What is an Acoustic Neuroma?
An acoustic neuroma -- also called vestibular schwannoma -- is a benign, non-cancerous tumor that develops on the vestibulocochlear (8th cranial) nerve that passes from the inner ear to the brain. The tumor originates (tumorigenesis) when Schwann cells malfunction. Normally, Schwann cells function beneficially to form an insulating myelin sheath on the nerve to protect and speed along balance and sound information to the brain. At times, however, things go awry. A by-chance (“out of the blue”) mutation in the NF2 gene on chromosome 22 results in a loss in the tumor suppressor cell protein named merlin, and the Schwann cells multiply uncontrolled to form a schwannoma on the vestibular portion of the nerve. These vestibular schwannomas (VS) are called “sporadic” tumors (i.e., non-hereditary) and the great majority (ca.95%) are unilateral, in one ear only. They do not spread to other parts of the body, but although non-malignant they can do harm or even become life-threatening when they enlarge and begin to press on other cranial nerves or vital structures such as the brain stem.
For the most part, these unilateral sporadic VSs are readily treated successfully by modern medical techniques. Having bilateral VSs is a more worrisome condition. Bilateralism is considered to be the hallmark and main diagnostic criterion for neurofibromatosis type 2 (NF2), a genetic disorder that is heritable, progressive, very difficult to manage, and has a 1 in 2 chance of being passed on to each offspring. NF2 is very rare and tends to develop in patients in their teens or early adulthood, whereas sporadic VSs are diagnosed mostly in patients between the ages of 40-60 years.
The Incidence of Acoustic Neuroma
Both varieties of VS are very rare, accounting for only about 8% of all primary brain tumors. A study by the Cleveland Clinic (V.R.Kshettry et al, Journal of Neurooncology, vol 124, Sept 2015) calculated that about 3,300 VS are diagnosed each year in the United States. The overall incidence of VS was 1.09 per 100,000 of population. The incidence increased to a peak of 2.93 per 100,000 in the 65-74 year old age group. Overall, there was no difference in incidence by gender. Incidence was higher in Asian Pacific Islanders and lower in African Americans and Hispanics. It seems safe to estimate that for New Jersey, with a population over 9 million, there are at least 90 new cases of VS each year.
Tumor Sizes (diameter)
Tumor size can be described as small (less than 1.5 cm), medium (1.5 to 2.5 cm), large (2.5 to 4.0 cm) and giant (more than 4.0 cm). One inch = 2.54 cm) Average tumor size at diagnosis has decreased steadily in the U.S. during the last 30 years due mainly to the introduction of MRI imaging. ANA patient surveys from 1998 to 2014 show that the percentage of small tumors 1.5 cm or less rose significantly from 23% to 47%.
Tumor Growth Rates
Tumor growth rates are highly variable. Some small tumors do not grow at all. There can be periods of rapid growth. But in general, VSs are slow growing at an average growth rate of 1.2 to 1.9 mm/year. There appears to be no difference in growth rates between older and younger individuals.
Tumor regrowth is possible after treatment by either microsurgery or radiosurgery/radiotherapy. A sequence of post-treatment checkups by MRI is recommended. For microsurgery, regrowth is more likely and occurs sooner in cases of partial removal. Regrowth following total removal by an experienced surgery team is rare and especially so after five years have passed. Tumor control rates of 95% or more are reported for radiosurgery/radiotherapy, even as radiation dosages have been lowered to 12-13Gy (Gy is a unit of absorbed radiation). Pre-treatment counseling should include the information that tumors may increase in size/swell temporarily following radiation. This rarely denotes a failed procedure. (See “Acoustic Neuromas that Enlarge after Gamma Knife Radiosurgery,” ANA/NJ Newsletter (Sept 2008).