Gamma knife thalamotomy for essential tremor in the third era of functional neurosurgery

We report our experience adopting high angular resolution diffusion imaging (HARDI) tractography to define the target in the Ventralis Intermedius (Vim) nucleus in patients with essential tremor (ET). Retrospective observational cohort study on 18 consecutive Gamma Knife Stereotactic (GKRS)-thalamotomies. Dento-Rubro-Thalamo-Cortical-tract (DRTT), Cortico-spinal tract (CST) and Medial Lemniscus (ML) were reconstructed and used to adjust the coordinates-based target. The DRTT received a median maximum dose of 116 Gy (IQR 92.8-124.2 Gy) and the median volume receiving > 20 Gy (V20) was 154 mm3 (IQR 63.5-190.2 mm3). The maximum dose to the Vim was 126 Gy (IQR 126.2-127 Gy) and the median volume covered by 100 Gy was 25 mm3 (IQR 23-28-2 mm3), corresponding to the 14.5% of the whole volume (IQR 10.7-18.0%). The median Euclidean distance between the indirect (in-Vim) and adjusted Vim (ad-Vim) was 1 mm (IQR 0.7-1.9) and the final position of the ad-Vim was 1 mm cranial compared to the in-Vim (p < 0.001). Tremor improved in 15 of 18 thalamotomies (83.3%) and in all 11 cases with follow-up longer than 12 months, with a median ΔT% of 63.7% (IQR 57.4-72.3). The only prognostic factor was higher dose rate (p = 0.007), associated with faster response. No patients presented adverse radio-induced events during the follow-up. GKRS thalamotomy adopting DRTT projection, as landmark for the Vim in addition to standard coordinates, and maintaining the CST outside the 20 Gy isodose line, is effective and safe in ET.