Acoustic Neuroma
Introduction
The term acoustic neuroma was coined years ago when these tumors were thought to arise from the acoustic or cochlear nerve. The cochlear nerve is part of the eighth cranial nerve called the vestibulocochlear nerve. More recent research demonstrated that the tumor actually arises from the vestibular portion of the nerve and therefore the accurate terminology is vestibular schwannoma. However, acoustic neuroma has become ingrained in the nomenclature of this tumor and remains widely used. These are benign and usually slowly growing tumors.
Acoustic neuromas usually are identified when individuals notice partial or complete hearing loss. This may develop gradually or suddenly. Others notice problems with ringing noise in the ear (tinnitus), balance troubles, dizziness, or facial numbness or weakness. The tumors may also be found coincidentally, during CT and MRI for investigation of unrelated complaints such as headache. Once neurological deficits develop they are often not reversible. The tumors also become more difficult and dangerous to treat as they enlarge. Therefore every patient with an acoustic neuroma, regardless of size or symptoms, should be presented with all the treatment options.
In the early 1900's, the first surgical resections were performed when large tumors began to exert life-threatening pressure on the adjacent brainstem. The mortality rate from these early surgeries was high, and the patient was certain to suffer deafness and facial paralysis. The results of surgery have improved tremendously, thanks to advanced surgical techniques and equipment, studies of microanatomy, and intraoperative cranial nerve and brainstem monitoring. In general, there is consensus that surgical resection should still be considered for tumors over 3cm that are causing pressure effects on surrounding brain structures.
Gamma Knife Surgery
GKS has become and increasingly popular treatment option for small and medium sized vestibular schwannomas / acoustic neuromas since its introduction in 1968. More surgeons and patients are electing this option over microsurgical resection or observation without treatment. GKS relies on the ability to precisely focus a series of gamma ray "shots" that cover the tumor with an effective dose of radiation. At the same time, the surrounding brain is exposed to a minimal amount of radiation. This form of radiosurgery is said to be extremely conformal and highly selective (see Defining Radiosurgery). The goal of GKS for VS /AN is to permanently stop the growth of the tumor, without causing any additional neurological problems.
In the 1980's, GKS treatments were based on poor resolution CT scans and simple dose planning algorithms. The prescribed dose of radiation was also much higher than used today. The results were good, with success in preventing further tumor growth for most patients. However, the occurrence of radiation related side effects or complications were not better than those achieved with surgical resection. Present day MRI has permitted clear delineation of the tumor target and surrounding structures. Modern sophisticated planning software has allowed creation of exquisitely conformal and selective dose plans. Equally important has been the results of careful outcome analyses that have demonstrated the effectiveness in tumor control and a dramatic reduction of complications associated with GKS. As more is understood about the radiobiology of single high doses of radiation. The occurrence of postoperative hearing loss, facial weakness and numbness and other complications is now rare, and significantly lower than expected with open surgical resection.
Another option is to observe the tumor for signs of growth. For all patients with vestibular schwannoma / acoustic neuroma, a thorough discussion of all the treatment options should be offered, including the potential advantages and disadvantages of each. Most acoustic neuromas / vestibular schwannomas are expected to grow without treatment and cause hearing loss and other damage. Therefore, GKS may be considered for all patients, regardless of age, symptoms or tumor size.
*Also see "Acoustic Neuroma/Cranial Base Tumors"
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