Treatment Options for Acoustic Neuromas

Radiosurgery for Acoustic Neuromas

Radiation therapy is a noninvasive procedure in which beams of radiation are targeted to the tumor to stop its growth while minimizing the risk of harming surrounding healthy tissue. This therapy differs from acoustic neuroma microsurgery in that there is no intention of curing the patient by removing or excising the tumor. Rather, the goal of radiation therapy is to control the growth of the tumor by destroying it with beams of high energy radiation which causes necrosis (tissue death). This type of therapy is usually reserved for small tumors, 3 cm. or less.

There are two modes of delivery of radiation to the patient with an acoustic neuroma:

  • Stereotactic radiosurgery (Gamma knife)
  • Fractionated stereotactic radiosurgery (FSR)

Stereotactic Radiosurgery

Stereotactic radiosurgery, also referred to as Gamma knife radiosurgery is a very precise type of radiation therapy that is used to treat a variety of tumors located in the brain. Although the term "radiosurgery" implies that this treatment is a type of surgical procedure, it's name actually derives from the "surgical" (pinpoint) precision that doctors can achieve with this technique by focusing x-rays directly to the area of a tumor. This highly sophisticated mode of radiation therapy involves several specialists, including a neurosurgeon and a radiation oncologist.

Stereotactic radiosurgery is typically performed under the following circumstances:

  • Patients who are elderly (older than 65 years)
  • Patients who cannot tolerate standard acoustic neuroma surgery
  • Patients who previously underwent partial tumor resection
  • Patients who refuse to undergo surgery for various reasons
  • Deafness in the unaffected ear
  • Presence of bilateral acoustic neuromas (both sides)

Gamma knife radiosurgery is a single-dose radiation treatment. It enables doctors to pinpoint abnormal tissue and deliver radiation with pinpoint accuracy. Radiation beams are merged from different locations outside of the head and merged into one beam at the target point. Accuracy is within a fraction of a millimeter. The source of radiation is typically either radioactive cobalt (gamma radiation) or a linear accelerator (LINAC). This type of treatment has been reported to have a high success rate of tumor control and a low incidence of facial weakness or numbness. Hearing preservation is equivalent to that following acoustic neuroma surgery. Stereotactic radiosurgery is contraindicated in cases where an acoustic neuroma is compressing the brainstem or cerebellum,.

Preparation for radiosurgery typically consists of not eating or drinking after midnight before the scheduled treatment. Patients are typically awake but may be sedated if they are anxious. The hair is usually pulled back but not shaven. At the time of the procedure, a local anesthetic is administered after which a special frame, similar to a halo, is attached to the head with 4 screws. These hold the head perfectly immobile so that the radiation beams converge and are delivered to a precise location. The frame is lightweight and once it is in place, imaging scans determine the exact location of the tumor. The patient then rests while calculations are performed regarding the dose of the radiation, the number of fields from which beams will be delivered, and the precise point where the beams will converge. During the actual treatment, the head is placed into a helmet which attaches to the head frame. The patient does not feel the radiation nor is there any noise during the treatment.

While some tumor cells die quickly within the first few weeks after stereotactic radiosurgery, others may die more gradually over a period of 6 to 18 months or longer. Radiosurgery effectively stops the growth of the tumor or shrinks the size of the tumor; however, the tumor still remains present. Periodic follow-up MRIs will be required after treatment for the remainder of the patient's life to make sure the tumor does not resume growth.

Doses of radiation in the 1980s, when the technique of stereotactic radiosurgery was still in its infancy, were about 35 Gray or Gy (a unit used to measure radiation dose) but since the 1990s the doses have been reduced to between 14 and 17 Gy to minimize side effects while maintaining or improving maximum effectiveness. Continuing research focuses on reducing the radiation doses even further. A study was recently published from the University of Pittsburgh reporting success rates of 13 Gy for the treatment of acoustic neuromas.

Side effects of stereotactic radiosurgery are usually mild and may include:

  • Nausea
  • Neck stiffness
  • Pain at the site of the screws used to attach the frame to the head
  • Radiation injury - damage to tissue caused by exposure to radiation. This may develop 3-12 months after stereotactic radiosurgery and results in swelling of brain tissue. Resolution of the swelling is usually without treatment though some patients may require steroid treatment for persistent swelling.
  • Some patients experience facial numbness and weakness or deafness on the side of treatment following radiosurgery which is usually temporary and typically occurs 6 to 18 months after treatment.

The cost of radiosurgery is 50% to 75% less than the cost of traditional acoustic neuroma microsurgery. It also involves a much shorter recovery time and some patients return to work the day after treatment.

