Object. The purpose of this study was to assess the relationship between the volume of brainstem that receives 20% or more of the maximum dose (VB20) and the volume of the trigeminal nerve that receives 50% or more of the maximum dose (VT50) on clinical outcome following gamma knife radiosurgery (GKS) for trigeminal neuralgia (TN).
Methods. Patients with TN were treated with a single 4-mm isocenter with a maximum dose of 75 Gy directed at the trigeminal nerve close to where it leaves the brainstem. The VB20 and VT50, as determined on dose—volume histograms, were correlated with clinical outcomes at 6 and 12 months, laterality, presence of multiple sclerosis (MS), and each other.
At 6 months excellent pain relief (no pain or required medicine) was achieved in 27 of 48 patients (p = 0.009) when VB20 was greater than or equal to 20 mm3 and in 25 of 78 when VB20 was less than 20 mm3, when all patients are considered. At 12 months excellent pain relief was achieved in 16 of 32 patients (p = 0.038) when VB20 was greater than or equal to 20 mm3 and in 14 of 52 when VB20 less than 20 mm3, when all patients are considered. When VB20 was less than 20 mm3 in MS patients, five of 21 had an excellent result at 6 months and two of 13 at 12 months. The VB20 was 20 mm3 or more in 38 of 64 on the right side and in eight of 41 on the left side (p < 0.001) in patients with TN and without MS. There is a difference between left and right dose—volume histograms even when the same isodose is placed on the surface of the brainstem.
The VB20 was 20 mm3 or more in 45 of 105 patients with TN and without MS but in only three of 21 patients with TN and MS (p = 0.014). There was an inverse relationship between VB20 and VT50 (p = 0.01).
Conclusions. Isocenter proximity to the brainstem, as reflected in a higher VB20, is associated with a greater chance of excellent outcome at 6 and 12 months. Worse results in patients with TN and MS may be partly explained by a lower VB20.