Choosing the best treatment is a decision that must be made by both the patient and the physician after careful review of the size of the tumor, the location of it, the patient's age and physical health. An open discussion should be encouraged about symptoms such as headaches, hearing loss, dizziness and facial numbness and the experience and treatment philosophy of the physicians involved in the patient’s care. ANA recommends treatment from a medical team with substantial acoustic neuroma experience.
Acoustic neuromas may be discovered when the tumor is small and symptoms are minor or incidentally, when an MRI is performed to evaluate another condition. Since acoustic neuromas are benign tumors and produce symptoms by slowly applying pressure on surrounding nerves, careful observation over a period of time may be appropriate for some patients. When a small tumor is discovered in an older patient, observation to determine the growth rate of the tumor may be indicated if serious symptoms are not present. There is now good evidence from large observational studies that suggest many small tumors in older individuals do not grow, thus allowing tumors with no growth to be observed successfully. If it appears that the tumor will not need to be treated during the patient's normal life expectancy, treatment and its potential complications may be avoided. In this case, MRI scans are performed periodically and if the tumor does not show growth, observation is continued. If the tumor grows, treatment may become necessary.
Another example of a group of patients for whom observation may be indicated includes patients with a tumor in their only hearing or better hearing ear, particularly when the tumor is of a size that hearing preservation with treatment would be unlikely. In this group, MRI is used to follow the growth pattern. Treatment is recommended if either hearing is lost or the tumor size becomes life-threatening, thus allowing the patient to retain hearing for as long as possible.
A. Subtotal Removal:
Subtotal removal is indicated when anything further risks life or neurological function. In these cases the residual tumor should be followed for risk of growth (approximately 35%). If the residual tumor grows further, treatment will likely be required. Periodic MRI studies are important to follow the potential growth rate of any tumor.
Older patients with large tumors causing a threat to life may elect to have their surgeon sub-totally remove their tumor. Partial tumor removal has also been advocated in some patients who have large tumors in their only hearing ear. This surgical management will reduce the tumor in size, so that it may cause no threat to the patient's health during his or her life expectancy. This approach may reduce the probability of facial nerve dysfunction as a result of the surgery.
B. Near Total Tumor Removal:
This approach is used by experienced centers when small areas of the tumor are so adherent to the facial nerve that total removal would result in facial weakness. The piece left is generally less than 1% of the original and poses a risk of regrowth of approximately 3%. Periodic MRI studies are important to follow the potential growth rate of any tumor.
C. Total Tumor Removal:
Many tumors can be entirely removed by surgery. Microsurgical techniques and instruments, along with the operating microscope, have greatly reduced the surgical risks of total tumor removal. Preservation of the facial nerve to prevent permanent facial paralysis is the primary task for the experienced acoustic neuroma surgeon. Preservation of hearing is an important goal for patients who present with functional hearing. Both facial nerve function and hearing is electrically monitored during surgery. This is a valuable aid for the surgeon while the tumor is being removed.
Figure 4. Comparison of partial and total tumor removal. Every effort is made to remove the tumor without damaging the adjacent nerves or vital brainstem functions. Sometimes it may be best to leave small pieces of tumor capsule attached to critical structures rather than risk damage. If over time the tumor remnant grows, futher treatment is warranted. (Printed with permission of the Mayfield Clinic – www.mayfieldclinic.com)
D. Surgical Procedures:
The surgery is performed by a team of physicians including a neurotologist (ear and skull base surgeon) and a neurosurgeon.There are three main microsurgical approaches for the removal of an acoustic neuroma: translabyrinthine, retrosigmoid/sub-occipital and middle fossa. The approach used for each individual patient is based on several factors such as tumor size, location, skill and experience of the surgeon, and whether or not hearing preservation is a goal. The surgeon and the patient should thoroughly discuss the reasons for a selected appoach. Each of the surgical approaches has advantages and disadvantages, and excellent results have been achieved using all three of the techniques.
The translabyrinthine approach may be preferred by the surgical team when the patient has no useful hearing, or when an attempt to preserve hearing would be impractical. The incision for this approach is located behind the ear. It involves removing the mastoid bone (the bone behind the ear) and the bone of the inner ear, allowing excellent exposure of the internal auditory canal and tumor. This also results in permanent and complete hearing loss in that ear. This approach facilitates the identification of the facial nerve in the temporal bone prior to any removal of the tumor. The surgeon has the advantage of knowing the location of the facial nerve prior to tumor dissection and removal. Any size tumor can be removed with this approach. A second, small incision typically is made in the abdomen to harvest fat. This fat is added during surgery and is the substance that is used to prevent a cerebral spinal fluid (CSF) leak after the tumor is removed. This approach affords the least likelihood of long-term postoperative headaches.
