An acoustic neuroma is a skull based nerve sheath tumor that constitutes about 6% of all primary intracranial tumors. The are usually benign and slow growing tumor 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.
It is a scary moment when your doctor tells you that you have a "brain tumor" 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 2500 and 3000 new acoustic neuroma patients are diagnosed in the United States 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 tumor 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 "radiosurgery" to describe this concept, since the way radiation was used differed greatly from conventional radiotherapy. He suggested radiosurgery for the treatment of deep-seated brain tumors.
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 tumor cells so they can no longer reproduce and eventually will die.
The Gamma Knife is precise down to half a millimeter 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 millimeters away or even adjacent to the surface. Stereotactic radiosurgery 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 tumors with an intracranial diameter of up to approximately 3 cm (1 1/4") qualify for Gamma Knife radiosurgery. Over the years, larger tumors occasionally have been treated successfully with this technique. However, there is a greater risk that these larger tumors, 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 tumor, induced by the Gamma Knife treatment, may occasionally result in hydrocephalus not present earlier. Surgical removal of a large tumor 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 radiosurgery and a shunt operation, a considerably less demanding procedure than microsurgical removal.
In fact, there are few reasons why Gamma Knife radiosurgery 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 tumor?
Very few acoustic tumors threaten the patient's general health initially. The rationale for treating the tumor is to avoid the risk that the tumor might cause serious health problems or even death down the road if left alone to grow. By treating the tumor 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 tumor itself.
Gamma Knife radiosurgery is different. The short-term and long-term risks are very low. The goal of the treatment is to kill or inactivate the tumor cells so they no longer duplicate. Since acoustic neuroma is a very benign type of tumor, it need not be completely destroyed. Instead, the aim is to stop further growth. An acoustic tumor that does not grow will not jeopardize the patient's health in the future.
In a benign tumor 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, radiosurgery has a less immediate effect than microsurgery.
Over a period of 26 years, I have personally treated almost 850 acoustic neuroma patients with Gamma Knife radiosurgery. I would like to share some of my experiences with you. Growth control - shrinkage or no growth - is achieved in at least 95% of the tumors.
Shrinkage actually is found in the vast majority of tumors when they are followed long enough. One year after the Gamma Knife treatment, shrinkage is confirmed in about one-third of the tumors. After four years, two-thirds of the tumors are smaller, and by 10 years, more than 90% have shrunk.
Signs of lack of response to radiosurgery, 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.
I 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 favorable 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 tumor 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 tumor 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 centers, 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 tumors. 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 tumor 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 radiosurgery.
Radiosurgery or microsurgery?
Traditionally, tumor treatment is defined as successful when the tumor 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 radiosurgery 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 radiosurgery 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 tumor 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 tumors that were first treated with radiosurgery without response were difficult to remove with microsurgery because surrounding nerves and other structures were more adherent to the tumor's surface. The radiosurgical treatment would stimulate the formation of scar tissue outside the tumor. The surgeons reporting these problems base them on the experience from a small number of tumors 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 radiosurgery sometimes mentioned is that the treatment would induce the formation of new tumors or change the character of the treated tumor 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 radiosurgery from young individuals who prefer radiosurgery to microsurgery.
One doctor's story of having an acoustic neuroma
In August 1991, Dr. Thomas F. Morgan noticed subtle signs of hearing loss and some high-pitched noise (tinnitus) in his right ear, along with a slight sense of imbalance. An experienced neurologist at Rhode Island Hospital, he knew he needed an MRI. It confirmed his initial self-diagnosis: he had an acoustic neuroma. So far, everything was relatively easy. Making a decision regarding the most appropriate treatment turned out to be more complicated.
Surgical removal of the tumor was the preferred standard treatment. But Dr. Morgan started doing research on his own, talked to a number of colleagues and participated in the Acoustic Neuroma National Institutes of Health Consensus Development Conference in Bethesda, Maryland, USA, in December 1991. Most acoustic neuroma surgeons attending that meeting favored microsurgery.
At a dinner meeting hosted by the Acoustic Neuroma Association, many already treated patients gave Dr. Morgan their perspective: "Doctors, please listen! Cure or control but do not harm." Dr. Morgan realized that there was an alternative: stereotactic radiosurgery with the Gamma Knife. It was an almost outpatient procedure and it could be performed without upsetting his busy daily routine. Additionally, he knew open surgery for acoustic neuroma could cause negative side effects, not to mention the risk of general anesthesia. After extensive research, he was convinced radiosurgery was the treatment of choice for acoustic tumors.
Much to his surprise, the Gamma Knife technique would soon be available at his own hospital in Providence, Rhode Island. He underwent Gamma Knife radiosurgery at Rhode Island Hospital in May 1992. Now, eight years later, the tumor has decreased in size and he still has hearing in that ear. His face has normal appearance, without even the slightest facial weakness. He feels great and is very active.
"My perspective now," he says, "is to help others in the decision-making process and to champion Gamma Knife radiosurgery for acoustic neuromas as the standard treatment with microsurgery as an adjunctive or alternative option."