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When radiotherapy is employed for central nervous system tumours, several clinical considerations relating to the special normal tissue environment in which they are located deserve recognition. First of all the radiosensitivity of normal brain is nearly equivalent to that of the majority of the primary tumours requiring irradiation. Secondly destroyed normal neural tissue never regenates, but partial recovery is possible following limited injury. Thirdly definition of the target volume is determined mostly by indirect means from a synthesis of neuroradiological findings (C.T. scans, M.R.I.). Finally the rigidity of the intact cranium confers greater clinical significance on mass effects including postradiotherapeutic edema. Most brain tumours respond to external irradiation which may be applied either postoperatively or definitively, e.g. gliomas, lymphomas, medulloblastomas, and metastases. New stereotactic techniques, including radiosurgery, interstitial brachytherapy, and proton beam radiotherapy allow the delivery of larger dose in a limited volume.

Mechanisms of death following irradiation of malignant cells are incompletely understood. Radiation-induced lesions and their consequences on the cancer cell are described. Factors conditioning malignant cell response to radiation are discussed, as well as the mechanisms of radiation-induced carcinogenesis. Finally, the recent molecular biology contributions to gene regulation during cell growth are presented.

The aim of radiation therapy is to deliver a dose resulting in eradication of the tumor (primary tumor, lymph node, metastasis) with minimal damage to surrounding healthy tissue. Once the decision has been made to treat a patient with radiation, some pretreatment procedures must be done. First, target volume localisation must be accurately determinated as well as the dose-limiting normal tissue volume, using a simulator and a contouring device. Second, accurate daily positioning of the patient in the treatment position and reduction of the patient movement during the treatment are required to enable the radiation therapist to deliver the prescribed dose. The dedicated radiation planning computers play an essential role in modern radiation therapy because it allows to plan complex treatment with selecting the particles (photons, electrons, neutrons, protons), particle energies, treatment machines (kilovoltage unit, cobalt therapy machine, linear accelerator, cyclotron), fields arrangements, critical organ shielding blocks, wedges, and filters. Finally, field delineation and tumor coverage are checked using either port films or portal imaging.