ACOUSTIC NEUROMA-THE BASIC FACTS Treatment

There are 3 treatment options available for AN

1) Observation

2) Microsurgical removal (partial or total)

3) Stereotactic radiation therapy (radiosurgery)

Observation

AN are occasionally discovered incidentally while evaluating another problem or when the tumor is very small with subtle symptoms. Since AN are benign tumors and produce symptoms due to pressure on surrounding structures, careful observation over a period of time may be appropriate for some patients. For instance, a small tumor diagnosed in an elderly patient may only require observation to study the growth rate of the tumor if acute symptoms are not present. If it appears that the tumor will not need to be treated during the patients normal life expectancy, treatment and its potential risks and complications maybe avoided. In these patients, MRI is performed periodically to monitor growth of the tumor. If there is no growth, observation is continued. On the other hand, if the tumor shows increase in size, treatment may become necessary. Another group of patients for whom observation is preferred is in patients who have a tumor in their only or better hearing ear, particularly if it is a size where hearing preservation is unlikely. In such cases, periodic MRI is done to monitor growth and surgery is considered only if the hearing is lost or the tumor size becomes life threatening.

Microsurgical removal

At the present time, the only treatment that can cure the patient is removal of the tumor by surgery. Within the last 2 decades, microsurgical techniques have been pioneered and refined. Use of the operating microscope, finely scaled surgical instruments, alternate cutting & tumor reducing tools, better anesthesia, have reduced the death rate extremely. In addition, the experience gained by trained surgeons in the delicate removal process of the tumor and minimizing contact with nerves and tissue adjacent to the tumor, have improved the overall results.

Three main surgical approaches are used depending upon the location, tumor size and hearing level of the patient. They are- middle fossa (MF), sub-occipital (SO), and the trans-labyrinthine (TL) approach. Surgery for AN's is done under general anesthesia using an operating microscope. Postoperatively, one to several days may be spent in the intensive care with careful monitoring. Problems that may develop in the immediate postoperative period including headache, dizziness, imbalance, vomiting and decreased mental alertness due to the development of a blood clot causing obstruction to the flow of cerebrospinal fluid (CSF).

Other early complications may include cerebrospinal fluid leak and meningitis, an infection controlled with antibiotics which requires a longer hospitalization. Some patients and their surgeons prefer incomplete removal of an AN in order to reduce the risk of complications, realizing that further surgery maybe needed in the future. Occasionally in cases with large tumors, disturbances in the vital brain centers during surgery require ending the surgery prior to complete tumor removal. In these cases, the tumor which was left behind is followed with MRI scans and if tumor growth is demonstrated, further surgery maybe necessary to remove the growing tumor. On the other hand, if the tumor shows no growth, observation is continued. Partial tumor removal maybe required in patients with Neurofibromatosis-2 (NF 2) who may have a tumor in the only hearing ear. Unfortunately, partial removal may result in substantial hearing loss in these patients and this risk must be considered.

Small tumor

Since the hearing is still preserved in such tumors, we prefer to do the middle fossa approach, incision for which is in front of the ear (figure). A small square piece of bone from the side of the skull is then removed (blue shaded area in the figure). The tumor is removed completely in most cases. On rare occasions, partial removal is possible. We attempt to preserve the hearing in all cases while removing the tumor. In about a third of the patients, the tumor involves the hearing nerve or the artery supplying the inner ear and in such cases, total loss of hearing occurs in the operated ear.

Medium tumor

The operation for medium sized tumors is performed by the sub-occipital and/or the trans-labyrinthine approach. The incision for these approaches is behind the ear, overlying the mastoid bone which is the bony projection felt behind the ear (figures). The mastoid and the inner ear structures are removed to expose the tumor, and remove it completely. The opening created in the mastoid bone is closed with fat taken from the abdomen. The translab-subocc approach sacrifices the hearing and balance mechanism since we go through the inner ear. Consequently, the ear is made permanently deaf. In such cases, the balance mechanism of the opposite ear compensates for the non-functioning operated ear and provides stabilization for the patient within few weeks to months.

