A halo-ring technique for fractionated stereotactic radiotherapy.

Stereotactic radiosurgery has become established as an effective treatment modality for certain non-malignant brain diseases such as arteriovenous malformations. This paper describes an extension of our linear accelerator-based radiosurgical technique to fractionated treatment of intracranial disease. The fractionated stereotactic radiotherapy technique expands the use of the modality by sparing normal cells within the treatment volume thus improving the therapeutic ratio. The first treatment is given using a stereotactic frame both for target localization and patient immobilization. The frame is then removed and subsequent treatments use a standard neurosurgical halo-ring for patient immobilization. The halo-ring is left in place on the skull for the duration of the course of treatment. Thus the physical requirements for fractionation pertain firstly to the patient immobilization and target localization using the halo-ring and secondly to the stringent quality assurance procedures required to maintain spatial accuracy under these new conditions. We describe a sensitive and effective technique for checking the rotational beam parameters and collimator alignment which we use immediately prior to treatment to ensure adequate accuracy of dose delivery to the target volume.