Non-invasive acoustic neuroma treatment via Gamma Knife stereotactic radiosurgery

Abstract

When a patient's audiometric evaluation or case history reveals unilateral or asymmetric sensorineural hearing loss, combined with other indicators such as absence of acoustic reflex, non-pulsatile tinnitus confined to the affected ear, loss of balance, dizziness, otalgia, facial pain or numbness (trigeminal neuralgia), clogged ear feeling, swallowing difficulty, vertigo, or mental confusion, the hearing healthcare professional should recommend further testing to rule out acoustic neuroma. Tests to establish a definitive diagnosis of acoustic neuroma include auditory brainstem response (ABR), electronystagmography, enhanced stereotactic computed tomography (CT), and magnetic resonance imaging (MRI).1 The latter technique is considered the most efficacious in determining the presence and size of a neuroma.2 The MRI can also rule out other retrocochlear causes of hearing loss such as viral neuronitis (damage to the sensory neurons of the vestibular ganglion), trauma, hemorrhage, or other neurologic disease.FigureACOUSTIC NEUROMA Acoustic neuromas (also known as vestibular schwannomas, peripheral fibroblastomas, or cerebellopontine angle tumors) are spontaneous, solitary, non-malignant, non-hereditary, slow-growing, primary brain tumors that diminish quality of life. In more than 80% of cases they arise from the superior vestibular division of the eighth cranial nerve (vestibulocochlear nerve).3 Acoustic neuroma is one of the most common types of brain tumors, representing approximately 6% of all intracranial tumors, 30% of brainstem tumors, and 85% of tumors in the region of the cerebellopontine angle at the base of the brain, where the vestibulocochlear nerve leaves the skull cavity and enters the bony structure of the inner ear.4 Tumors large enough to cause retrocochlear hearing loss and other neurologic symptoms occur in about one person in 100,000, or approximately 3000 cases a year.5 The hearing loss is due to direct compression of the cochlear nerve or secondary to interruption of the blood supply to the cochlea. An acoustic neuroma is created by an over-proliferation of Schwann cells covering the vestibular branch of the eighth cranial nerve as it passes through the internal auditory canal (IAC). In 95% of cases they occur unilaterally; they are rarely found in children and occur more frequently in women than in men in the range of 30 to 60 years of age. Some studies have suggested that long-term exposure to loud noise or radiation from cell phones may be causal factors in tumor development. As the acoustic neuroma grows, it can cause progressive hearing loss (usually the first symptom), taste dysfunction, visual deficits, facial weakness, pain or paralysis, and can eventually press on the trigeminal nerve, causing facial numbness or pain.6 Large neuromas can apply pressure on the lower cranial nerves causing swallowing and speech problems. If the acoustic neuroma becomes extremely large, its pressure on the brainstem can cause life-threatening hydrocephalus. Early diagnosis of acoustic neuroma is the key to preventing its serious consequences. Approximately 5% of acoustic neuromas are bilateral. They are most commonly found in patients with the inherited genetic disorder known as neurofibromatosis–type 2 (NF-2).7 Persons with NF-2 have a 50% chance of passing this disorder to their children. Whereas most spontaneous unilateral acoustic neuromas occur in patients between age 30 and 60 (mean age of onset is 50 years), bilateral acoustic neuromas can occur in children as young as 8 or 9 (mean age of onset is 31 years) and ultimately cause deafness in both ears. In addition to acoustic neuromas, individuals with NF-2 usually develop tumors of the nerves that control swallowing, speech, sight, and facial movement/sensation. In some patients, an auditory prosthesis known as an auditory brainstem implant can help restore hearing.8 Types of acoustic neuromas Based on the MRI scan, acoustic neuromas can be placed in one of three classifications: Entirely intracanalicular: The entire tumor is approximately 1 cm or less in size and is confined to the bony canal. This tumor poses no threat to the brain or cranial nerves other than VII and VIII, which are in the bony canal. Intracranial extension without brain stem distortion: The intracranial portion of the tumor is less than 1–2 cm. These cisternal tumors have demonstrated a propensity to grow and should be removed before they become life threatening Intracranial extension with brain stem distortion: The intracranial portion of the tumor is larger than 2 cm and is pressing on the brainstem. These compressive tumors should be addressed as soon as possible, taking into account the patient's age and condition of health. TREATMENT OPTIONS Once it has been determined that a patient has an acoustic neuroma and its size and location have been evaluated, the patient (and physician) can consider one of three treatment options: Medical management/observation: small to medium-sized tumor, less than 2 to 3.9 cm. Microsurgical excision: small to large tumor, less than 2 cm to greater than 4 cm Single-event Gamma Knife® stereotactic radiosurgery: small to medium tumor, less than 2 to 3.9 cm, or Fractionated Stereotactic Radiotherapy (FSR) on medium to large tumors. FSR allows the Gamma Knife to deliver smaller doses of radiation over a longer period, thereby