Radiotherapy and cerebral stereotaxis. Examples of dosimetry with 3D reconstruction

External stereotactic radiotherapy allows to irradiate a small and carefully delimited intracranial volume according to the spatial definition of the target. To determine the distribution of the dose in the volume irradiated, a dosimetric programme adapted to particular treatment conditions (arc therapy in the frontal and oblique planes converging onto the centre of the target volume with circular beams 8 to 20 mm in diameter using 18 MV X photons) is developed. The principle of the programme is a 3D reconstruction based on ten transverse CT slices. This reconstruction, related to the stereotactic coordinates defined during stereotactic localization, visualises the outline of each oblique frontal treatment plane and the outline of the three perpendicular reference planes passing through the centre of the target volume (i.e. transverse, sagittal, coronal). The isodose distribution is then calculated in the planes defined by these reconstructions. Under treatment conditions, the parameters measured for each beam with their additional collimation are used. An evaluation of this software performed on a phantom consisting of a skull containing a defined target, is presented