Large tumor

Surgery for large tumors require extensive removal of bone to properly expose the tumor and control the large blood vessels which make access to the tumor difficult. For this reason, special studies of the arteries (arteriograms) may be required in addition to the other investigations, in order to diagnose and establish the size of the acoustic tumor. The operation for large tumors is performed by the TL-SO approach as described for medium tumors. The figure to the right shows the area of the skull approached via the TL and the SO approaches. In these patients, total removal is attempted unless changes in vital signs occur. If there are changes in blood pressure, pulse rate, or respiratory rate, the surgery must be terminated even if the tumor has not been totally removed. The opening in the mastoid is closed with abdominal fat. For large tumors, it is often necessary to monitor the patients general status by inserting a small tube (arterial line) into an artery in the arm or leg. In these cases, occasionally a blood clot may form in the artery following surgery. In case this complication occurs, further surgery may be required to remove the blood clot.

Stereotactic Radiation Therapy (Radiosurgery):

This is a technique based on the principle that a single relatively high dose of radiation delivered precisely to a small area will arrest or kill the tumor while minimizing injury to the surrounding nerves & brain tissue. The source of radiation is from either radioactive cobalt (called gamma ray) or a linear accelerator (LINAC). The treatment team consists of a neurosurgeon or neurotologist, radiophysicist and a radiation oncologist working together to develop a treatment plan based on the size & shape of the tumor. Radiation, even at relatively high doses such as those used in radiosurgery, does not kill or injure cells immediately. Some tumor cells die in weeks while others die more gradually over 6-18 months after radiation. This treatment usually arrests growth of the tumor and some tumors shrink, but they rarely disappear.

Follow-up of these patients is important because approximately 20% of tumors continue to grow after radiosurgery or at some time in the future. The chance of recurrence increases over time. A tumor which has been irradiated and grows may be more difficult to remove than an un-radiated tumor. Symptoms such as dizziness & disturbances in balance are improved earlier after microsurgical tumor removal than after radiosurgery. This is because effects of radiosurgery may require up to 18 months. Residual dizziness & imbalance are less after microsurgical treatment. The side effects of radiosurgery may be headache, dizziness, nausea, facial numbness, or rarely, cranial nerve paralysis. In the long term radiosurgery may be more expensive than microsurgery because it requires more follow-up MRI's over the years and there is a potential for additional treatment in cases of continued growth or later re-growth.

Radiosurgery may be considered in selected patients in whom the risk of surgery is excessive because of advanced age or pre-existing health problems, patients having small to moderate sized tumors or patients with tumors on both sides, or in the only hearing ear. Radiosurgery can be done for patients taking blood thinners that make microsurgery unsafe. Microsurgical treatment remains the treatment of choice for AN's. An experienced microsurgeon, operating on tumors in the radiosurgery size range, can produce results comparable to radiosurgery and the long term tumor cure or control rate appears more certain with microsurgery. Radiosurgery has all the risks of surgery and most of these risks are lower. A few issues are important to recall. Dizziness may not be treated successfully with radiosurgery. Second, the complications of facial numbness are more likely to occur with radiosurgery but facial weakness is less likely. Just like with microsurgery, as tumor size increases, complications are more likely to occur. Tumors over 3 cm. are very difficult to treat and many centers avoid radiosurgery in large tumors.

One issue that is always a concern with radiation is the potential for malignant changes. A non-cancerous tumor can be made malignant. This does occur but relatively rarely. This appears to be an unusual or very late complication. Rates are debated among doctors but it underlines the seriousness of this treatment. Although there is no incision there is still a remote and late chance of death.

Choosing the "best" method of treatment

There is no single "best" method for treating acoustic neuroma. For patients with small tumors in whom the preservation of hearing is not a major issue, watchful waiting may be appropriate, particularly in the elderly. If long term hearing preservation is very important to a patient with a very small tumor, microsurgical removal via the middle fossa approach may be best. For patients who desire "cure" of their tumor (complete absence with a very low chance of recurrence), microsurgical excision has the best chance of producing that result. For those who can accept the long term presence of a tumor that doesn't grow (or grows very slowly) radiosurgery can produce that result in 70 to 80 percent of patients.

The "best" choice for an individual requires careful thought, good understanding of the tumor and options for treatment and an honest and frank discussion with the surgical team. The issues of hearing preservation, tumor size and "cure" versus "control" must be carefully assessed. The "right" choice for one person may be the "wrong" choice for another.

Department of Otolaryngology, University of Minnesota

University of Minnesota Physicians - Clinic and Appointment Information