minimizing the effect of radiation on the normal brain. Option 1: Medical management/ observation If a patient is found to have a very small acoustic neuroma, or the tumor is in the patient's only or better hearing ear, careful observation over time may be the preferred management modality. This is particularly appropriate for older patients whose tumors are static or slow-growing and are not expected to present serious symptoms during the patient's normal life expectancy.10 In this approach, the potential complications of surgery, and to a lesser extent those of stereotactic radiosurgery, can be avoided, although the potential loss of hearing is significant.11 Periodic MRI scans to track tumor growth, use of hearing aids where appropriate, and monitoring of the patient's neurologic status are recommended.12 Option 2: Microsurgical excision The first successful surgical removal of an acoustic neuroma, i.e., the patient survived, is reported to have occurred in 1895. During the early 20th century, patients with acoustic neuroma usually presented with very large tumors, causing significant brainstem compression. In these instances, the surgical mortality rate often exceeded 80%. In the second half of the century the introduction of the operating microscope and advanced medical imaging technologies (CT and MRI) allowed the development of microsurgical excision techniques that shifted the focus of surgery from prolonging life to preserving hearing and facial nerve function.13 There are three basic approaches to microsurgical removal of an acoustic neuroma: (1) translabyrinthine, (2) suboccipital/retrosigmoid, and (3) middle fossa. Otologists/neurotologists, ENTs, and neurosurgeons may use variations of the basic approaches, depending on their experience and skill level. The choice of surgical approach is generally based on the size of the tumor; the patient's age, medical condition, pre-operative hearing thresholds, and speech-discrimination abilities; and the surgeon's preference. The translabyrinthine craniotomy (TLC) approach, developed by William F. House, MD, of the House Ear Institute, is employed when there is no useful hearing in the affected ear or when attempts to preserve hearing ability would be impractical. In the TLC approach, the surgeon enters the skull behind the ear and removes the mastoid bone and the hearing and balance structures of the inner ear. This method allows for complete removal of tumors of any size and permits the surgeon to identify the facial nerve in the temporal bone prior to excising the tumor. No attempt is made to save hearing ability in the affected ear since the approach through the temporal bone destroys the cochlea.14-15 Because there is no need to retract the brain with this approach, TLC is considered the safest of the surgical alternatives. The suboccipital/retrosigmoid craniotomy (SRC) approach is used on small to very large acoustic neuromas where the tumor extends into the posterior fossa. Instead of drilling through the mastoid and cochlea to access the neuroma as in the TLC approach, the SRC entails opening the skull behind the mastoid bone and pushing the cerebellum toward the midline to visualize and remove the tumor. When the tumor extends into the interior auditory canal (IAC), a small piece of the bone of the IAC is removed so the surgeon can access the portion of the tumor in the canal. A plug of soft tissue is used to close the IAC. The size of the tumor plays a significant role in determining if hearing or facial nerve function will be preserved. The middle fossa craniotomy approach, also developed at the House Ear Clinic, is employed when the primary goal is preservation of hearing.16 It is generally limited to tumors less than 2 cm in size contained in the inner auditory canal. The surgeon enters the skull above the ear and removes the bone overlying the tumor. The inner ear is not entered. Not only is hearing usually preserved with this approach, but also facial nerve dysfunction seldom occurs. Advantages and disadvantages of microsurgical excision The acoustic neuroma craniotomy procedures have both advantages and disadvantages. The advantages include complete removal of the tumor with slight risk of recurrence.17 If hearing has been preserved in a middle fossa or suboccipital retrosigmoid approach, it usually remains stable over time; hearing improvement occurs infrequently. When facial nerve dysfunction occurs following surgery, the condition may be transitory and improve over time. If facial nerve integrity is compromised, the patient may experience delayed onset of facial palsy and paralysis. In this case, a hypo-glossal nerve to facial nerve graft may be undertaken. Patient age and tumor size are associated with poor facial nerve outcome when the tumor was removed via either a translabyrinthine or a retrosigmoid approach.18 Among the disadvantages of craniotomy procedures are the risks associated with any neurosurgical procedure: infection, hemorrhage, seizures, reaction to anesthesia, trigeminal nerve (cranial nerve V) dysfunction, meningitis, cerebral spinal fluid leakage, psychosocial problems, and, in rare instances, surgical re-intervention.19 Removal of the tumor does not necessarily change, improve, or diminish the patient's tinnitus. Post-operative headache rate can exceed 50%, especially with the SRC approach.20 The risk of hearing loss is substantial for all methods of microsurgical excision: translabyrinthine, 