Efficacy of Stereotactic Radiosurgery

Efficacy rates of stereotactic radiosurgery continue to improve due to:

  • Improvement of MRI technology that allows tumors to be visualized earlier and in greater detail
  • MRI allows for highly sophisticated treatment plans to deliver radiation to the tumor with pinpoint accuracy
  • Improved technology has allowed for a reduction of the radiation dose needed for effective treatment

A study published in 2005 by investigators at the University of Pittsburgh involving 829 patients with acoustic neuromas who had been treated with stereotactic radiosurgery reported the following:

  • Tumor control rates of 97% at 10 years
  • Hearing preservation in 50-77% of patients
  • 70% of tumors decreased in size at 10 years
  • Less than 1% risk of facial neuropathy
  • Less than 3% risk of trigeminal neuropathy
  • All tumors which recurred were identified within the first three years of treatment
  • Side effects occurred within the first 2 years following treatment

To read more about this study, please follow this link:

http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=Abstract&listuids=15662809&queryhl=6&itool=pubmed_docsum

In 2005, the results of a study conducted in Japan were published regarding the outcome of 301 patients with acoustic neuroma who underwent Gamma knife radiosurgery. Patients were followed for an average of 7 years. Findings from this study included:

  • 5 year tumor control rate of 93%
  • 7% tumor recurrence rate
  • Hearing preservation rate of 68%
  • 2% of the patients developed facial numbness

To read more about this study, please click on this link: http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=Abstract&listuids=16094154&queryhl=24&itool=pubmed_docsum

Complications Following Stereotactic Radiosurgery

The rate of complications following stereotactic radiosurgery continues to decline with advances in technology, dose flexibility, and precision of radiation delivery. Some complications commonly associated with stereotactic radiosurgery are facial nerve neuropathy and trigeminal nerve neuropathy. The rates of these complications currently are estimated to occur in up to 3% of patients.

Vestibular dysfunction is estimated to occur in up to 50% of patients following stereotactic radiosurgery. Another potential problem of great significance, though its incidence is not well known yet, is the development of a secondary malignancy due to radiation exposure. It is, therefore, very important for patients to be carefully followed by their health care provider following radiosurgery in order to identify any significant delayed complications that may develop even several years after radiosurgery.

Newer studies with low-dose stereotactic radiosurgery have shown improved results for facial nerve complications, hearing preservation rates of 65%, and no reports of trigeminal nerve complications. Long-term follow-up of patients, however, is necessary to confirm these observations.

There are indications that facial or trigeminal nerve injury following radiosurgery may be increased by factors such as:

  • Age of the patient - patients less than 65 years of age are more likely to experience facial and/or trigeminal nerve injury following stereotactic radiosurgery
  • High doses of radiation - estimates are that at doses of 16 Gy or higher, 29% of patients experienced facial palsy and 44% of patients experienced symptoms of trigeminal nerve dysfunction. With doses less than 16 Gy, 9.5 % of patients experienced facial palsy while 15.5% of patients experienced trigeminal nerve dysfunction.
  • Tumor size of 2 cm or larger
  • Prior surgical resection

The risk for developing a secondary malignancy is thought to be approximately 1% at 10 years after stereotactic radiosurgery. These radiation-induced tumors tend to be more aggressive than non-radiation induced tumors and carry a very poor prognosis. This is of particular importance for young people considering stereotactic radiosurgery.

In summary, the advantages of stereotactic radiosurgery include:

  • It is less expensive than conventional acoustic neuroma surgery
  • Hospital stay following stereotactic radiosurgery is shorter than following conventional surgery
  • Entire treatment is completed in one session under local anesthesia with same day discharge
  • There are fewer complications and problems immediately following stereotactic radiosurgery resulting in a better quality of life
  • Similar rates of hearing preservation and facial nerve injury (with lower dose radiation) as conventional acoustic neuroma surgery
  • Patients can usually go back to work the day after treatment

Disadvantages of stereotactic radiosurgery include:

  • Costs are higher in the long term since follow-up with periodic imaging studies is critical in post-treatment care and 5% of patients must undergo salvage surgery for tumors that begin to grow again after treatment has been completed
  • The tumor is not removed
  • Higher tumor recurrence rate than after conventional surgery
  • Higher incidence of trigeminal nerve injury with higher dose stereotactic radiosurgery than following conventional surgery
  • The incidence of post-radiation tumors (secondary malignancy) is unknown in the long term but it could be a potentially life threatening problem. This is a consideration which is especially important for young people who are considering stereotactic radiosurgery vs. conventional surgery
  • Higher incidence of post-treatment disabling vestibular dysfunction compared to conventional acoustic neuroma surgery
  • Hydrocephalus (an accumulation of cerebrospinal fluid in the skull) may occur in up to 5% of patients who undergo stereotactic radiosurgery
  • Surgery to remove residual tumor following stereotactic radiosurgery is very difficult
  • Long-term ramifications of using low-dose radiation is not yet known

