[Progress in optimizing dosimetry plans in stereotactic radiotherapy in the Salt Group (Saint-Anne-Lariboisière-Tenon].

We began intracranial stereotactic irradiation under the direction of O Betti 11 years ago. At the present time, we believe it is interesting to present the methodologies of the SALT (Saint-Anne-Lariboisière-Tenon) group. Up to the present time we have irradiated 693 patients using a single fraction. Arteriovenous malformations (AVMs) represented the majority (90%) of treated lesions. Irradiation protocol has little changed since 1986, and the localization of the target volume was performed in the neurosurgery department of St Anne Hospital, France. The stereotactic images (computerized tomography [CT], angiography) were sent to the radiotherapy department of Tenon Hospital through the French public digital network NUMERIS. Protocol was realized using the stereotactic ARTEMIS-3D/Dosigray TPS. The lesion volume was filled by one or more spherical or elliptical subvolumes using the "Associated Target Methodology". The interactive adjustment of subvolumes was based on the 3D graphical representations of the lesion. The direct optimization of the irradiation space was performed by managing parameters provided by the DDB (Dosimetric Data Base) such as the number of arcs, their angular position, as well as the starting and the ending point of each arc. The evaluation of the calculated dose distribution was made using quantitative parameters. The second method of optimization was based on the minibeam intensity modulation using a mathematical theory of inverse problems and singular value decomposition (SVD) analysis. At the present time, due to technical reasons, linear accelerators do not permit the modulation of intensity of arctherapy. Thus we transformed the profiles of irregular forms into rectangular profiles of modulated ponderation, with each optimized plan being evaluated before its implementation. The criteria of evaluation were derived from the differential and cumulative dose volume histograms (DVH). The DVHs permitted the evaluation of the volumes of underdosage and overdosage inside the lesion and in the healthy tissue, respectively. Using DVHs, we have defined parameters such as the conform factor and the homogeneity index. We stress that the methodology of protocol optimization is valid for single or multiple fractions as well as for intra- and extra-cranial irradiation.