100%; suboccipital/retrosigmoid, 60%; and, middle fossa, 40%. Acoustic neuroma surgery usually requires several days or more of hospitalization, a period of in-home convalescence, and limitations on physical activity. Patients may experience post-surgical vertigo and balance problems requiring vestibular rehabilitation. Option 3: Gamma Knife stereotactic radiosurgery For patients found to have variably shaped acoustic neuroma ranging in diameter from several millimeters to more than 3 centimeters, the use of radiation, particularly Gamma Knife stereotactic radiosurgery, is often a preferred treatment method.21 The Gamma Knife is a non-invasive, precision tool that treats brain lesions with such intensely focused gamma radiation that healthy tissue less than 2 millimeters from the target is essentially unaffected.22 The goal of stereotactic radiosurgery is to arrest tumor growth while preserving neurologic function.23 Long-term success of the procedure is predicated on the failure of the tumor to grow, and in some instances, shrinkage of the tumor. The Gamma Knife was developed by Lars Leksell, a professor of neurosurgery at the Karolinska Institute in Stockholm, and the physicist and radiation biologist Borje Larsson of Uppsala University's Gustaf Werner Institute. The device was based on their research into combining radiation beams with stereotactic identification devices to isolate targets within the brain. Patient treatment via stereotactic radiosurgery began in 1967. The first Gamma Knife in the U.S. was installed at the University of Pittsburgh Medical Center in 1987. That device, and subsequent model upgrades, uses 201 cobalt 60 radiation emitters of approximately 30 curies each placed in a circular array in a heavily shielded steel collimator helmet that directs the gamma radiation to the target.24 To focus the beams of radiation precisely a light-weight stereotactic frame is secured to the patient's head.25 The patient is given local anesthesia or intravenous sedation and experiences only slight discomfort during frame application. Once the stereotactic frame is attached to the patient, a remote-controlled positioning system moves the patient into a shielded base and the 201 beams of gamma radiation are delivered. Radiation from the Gamma Knife causes tumor cell destruction by destroying the DNA intracellularly without the induction of an inflammatory response and by proliferative vasculopathy, in which the endothelial lining of the blood vessel is injured so that the smooth muscle cells proliferate and cause the vessel to close. Advantages and disadvantages of Gamma Knife The advantages of Gamma Knife® stereotactic radiosurgery include the following: It is non-invasive and may be used on residual tumors following surgery or portions of tumors that cannot be removed surgically without damaging brain tissue. The absence of an incision minimizes the risk of hemorrhage, infection, and adverse reaction to anesthesia. Facial and trigeminal nerve function can be preserved in most patients and in cases of small to medium-sized tumors, when pre-operative hearing is present, it may also be preserved. Gamma Knife treatments are usually performed in one session, which typically lasts from 12 to 60 minutes, depending on the type, size, and location of the target. Convalescent time is extremely short. There is no post-operative pain and post-operative rehabilitation is not required. No hair is shaved from the head. The cost of a Gamma Knife procedure is often 20% to 35% less than conventional neurosurgery and the procedure is usually performed on an out-patient basis. If hospitalization is required it is typically an overnight stay. Patients experience little discomfort and can immediately resume their previous activities. The disadvantages of Gamma Knife stereotactic radiosurgery are as follows: Risk of radiation-induced tumors developing in the future: To date, no documented cases secondary to stereotactic radiation have been reported. Radiation may not kill all the tumor cells. There is a slight risk that the tumor will grow in the future, so periodic MRI scans may be required to monitor tumor growth. If the radiation treatment is unsuccessful, the tumor is usually surgically removed. There is a very low to non-existent risk that the tumor will change biological characteristics and become malignant following radiographic treatment. CONCLUSION Gamma Knife stereotactic radiosurgery is a highly effective, integrated solution for the non-invasive treatment of acoustic neuroma. It allows patients the option of tumor control versus tumor removal, thereby avoiding the need for brain surgery. The precision of Gamma Knife surgery makes it possible to administer a high radiation dose to the lesion, while minimizing the risk of collateral damage to adjacent healthy brain tissue. Stereotactic radiosurgery has also proven to be an extremely effective treatment modality for other benign brain tumors, malignant tumors, metastatic tumors, focal epilepsy, trigeminal neuralgia, and arteriovenous malformations. More than 50,000 patients a year undergo Gamma Knife stereotactic radiosurgery at more than 1100 treatment centers worldwide.28 The House Ear Clinic, which treats more acoustic neuromas than any other center in the world, has historically recommended, with rare exception, the surgical removal of the acoustic neuroma. It began offering Gamma Knife surgery as a treatment option in July 2006.