Objective:

To correlate outcomes of Gamma Knife Thalamotomy (GKT) for Essential Tremor (ET) to the Biological Equivalent Dose (BED) delivered to the nucleus ventralis intermedius (Vim) of the thalamus.

Background:

Prescriptions for GKT are traditionally fixed doses rather than BED adjusted. Gamma Knife source decay prolongs delivery time. BED incorporates both the actual dose prescribed and the tissue repair that occurs over the course of radiation delivery as a function of delivery time.

Design/Methods:

43 patients underwent GKT between 2003 and 2023. Clinical, imaging, and dose parameters were compiled into a database for this analysis. 38 patients had clinical follow-up ranging from 2 to 61 months. Imaging follow-up was performed with MRI in 20 patients and CT in 3. DTI was performed in 4 patients.

Results:

Tremor reduction was seen in 84% of patients. Response was 90% in patients receiving a BED ≥ 4600 Gy_2.47 vs. 65% in patients receiving less (p<0.004). Complications occurred in 5 patients: apraxia (2), hemiparesis (2), and hemorrhage (1). MRI FLAIR signal changes exceeding 4 mm in diameter at the thalamotomy site were associated with clinical response. Extensive FLAIR change involving the genu and anterior internal capsule and/or the cerebral peduncle and insula with or without sylvian fissure deformation were associated with motor deficits. Expectant and corticosteroid management was used in all patients, and recovery was near complete in all cases in terms of motor function except for the patient with hemorrhage who remained weak on the contralateral side. DTI analysis showed significant reduction in the volume of ipsilateral dentato-rubro-thalamic tract fibers after successful radio-surgical lesioning and was associated with good tremor response.

Conclusions:

We conclude that BED is a more effective predictor of treatment efficacy with a clearly demonstrable response threshold. Occurrence of side effects corresponded to findings on imaging. Tractography changes also correlated to response.