[Radiotherapeutic basis of radiosurgery as applied to cerebral arteriovenous malformations: dose-volume effects]

To attempt to analyze the basic effects of radiosurgery as applied to cerebral arteriovenous malformations.

We reviewed and discussed the previous issues of the international literature concerning radiobiological principles of radiosurgery of arteriovenous malformations.

The irradiated volume contained both a target volume consisting of abnormal vessels of the nidus often embedded within connective tissue stroma and the surrounding normal brain tissue. These were late-responding tissues after the radiation and sensitive tissues to high dose per fraction. There was not resistant hypoxic cell that could be killed efficiently with fractionated treatment. The therapeutic ratio between damage in the arteriovenous malformation nidus and late complications in the surrounding tissues was independent of fractionation. Furthermore, sterilization of all clonogenic cells in the target volume was not necessary to obtain complete obliteration of arteriovenous malformation. Consequently, there was nothing to be gained by a fractionated course relative to a stereotactic high single-dose irradiation for a small targeted volume (<=25 mm in diameter or<=4 200 mm(3)). The minimum target dose (Dmin) seemed to be the dosimetric independent predicting factor for complete obliteration. The Dmin-response relationship suggested a threshold dose of 5 Gy to 10 Gy and the better effects following Dmin from 20 Gy to 25 Gy. A Dmin above 25 Gy significantly increased the risk of serious late complications. The treated volume seemed to be the significant independent predicting dosimetric factor for developing persistent symptomatic sequelae. The optimum dose to deliver to obtain complete obliteration of an arteriovenous malformation was selected by balancing the expected obliteration rate for each dose with the corresponding risks from radiosurgery depending on size, location, shape and dose distribution.

Although the basic pathological process is still uncertain there is a strong rationale for stereotactic radiotherapy delivered in a high single-dose for small arteriovenous malformations.