some variable concern about tumour growth, but that they will have been managed conservatively.
• Less than 10% of patients underwent surgery or further salvage therapy for the treated tumour.
• 61% of patient’s hearing remained serviceable.
• 39% became deaf.
• 1 patient experienced a temporary facial palsy post treatment, and 2 patients had a facial spasm occur. All 3 cases resolved.

Whilst these results are not as good as those achieved treating unilateral sporadic tumours, the evidence is that radiosurgery does slow or stop the growth of the majority of NF2 tumours.  The hope is to avoid the need for surgery in most patients. Given the morbidity and mortality associated with surgery, we believe that radiosurgery is an attractive treatment option for selected NF2 patients with growing tumours

The second most common tumour requiring treatment in the setting of NF2 is meningioma disease. In Sheffield the gamma knife to date has treated around 3100 sporadic meningiomas, again making this a routine treatment. In NF2 the meningioma disease is more likely to be multifocal. Reviewing our practice we have treated 170 NF2 meningiomas in 63 patients between 1987 to 2023.

To some extent radiosurgery has been seen as competition for and taking away work from skull base surgical practices. Although technically becoming more accepted, individual surgeons vary greatly in how receptive they are towards radiosurgery and how they perceive its risks and benefits. This of course influences how the radiosurgery treatment option is portrayed to patients.

Questions which we are commonly asked include:

1. What is the risk of a radiation treatment causing malignancy?

We believe that the risk of this in the general population is very low indeed. To date there are only a few single patient case reports describing altered tumour behaviour after radiosurgery. World-wide over 1 million patients have been treated with the Gamma Knife, over 100,00 for vestibular schwannomas.

In a systematic UK population-based study we have not found any increased risk despite accumulating most recently 60,000 patient-years of data.

Of course, the majority of these patients will not have NF2. Potentially the risk in NF2 may be different. The reason for raising this is the notion that a NF2 patient may inherit one faulty gene but require a mutation in the other healthy gene to develop a tumour. Radiation causes gene damage and is therefore potentially a concern. To address this safety issue, we are actively monitoring these events. To date we have treated over 300 NF2 patients and two have had malignant tumours. One tumour was growing rapidly (increasing in volume 15 fold in the 2 years before radiosurgery) and was found  subsequently to be malignant. The rapid growth, however, preceded the radiosurgery and was not caused by it. The second patient developed a glioma 3 years after Radiosurgery but it is impossible to say if the Radiosurgery was the cause of the 2^nd tumour or whether this would have developed anyway. Overall the risks would appear to be low, and in practice have to be weighed against the morbidity and mortality associated with other treatment options.

2. What is the risk of radiosurgery causing scarring, and making subsequent surgery more difficult?

This is even harder to assess and quantify. The published literature is extremely anecdotal and relates to vestibular schwannomas in general, rather than to NF2. Individual surgeons have both reported surgery being harder and easier after radiosurgery (radiosurgery may decrease the blood flow through a tumour making an operation easier but may cause scaring making it harder).

We are aware that some surgeons claim that there is always damage to the facial nerve after surgery if there has been previous radiosurgery. In evaluating these claims and making decisions about treatment, it is important to remember that it is the minority of patients who undergo radiosurgery who subsequently undergo surgery. Furthermore, in a review of British NF2 patients, most of whom had not undergone radiosurgery, 70% of patients had some degree of facial nerve dysfunction after surgery.

3. Can radiosurgery be repeated?

This is often asked, particularly by patients who have had unfavourable surgical experiences. The answer is yes: repeat treatments in other conditions is safe and there is considerable experience of this. In practice it is rare to retreat VS.  We would stress that this practice of repeating a treatment is very much the exception and that each case must be judged on its individual merits.

Management decisions in NF2 are difficult because there is no ideal treatment option for all patients. Factors in making a decision include tumour size and how fast they are growing; the hearing in both ears, and other neurological deficits; and the individual’s past experience either of their own care or other family members of the different management options.

The final decision rests with the patient. In making this decision we would however encourage the individual to have as many informed opinions as he/she finds helpful. We would hope that this information would clarify some of the issues surrounding radiation treatments

The information summarised in this fact sheet has drawn on a number of sources including;

www.gammaknife.org.uk

Rowe JG, Radatz M, Walton L & Kemeny AA

Stereotactic Radiosurgery for type II neurofibromatosis acoustic neuromas: patient selection and tumour size.

Stereotact Funct Neurosurg. 2002;79(2):107-116

Rowe JG. Radatz MW. Walton L. Soanes T. Rodgers J. Kemeny AA.

Clinical experience with gamma knife stereotactic radiosurgery in the management of vestibular schwannomas secondary to type 2 neurofibromatosis.

Journal of Neurology, Neurosurgery & Psychiatry. 74(9):1288-93, 2003 Sep.

Rowe J, Radatz M, Walton L , Kemeny A

Gamma Knife Radiosurgery for type 2 Neurofibromatosis Acoustic Neuroma Radiosurgery Volume 5.  Karger, Ed. D Kondziolka, 2004

Dean Winter, Specialist NF2 Therapeutic Radiographer.

dean.winter@nhs.net

National Centre for Stereotactic Radiosurgery

Royal Hallamshire Hospital

Glossop Road

Sheffield

S10 2JF