Neuropathology of intracranial arteriovenous malformations following conventional radiation therapy

Conventional fractionated radiation therapy was used in 15 patients to treat arteriovenous malformations (AVMs) of the brain deemed inoperable or incompletely obliterated using endovascular or surgical techniques. The AVMs measured from 0.8 to 85 cm3 (mean 28.2 cm3; median 24 cm3). Angiography in 12 patients 1-21 years following radiation therapy demonstrated no significant change in 7, slight reduction in 2, near complete obliteration in 1, and complete obliteration in 2. Magnetic resonance imaging further demonstrated apparent obliteration in one other case. Three irradiated AVMs were available for pathological examination following fatal recurrent hemorrhage after 21 years in case 1 and following surgery after 5 years in cases 2 and 3. Doses of 20 and 45 Gy were delivered to the area of the AVM in 10 and 15 fractions with a 6-MV linear accelerator in case 1 and in cases 2 and 3, respectively. A blinded histopathological comparison was made of the latter cases and three AVMs removed at surgery that were not previously irradiated and that were comparable in size, number of arterial feeders, and location within the brain. Segmental hyalinization of some blood vessels was seen in both irradiated and nonirradiated cases. The single postmortem specimen showed extensive thrombosis but a patent nidus. The findings are in keeping with the clinical impression that conventional fractionated radiation therapy fails to alter the natural history of cerebral AVMs. The favorable outcome of radiosurgery on small- to medium-sized AVMs appears attributable to the shorter duration of therapy using relatively high-dose prescriptions to the nidus.