Cerebral autoregulation involves adaptive mechanisms at subcellular, cellular and intercellular levels as well as regulation of local, regional and global cerebral circulation (Bruce et al. 1973, Grubb et al. 1975, Harper et al. 1966, McGillicuddy et al. 1978). Autoregulation also includes systemic responses of metabolism, respiration and circulation. All of those cerebral autoregulatory mechanisms may be severly impaired in head injured patients (Langfitt et al. 1965, Höper et al. 1994, McGillicuddy et al. 1978) and in patients with brain tumors (DeChiro et al. 1986/1987, Kusske et al. 1974, Jeske et al. 1993).

Intriguing differences in the results of brain arteriovenous malformation (AVM) therapy with ionizing radiation are reported in the literature, i.e. AVMs obliterate at different time intervals from the initiation of radiotherapy although presenting with similar nidus volumes that are treated with the same radiation dose. The purpose of this paper is to present a new concept that explains the variety of results after radiotherapy.To account for an individual AVM's responsiveness to radiotherapy, four variables havebeen identified from the literature. The variables are factors that are generally accepted to influence the biological effect of radiation. Combined, they can visualize a variety of AVM tissues as well as surrounding normal brain. These hypothesized variations in the type of tissues are then matched with the incongruent results of radiotherapy in order to clarify the issue.Future clinical research programs and possible therapeutical implications of this concept are proposed and discussed.