Summary of Results of Gamma Knife Surgery
for Vestibular Schwannoma (ANs)
at the University of Virginia Gamma Knife Center, 1989-99
Source: D. Prasad, M. Steiner, L. Steiner, "Gamma Surgery for Vestibular Schwannoma," unpublished manuscript, 1999 (republished with permission)
Lars Leksell Center for Gamma Knife Surgery.
Department of Neurological Surgery,
University of Virginia, Charlottesville, VA 22908.
(boldface and italics ours)
Of 200 cases of vestibular schwannoma treated 3/89-3/99, follow-up for 1-10 years (mean 4.27) was available for 153 cases, and 95 cases out of the 153 had follow-ups of over 5 years. The 153 cases ranged in age from 18 to 88 years (mean 60). There were 73 females, 80 males. Gamma surgery was the initial treatment in 96 cases; the remaining 57 cases had undergone prior microsurgery. Tumor diameters at the time of treatment ranged from 0.3 cm to 3.2 cm (mean 1.74). The prescribed minimum dose (dose to the tumor margin) varied from 9 to 20 Gy (mean 13.3); maximum dose was 17 to 53 Gy (mean 34.3).
Of the total 153 cases, reduction in size was seen in 115 (75%); no change in 26 (17%); and increase in size in 12 (7%). For the 96 cases treated initially with Gamma Knife, reduction in size was seen in 78 (82%); no change in 12 (12%); and increase in size in 6 (6%). For the 57 cases with prior microsurgery, reduction in size was seen in 37 (65%); no change in 14 (25%); and increase in size in 6 (10%). In the 95 cases with follow-up of 5-10 years, there were 70 (74%) with shrinkage, 16 (17%) unchanged, and 6 (6%) increased in size. In 19 cases with tumors larger than 3 cm in diameter, 95% showed shrinkage.
A total of 47 patients exhibited increase in tumor size preceding shrinkage. Such transient increase in tumor size should not be confused with tumor growth. The onset of shrinkage was observed as early as 3 months and as late as 7 years after treatment. Most tumors showed shrinkage in the first two years.
Earlier reports from Gamma Knife centers put the incidence of initial facial weakness following treatment at 17 to 33%. At Virginia, low doses to the tumor margin (mean of 13.2 Gy) greatly reduced the incidence. Only 3 cases of new but transient facial weakness were observed (2.2%), and the risk of some degree of permanent facial weakness was 0 to 1.3%.
The low average dose of 13.2 Gy also yielded a lower incidence of trigeminal neuropathy. New trigeminal (sensory) symptoms were observed in 5 cases, with onset from 6 months to 1.2 years after treat-ment. Three cases had complete remission, while two persisted in follow-ups of 3-4 years. The risk for transient trigeminal neuropathy was 4%, and permanent 1.6%.
Hearing deteriorated in 60% of patients over 6 years. For the 36 cases with useful hearing before treatment, 58% retained their hearing at a mean follow-up of 4.27 years; and 42% showed deterioration in hearing. Hearing changes began to be seen after two years of follow-up. Small tumors did best for hearing preservation. Four patients with useful hearing but with tumors larger than 3 cm lost their hearing over a mean period of 4.3 years. The minimum prescribed dose (dose to the tumor margin) was important. Patients who received 13 Gy or less did best in hearing preservation.
Tinnitus was present in 26 cases. Three improved, one new tinnitus developed, and the rest were un-changed. No cases of post-treatment brainstem edema or of hydrocephalus were observed.©
Last Edited: Friday, November 01, 2002