Dosimetric analysis of trigeminal nerve, brain stem doses in CyberKnife radiosurgery of trigeminal neuralgia

CyberKnife radiosurgery treatment of Trigeminal neuralgia (TN) is performed as a non-invasive image guided procedure. The prescription dose for TN is very high. The brainstem is the adjacent critical organ at risk (OAR) which is prone to receive the very high target dose of TN. The present study is to analyze the dose distribution inside the tiny trigeminal nerve target and also to analyze the dose fall off in the brain stem. Seven TN cases treated between November 2010 and January 2012 were taken for this study retrospectively. The treatment plans were analyzed for target dose conformity, homogeneity and dose coverage. In the brainstem the volume doses D(1%), D(2%) were taken for analyzing the higher doses in the brain stem. The dose fall off was analyzed in terms of D(5%) and D(10%). The mean value of maximum dose within the trigeminal nerve target was 73.5±2.1Gy (P=0.0007) and the minimum dose was 50.0±4.1Gy (P=0.1315). The mean conformity index was 2.19 and the probable reason could be the smallest CyberKnife collimator of 5mm used in the treatment plan. The mean D(1%), of the brainstem was 10.5± 2.1Gy (P=0.5316) and the mean value of the maximum point dose within the brainstem was 35.6±3.8Gy. This shows the degree of dose fall off within the brainstem. Though the results of the present study are showing superior sparing of brain stem and reasonable of target coverage, it is necessary to execute the treatment plan with greater accuracy in CyberKnife as the immobilization is noninvasive and frameless.