GAMMA-KNIFE RADIOSURGERY FOR THALAMOTOMY IN PARKINSONIAN TREMOR - A 5-YEAR EXPERIENCE

Object. Certain patients, for example, elderly high-risk surgical pati ents, may be unfit for radiofrequency thalamotomy to treat parkinsonia n tremor. Some patients, when given the opportunity, may choose to avo id an invasive surgical procedure. The authors retrospectively reviewe d their experience using gamma knife radiosurgery for thalamotomies in this patient subpopulation: 1) to determine the efficacy of the proce dure; 2) to see if there is a dose-response relationship; 3) to review radiological findings of radiosugical lesioning; acid 4) to assess th e risks of complications. Methods. Radiosurgical nucleus ventralis int ermedius thalamotomy using the gamma knife unit was performed to make 38 lesions in 24 men and 10 women (median age 73 years, range 58-87 ye ars) over a 5-year period. A median radiation dose of 130 Gy (range 10 0-165 Gy) was delivered to 38 nuclei (four patients underwent bilatera l thalamotomy) using a single 4-mm collimator following classic anatom ical landmarks. Twenty-nine lesions were made in the left nucleus vent ralis intermedius thalamus for right-sided tremor. Patients were follo wed for a median of 28 months (range 6-58 months). Independent neurolo gical evaluation of tremor based on the change in the Unified Parkinso n's Disease Rating Scale tremor score was correlated with subjective p atient evaluation. Comparison was made between a subgroup of patients in whom ''low-dose'' lesions were made (range 110-135 Gy, mean 120 Gy) and those in whom ''high-dose'' lesions were made (range 140-165 Gy, mean 160 Gy) for purposes of dose-response information. Four thalamoto mies (10.5%) failed, four (10.5%) produced mild improvement, 11 (29%) produced good improvement, and 10 (26%) produced excellent relief of t remor. In nine thalamotomies (24%) the tremor was eliminated completel y. The median time to onset of improvement was 2 months (range 1 week- 8 months). Concordance between an independent neurologist's evaluation and that of the patient was statistically significant (p < 0.001). Tw o patients who underwent unilateral thalamotomy experienced bilateral improvement in their tremor. There were no neurological complications. There was better tremor reduction in the high-dose group than in the low-dose group (p < 0.04). Conclusions. Although less effective than o ther stereotactic techniques, gamma knife radiosurgery for thalamotomy offers tremor control with minimal risk to patients unsuited for open surgery.