While some authors feel that stereotactic radiosurgery will eventually become the first- line treatment for acoustic neuromas, others believe that the higher rate of tumor recurrence and increased chance for secondary malignances related to radiation will limit its role to that of a second-line treatment modality.

Fractionated Stereotactic Radiosurgery

Fractionated stereotactic radiosurgery (FSR) involves short, multi-dose treatments in which smaller doses of radiation are delivered but the tissue included in the area which is targeted is larger than for standard stereotactic radiosurgery. Treatment continues daily for several weeks. Fractionated stereotactic radiosurgery was developed in an attempt to minimize cranial nerve complications following the delivery of radiation. It may be performed on an outpatient basis at special centers or in the radiology department of a hospital. Currently, there is limited data regarding the long term effects of fractionated stereotactic radiation and whether it has any advantage for treatment outcome over single-dose stereotactic radiosurgery.

Researchers from Heidelberg, Germany reported results for 106 patients with acoustic neuromas who underwent fractionated stereotactic radiosurgery. With a median follow up period of 4 years, they reported the following findings:

  • Treatment was well tolerated by all patients
  • Local tumor control at 5 years was 93%
  • Hearing preservation was achieved in 94% of patients at 5 years and was 98% overall.
  • Rates of successful hearing preservation were significantly reduced in the presence of neurofibromatosis
  • Rate of facial nerve complications was 2.3%
  • Rate of trigeminal nerve complications was 3.4%

To read more about this study, click on the following link: http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=Abstract&listuids=16111574&queryhl=4&itool=pubmed_DocSum

Microsurgery vs. Stereotactic Surgery

The 1991 NIH Consensus Statement recommended that first-line treatment for acoustic neuromas should be microsurgery for most patients and that radiotherapy should be considered only for a subset of patients who are unable or unwilling to undergo surgery. The statement also noted the delayed occurrence of complications following stereotactic radiosurgery and the limited available long-term data regarding other aspects of this treatment.

Information regarding the comparative advantages for each treatment modality may be summarized as follows:

  • Though the goals of each treatment are different regarding tumor control, many surgical studies report close to 100% tumor removal and the latest results of tumor control hover at approximately 97%. There is no tumor removal with stereotactic radiosurgery, only tumor reduction.

  • Tumor control rate is superior following total resection than following stereotactic radiosurgery

  • There is no long term data regarding efficacy of lower radiation doses that may be used during stereotactic radiosurgery

  • According to available data, it appears that for tumors less than 4 cm. there is no significant difference between Gamma knife radiosurgery and microsurgery regarding facial nerve preservation and hearing preservation. However, complications involving the trigeminal nerve are significantly lower for patients undergoing microsurgery.

  • Mortality rates for surgical therapy are very low and for radiosurgery it is zero. However, there is a higher risk of postradiation secondary tumor development with stereotactic radiosurgery which usually carries a very poor prognosis.

  • The most frequent complication of surgery for acoustic neuroma is CSF leakage which can be repaired.

  • Approximately 5% of patients undergoing stereotactic radiosurgery for acoustic neuromas require subsequent surgical excision of the tumor. This is very difficult surgery for which hearing preservation is not an option and the level of facial function following this surgery is low on the House-Brackmann scale.

The term "salvage surgery" refers to surgery used to treat a patient who had not responded to other treatments. Salvage surgery after failed stereotactic radiosurgery is technically very difficult. The American Neurotology Society has noted that neurosurgeons who performed salvage surgery approximately 35 months after radiotherapy experienced the following complications:

  • Very friable tissue (easily broken into small fragments)
  • Anatomical landmarks were difficult to identify
  • Tumors tended to adhere to the facial nerve resulting in the majority of patients experiencing postoperative facial palsy.

For more information regarding the opinion of American Neurotology Society, please click on the following link: http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=pubmed&cmd=Retrieve&dopt=Abstract&listuids=10821551&queryhl=1&itool=pubmed_DocSum

Some surgeons recommend that salvage surgery should be delayed at least 6-18 months after the stereotactic radiosurgery which is the period of time for highest risk of post-radiation injury to cranial nerves.