Radiation injury is the most serious complication after gamma knife radiosurgery. To reduce the rate of radiation injury, it is important to minimize the irradiated volume of the surrounding normal brain tissue. Especially, when AVMs are located adjacent to eloquent areas such as the brain stem or optic pathway, sparing the surrounding vital structures is one of the most important factors to minimize the radiation injury. However, the three dimensional shape of the nidus cannot be fully delineated with only angiograms of frontal and lateral views. Morever, it is impossible to determine the accurate dose at surrounding vital structures from angiograms alone. To solve these problems, dose planning using axial MRI combined with angiograms was performed in 40 cases from July 1991 to June 1992. Axial MRI combined with angiograms gave us more information about the shape of the nidus than only angiograms of frontal and lateral views in all cases. Because MRI showed accurate irradiated dose to surrounding vital structures, we were able to minimize the dose irradiated to optic pathways in 7 cases and the brain stem in 9 cases.
To minimize radiation injury, dose planning using axial MRI combined with angiograms should be carried out especially in cases of AVMs located adjacent to the optic pathway and brain stem.
