[Laitinen's stereo-adapter: application to the fractionated cerebral irradiation under stereotaxic conditions].

Radiosurgery is a technique which provides a single high dose of radiation very precisely to an intracranial lesion. The volume of tissue treated is small enough to avoid undue biological damage. This treatment requires very sophisticated apparatus and often considerable modifications in the treatment apparatus. The authors have closen to elaborate a system which makes it possible to deliver conventional fractionated irradiation, using a non-invasive stereotactic head frame called "Laitinen stereo-adapter". By fractionating the dose, most benign or low-grade malignant brain tumours can be treated, whatever their volume. The stereo-adapter and the target localization process are described. It can be used with C.T. scanning, angiography or M.R.I. The treatment apparatus is a linear accelerator delivering a 10 M.V. photon beam and treatment is made in the precise positioning of the target localization. The geometrical accuracy of the repeated mountings was studied in a preliminary study, showing a mean discrepancy of less than 1 mm in the three space planes. The geometrical accuracy of the radiation treatment was also studied on a phantom and the precision evaluated at less than 1 mm. The addition of extra collimators with a precollimation system decreased the lateral penumbra to 2-3 mm. Careful dosimetry was performed for each collimator. The use of non-coplanar arcs makes it possible to obtain a steep dose fall off outside the target volume. The dose planning calculations were compared to photodensitometry and thermoluminescent measurements, showing a good correlation. Each technical point will be discussed.(ABSTRACT TRUNCATED AT 250 WORDS)