Only about 2,000 to 3,000 cases of acoustic schwannomas occur annually in the United States. Because it's a slow-growing tumor that develops in the vestibular nerve that lies very close to the auditory or hearing, nerve, the most common symptom is hearing loss. Left untreated, the hearing loss can be severe.
While surgery is the only way to remove the tumor, it's tricky and can potentially damage surrounding cranial nerves. Treating the tumor with radiation is usually effective, less risky, and the vast majority of tumors do not grow back, says Maria Werner-Wasik, M.D., associate professor of radiation oncology at Jefferson Medical College of Thomas Jefferson University.
But higher doses of radiation carry some risks of hearing damage too. Dr. Werner-Wasik, David Andrews, M.D., professor of neurological surgery at Jefferson Medical College, and their co-workers wanted to find out if lower doses would work just as well. They compared two groups among 115 patients with acoustic schwannomas treated at Thomas Jefferson University Hospital between 1994 and 2005. One group of 74 patients received fractionated doses – small, daily doses of radiation – resulting in "excellent tumor control and reasonable hearing preservation," she says. The remaining 41 patients also received fractionated doses, but at a lower radiation dose.
The researchers found that the local tumor control was the same for both groups, around 98 to 100 percent. The fifth and seventh nerve toxicity was the same – between 0 and 2 percent, but the hearing seemed slightly better in the lower dose group than in the higher dose. They report their results on November 7, 2006 at the annual meeting of the American Society for Therapeutic Radiology and Oncology in Philadelphia.
When the researchers adjusted statistically for patients in both groups that had only up to three years of follow-up, they found a significant improvement in hearing as gauged by "pure tone average," or PTA, which is the loudness in decibels that an individual patient can hear. Patients who had the higher radiation dose had a mean difference in PTA before and after treatment of 15.4, versus those in the lower dose level, at 5.7.
"We think that the lower dose level is associated with equally good local control and better hearing preservation, and we'll continue to make these observations and perhaps further reduce the doses," says Dr. Werner-Wasik. Lower radiation doses mean shorter treatment times and fewer visits. Patients received the lower dose over 26 treatments, while those who received the higher dose required 30 treatments. She notes that similar results were found by other researchers using Gamma Knife technology.
"While the standard of care has been surgery for most patients, the pendulum is swinging to radiation becoming the new standard for most as an initial treatment," she says.
Steve Benowitz | EurekAlert!
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