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Objective : The authors conducted this study to present the long-term treatment outcomes (minimum 2 years) of Gamma kniferadiosurgery(GKS) for trigeminal neuralgia(TN) and to demonstrate the correlation of treatment outcomes and the anatomicalcharacteristics of TN. Methods : From 1997 to 2003, 44 consecutive patients suffering from medically intractable pain underwent GKS for TN. A single 4mmcollimator was used with a median maximum dose of 80Gy (range 75~80Gy) prescribed to the root entry zone of the trigeminal nerve. Median follow up duration was 30 months (range 24~78 months). Anatomical measurements of trigeminal nerve in magnetic resonanceimages during GKS planning were correlated with clinical outcome. Results : Twenty-two patients (50%) achieved an excellent outcome (BNI grade I & II), 20 patients (45.5%) a good outcome (grade IIIa &IIIb), and only 2 patients (4.5%) a poor outcome (grade IV & V). Eleven patients (25.0%) experienced pain recurrence after initial painrelief. Smaller volume of trigeminal nerve area irradiated more than 40Gy was significantly correlated with excellent outcome in bothunivariate and multivariate analyses respectively (P=0.033 and 0.040). Conclusion : Anatomical considerations during the planning of GKS would be helpful for predicting clinical outcome in TN.