An acoustic neuroma is a skull based nerve sheath tumour that constitutes about 6% of all primary intracranial tumours. They are usually benign and a slow growing tumour which arise primarily from the vestibular portion of the VIII cranial nerve and lie in the cerebellopontine angle - a wedge shaped area bounded by the petrous bone, the pons and the cerebellum.
Diagnosis
It is a scary moment when your doctor tells you that you have a "brain tumour" called acoustic neuroma (vestibular schwannoma). You think you are the only one with this disease and you will soon die or at least become a physical wreck. You fear the only treatments available are either ineffective or very dangerous.
Fortunately, this is all wrong. You are not alone: between 1000 and 1500 new acoustic neuroma patients are diagnosed in the United Kingdom alone every year. And effective, low-risk treatment is available.
Acoustic neuroma was described for the first time in Holland in 1777. A comprehensive clinical description was presented in 1830. Although the first successful removal of an acoustic neuroma was performed in 1894, the mortality following surgery at the turn of the century was at least 80%.
Excision of the tumour was the standard treatment and the only available option for many years. The results improved gradually but were still far from satisfactory in the early 1960s, when microsurgical techniques were gradually introduced into this field in the USA.
In 1951, the Swedish neurosurgeon Lars Leksell presented the idea of letting a large number of converging beams of ionizing radiation crossfire targets in the brain. He coined the term "radio surgery" to describe this concept, since the way radiation was used differed greatly from conventional radiotherapy. He suggested radio surgery for the treatment of deep-seated brain tumours.
The first device for routine clinical use based on this idea was the prototype Gamma Knife constructed in 1967-68. Dr. Leksell treated the first acoustic neuroma with the technique in June 1969 at Karolinska Hospital in Stockholm, Sweden. Since then, more than 10,000 acoustic neuroma patients have been treated with this technique worldwide.
The Gamma Knife
This is an 18-ton machine with 201 permanently mounted cobalt-60 sources arranged spherically around the patient's head. These sources emit gamma radiation, which is similar to diagnostic X-ray (not laser as sometimes assumed) but with higher energy. These beams are precisely shaped through two consecutive sets of tungsten channels (collimators). They all focus on one point. Here, the radiation is very powerful. However, each individual beam on its way through the skull is weak and will not cause any detectable biological effects. The gamma radiation destroys molecules in the tumour cells so they can no longer reproduce and eventually will die.
The Gamma Knife is precise down to half a millimetre or even less (about 1/50 of an inch). Thus, a high dose of radiation can be delivered to targets with little harm to important sensitive structures just millimetres away or even adjacent to the surface. Stereotactic radio surgery is performed by a team composed of neurosurgeons, radiation oncologists, medical physicists and a nursing staff. Specialists in neuroimaging join the team when required.
Who can be treated?
In general, all acoustic tumours with an intracranial diameter of up to approximately 3 cm (1 1/4") qualify for Gamma Knife radio surgery. Over the years, larger tumours occasionally have been treated successfully with this technique. However, there is a greater risk that these larger tumours, even before any treatment, interfere with the circulation of the cerebrospinal fluid (CSF), causing hydrocephalus (an excessive accumulation of CSF). In this case, a shunt may be required to divert the CSF. Temporary swelling of a large tumour, induced by the Gamma Knife treatment, may occasionally result in hydrocephalus not present earlier. Surgical removals of a large tumour will frequently, though not always, eliminate the need for a shunt.
Patients with large acoustic neuromas - especially older patients - may still prefer the combination of Gamma Knife radio surgery and a shunt operation, a considerably less demanding procedure than microsurgical removal.
In fact, there are few reasons why Gamma Knife radio surgery should not be considered first instead of microsurgery for the vast majority of acoustic neuroma patients, including young and otherwise healthy ones.
What happens to the tumour?
Very few acoustic tumours threaten the patient's general health initially. The rationale for treating the tumour is to avoid the risk that the tumour might cause serious health problems or even death down the road if left alone to grow. By treating the tumour when it is still small, the risk of complications from treatment is generally smaller. Even so, a microsurgical procedure usually poses a greater immediate risk to the patient's health in terms of morbidity than does the tumour itself.
Gamma Knife radio surgery is different. The short-term and long-term risks are very low. The goal of the treatment is to kill or inactivate the tumour cells so they no longer duplicate. Since acoustic neuroma is a very benign type of tumour, it need not be completely destroyed. Instead, the aim is to stop further growth. An acoustic tumour that does not grow will not jeopardize the patient's health in the future.