The incision for this approach is located in a slightly different location. This approach creates an opening in the skull behind the mastoid part of the ear, near the back of the head on the side of the tumor. The surgeon exposes the tumor from its posterior (back) surface, thereby getting a very good view of the tumor in relation to the brainstem. When removing large tumors through this approach, the facial nerve can be exposed by early opening of the internal auditory canal. Any size tumor can be removed with this approach. One of the main advantages of the retrosigmoid approach is the possibility of preserving hearing. For small tumors, a disadvantage lies in the risk of long-term postoperative headache.
This approach utilizes a different incision location and is utilized primarily for the purpose of hearing preservation in patients with small tumors, typically confined to the internal auditory canal. A small window of bone is removed above the ear canal to allow exposure of the tumor from the upper surface of the internal auditory canal, preserving the inner ear structures.
Another treatment option for an acoustic neuroma is radiation. Stereotactic radiation can be delivered as single-fraction stereotactic radiosurgery (SRS) or as multi-session fractionated stereotactic radiotherapy (FSR). Both techniques are performed in the outpatient setting, not requiring general anesthesia or a hospital stay. The purpose of these techniques is to arrest the growth of the tumor causing the tumor to die, which is called necrosis.
In single-dose treatments, many hundreds of small beams of radiation are aimed at the tumor. This results in a high dose of radiation to the tumor and very little to any surrounding brain structures. Many patients have been treated this way with high success rates. Facial weakness or numbness, in the hands of experienced radiation physicians, occurs in only a small percent of cases. Hearing can be preserved in some cases, with a slightly greater opportunity with FSR.
The multi-dose treatment, FSR, delivers smaller doses of radiation over a period of time, requiring the patient to return to the treatment location on a daily basis, from 3 to 30 times, generally over several weeks. Each visit lasts a few minutes and most patients are free to go about their daily business before and after each treatment session.
The treatment team should consist of a neurosurgeon, and/or a neurotologist (ear and skull base surgeon), a radiation oncologist and a physicist. Follow-up after SRS and FSR typically involves an MRI scan and an audiogram at six months, one year, then yearly for several years, then every second or third year indefinitely to make sure the tumor does not start to grow again.
Patients should understand that all types of radiation therapy for acoustic neuromas may result in “tumor control” in which the tumor cells die and necrosis occurs. Tumor control means that the tumor growth may slow or stop and, in some cases, the tumor may shrink in size. In almost no cases have acoustic neuroma tumors been completely eliminated by radiation treatments. In other words, radiation does not remove the tumor like microsurgery can. Furthermore, radiated patients require lifetime follow-up with MRI scans. Tumors under 2.5 – 3.0 cm, without significant involvement of the brainstem, are more favorable for radiation treatment. Side effects can occur when the brainstem is irradiated and in cases of large tumors, radiation is contraindicated. Patients should understand there have been rare reports of malignant degeneration (a benign tumor becoming malignant) after radiotherapy.
In some cases, the tumor does not die and continues to grow. In those instances, another treatment is necessary – either microsurgery or another dose of radiation. Retreatment must be done as always, in the hands of experienced physicians.
Several types of machines deliver focused radiation treatment suitable for treating acoustic neuromas, such as Gamma Knife® and linear accelerator (LINAC), such as CyberKnife®, Novalis® and Trilogy®. The underlying premise is to treat the tumor with a high dose of radiation while sparing the nerves and brain tissues. Much of the long-term data comes from the Gamma Knife literature since this was one of the earliest techniques used to radiate acoustic neuromas on a large scale.
The Gamma Knife uses 195-201 fixed Cobalt-60 radiation sources that are “collimated” to intersect at the site of the tumor and is a single-dose treatment. In this way, each individual beam of radiation has very little effect, but where they all intersect produces a maximum effect on the tumor. Very similar results can be obtained using a linear accelerator (LINAC) as the radiation source, such as with the Novalis or CyberKnife with multi-dose treatment.
Studies are beginning to appear for the other modalities. All of the techniques use computers to create three-dimensional models of the tumor and surrounding neural structures. Radiation physicists then create dosimetry maps showing the level of radiation to be received by the tumor and the normal tissues. Surgeons, radiation therapists and physicists then modify the dosimetry to maximize tumor doses and minimize radiation toxicity to surrounding normal tissues. The head is stabilized with a metal frame pinned to the head (Gamma Knife) or a fitted mask shield (CyberKnife, linear accelerator, fractionated XRT). Treatments generally last 30-60 minutes. Just like for surgery, the experience of the team in treating acoustic neuromas with all modalities (surgery and radiation) can affect outcomes.
There are a multitude of studies supporting short-term (<5 yrs.) and long-term (over 10 yrs.) tumor control with radiation. Unfortunately, as is the case with microsurgical studies, most have inconsistent follow-up to draw definitive conclusions.