Role of gamma angle in treatment planning of vestibular schwannoma in Gamma Knife: A retrospective study.

Gamma angle plays a major role in Gamma Knife Radiosurgery (GKRS) treatment planning. Selecting an appropriate gamma angle may help in mitigating unnecessary radiation exposure to organs at risk (OARs). The aims in GKRS of vestibular schwannoma (VS) is to deliver sufficient radiation to the tumor extending into internal auditory canal (IAC) while keeping basal turn of cochlea and brain stem away from 4 and 12 Gy radiation exposure, respectively. This study analyses the optimal gamma angle in GKRS for VS treatment planning. The study was performed using old MRI datasets of 16 patients of VS in Leksell GammaPlan version 10.1.1. T2 weighted contrast MRIs were used for the planning purposes. Three different plans were made for each patient at gamma angles 90°, 110° and 70° using hybrid inverse planning technique. Dynamic shaping was used to achieve as low as reasonably achievable (ALARA) doses to the cochlea without compromising target coverage (i.e. coverage of more than 97% of tumor volume). This comparative analysis shows minimal radiation exposure to cochlea for plans made at gamma angle 110° compared to 90° and 70°. Average percentage volume of cochlea receiving 4 Gy were 9.63 ± 12.32%, 6.19 ± 8.24%, and 25.25 ± 31.82% at gamma angles 90°, 110° and 70°, respectively (one-way ANOVA p = 0.0247). The average selectivity indices were 83.44 ± 7.13, 84.06 ± 7.84 and 83.56 ± 7.22 at gamma angles 90°, 110° and 70° respectively. Similarly, the gradient indices and beam on time were 2.80 ± 0.23, 2.81 ± 0.23 and 2.80 ± 0.25 and 120.65 ± 59.63, 117.95 ± 58.06 and 123.99 ± 61.61 min, respectively, at 90°, 110° and 70°. The selectivity index, gradient index and beam on time were minimal at gamma angle 110° compared to the other two angles, but not statistically significant (one-way ANOVA p-values were 0.9686, 0.9942 and 0.9598, respectively). The gamma angle of 110° is a good choice for treatment planning of VS patient in Gamma Knife as it gives better treatment plans (minimal cochlea doses).