Gamma knife radiosurgery for trigeminal neuralgia: role of trigeminal length and ponto-trigeminal angle on target definition and on clinical effects

ABSTRACT Objective Gamma knife radiosurgery (GKRS) is a well-defined treatment for trigeminal neuralgia (TN). The aim of this study is to determine how the GKRS planning might change on the basis of the patient’s own anatomy and how to best choose the target location. Methods Trigeminal cisternal length, ponto-trigeminal angle and distance between middle of the shot and emergence were evaluated in 112 consecutive GKRS plans for TN. Correlations with pain outcomes and facial hypoesthesia were analyzed. Results The mean angle was 29° ± 4.4° and 37° ± 0.9°, respectively, in patients developing and not-developing severe hypoesthesia (p = 0.045), despite no significant difference on brainstem dose (11.9 ± 0.8 and 10.5 ± 0.3 Gy; p = 0.22). The length of the nerve was not relevant on clinical outcomes but the shot-emergence distance (mean 8.1 ± 0.2 mm) depended on both trigeminal length and angle (p = 0.01). At constant prescription dose, 6 months cumulative rates of pain relief and control without therapy were 52.9% when the shot-emergence distance was ≤8mm whereas 25% when this distance was >8mm (p = 0.017). The maintenance of good pain control was more long lasting in the first group (49.5 ± 6.6 vs. 25.4 ± 5 months; p = 0.006) with a 5 years cumulative rate of 70% and 26%, respectively (p Conclusions The ponto-trigeminal angle and the shot-emergence distance should be considered during GKRS planning: the first as a potential risk factor for hypoesthesia and the second should not exceed 8 mm.