In a benign tumour such as acoustic neuroma, with a very slow cell turnover, it will take some time for the radiation to affect the cells in a way that can be detected clinically or by imaging. Therefore, radio surgery has a less immediate effect than microsurgery.
Shrinkage actually is found in the vast majority of tumours when they are followed long enough. One year after the Gamma Knife treatment, shrinkage is confirmed in about one-third of the tumours. After four years, two-thirds of the tumours are smaller, and by 10 years, more than 90% have shrunk.
Signs of lack of response to radio surgery, in general, appear within one to three years of treatment. At least in my experience, failure is extremely unlikely to occur when five years or more have elapsed.
This statement may not apply for acoustic neuromas associated with neurofibromatosis 2 (NF2) in which case recurrence may occur later following Gamma Knife treatment as well as microsurgery.
Specialist Surgeons have found that Gamma Knife treatment can be repeated without increased risks if the acoustic neuroma did not respond as expected (unchanged size/shrinkage) to the first treatment. Microsurgery can also be selected, depending on the patient's preference.
Acoustic neuromas sometimes increase in size temporarily as a reaction to the Gamma Knife treatment. This is actually a favourable sign indicating a brisk response. Such swelling usually is most obvious between 6 and 18 months after the procedure. It should not be confused with increase due to lack of response in which case the tumour size will not return to the baseline but continue to increase. A definite assessment should be made two years after the treatment: was the swelling merely temporary or did the tumour fail to respond to the treatment? In any case, resection should not be considered during this two-year wait.
Cranial nerve function
At experienced Gamma Knife centres, the incidence of temporary facial and trigeminal nerve dysfunction among acoustic neuroma patients is as low as less than 2-3%. Preservation of useful hearing currently is achieved in 55-75% in different series with the better results usually in smaller tumours. Hearing tends to remain stable when the first one to two years have elapsed after treatment.
The tinnitus (spontaneous noise) so frequently associated with the hearing loss in acoustic neuroma patients is usually not affected, for better or worse, by Gamma Knife treatment initially. Over time, some patients say they have experienced some improvement. It is hard to say whether this is a real reduction of the intensity of the noise or an adaptation to a steady noise level. Even though most acoustic neuromas arise from the balance nerve (and not from the adjacent hearing nerve), hearing loss in the affected ear is a much more frequent presenting symptom than balance disturbance. When asked about it, however, most acoustic tumour patients admit to some feeling of unsteadiness or episodes of dizziness. Sometimes these symptoms may increase temporarily after the Gamma Knife treatment, indicating a transient reaction in the balance nerve to the radio surgery.
Radio surgery or microsurgery?
Traditionally, tumour treatment is defined as successful when the tumour has been completely removed. This apparently does not apply for Gamma Knife radiosurgery for which other standards have to be accepted when the results are evaluated.
Stereotactic radio surgery has a number of evident advantages over microsurgery including no mortality, no risk of intracranial bleeding or infection, no post-surgical complications, short or no inpatient time, and almost no recovery period. In addition, Gamma Knife treatment almost eliminates the risk of permanent facial weakness and the need for further surgery to restore proper facial functioning including eyelid closing, excessive tearing or dry eye.
These features are in themselves usually so attractive to the patient that they may decide on radio surgery based on the very low risk of side effects. Of course, such a decision should focus primarily on the best way of eliminating the impact of the tumour and secondly on the risk of disturbances in adjacent structures, such as surrounding cranial nerves, induced by the treatment.
It has been reported that acoustic tumours that were first treated with radio surgery without response were difficult to remove with microsurgery because surrounding nerves and other structures were more adherent to the tumour's surface. The radio surgical treatment would stimulate the formation of scar tissue outside the tumour. The surgeons reporting these problems base them on the experience from a small number of tumours resected. The experiences reported are far from consistent. Because of lack of experience we do not conclusively know to what extent and how often this is a real problem. We should not be able to expand our experience very much since a second Gamma Knife treatment is almost always possible to perform in those few patients in whom the response to the first treatment was inadequate.
Another objection against radio surgery sometimes mentioned is that the treatment would induce the formation of new tumours or change the character of the treated tumour to become more aggressive. These are known risks with radiation in general. Based on known clinical data and theoretical considerations that risk is equal to or less than 1 per 1000 persons treated. This is an extremely low risk level, which in my mind is not a reason to withhold Gamma Knife radio surgery from young individuals who prefer radio surgery to microsurgery.
Why after gamma knife surgery did my taste buds change, many things are so bitter now!!!?? It was done in February of this year