facilities (surgery, embolization and radio-surgery) available. Those centers where one or the other is unavailable should obtain an outside opinion before any treatment is carried out, even if this involves transmitting or posting images in order to utilize distant expertise.

The likelihood of complete AVM obliteration must be balanced against the need to avoid an unacceptable rate of radiation-related complications after radiosurgery. Flickinger et al. [13] found that 30% of AVM patients had MRI changes adjacent to or within the irradiated volume at a median of 8 months after radio-surgery. However, though some mistakenly describe this as "radionecrosis", these radiological changes resolve within 2-3 years in the majority of patients. The explanation is probably that the signal change is due to altered per-ilesional blood flow as the nidus gradually obliterates rather than to tissue damage [14]. It is more important to consider the neurological symptoms and signs rather than the radiological appearance. In a multicenter study of AVM patients, 8% of patients developed neurological sequelae (cranial nerve deficits, seizures, cyst formation) after radiosurgery [15]. Symptoms resolved completely in 54% of these at 3 years post onset. Good prognostic factors were no prior history of hemorrhage and symptoms of minimal severity. According to the prospec-tively maintained database of over 3,000 cases treated in Sheffield, the permanent clinical complication rate is 3.8% (data on file). These low-complication figures were obtained with gamma knife treatment. Linear-accelerator (LINAC) technology results in somewhat worse statistics [16].

Earlier reports on AVM radiosurgery suggested that the annual hemorrhage rate after radiosurgery prior to obliteration was greater than that occurring in the natural history of untreated AVMs. More recent reports, analyzing radiosurgical hemorrhage rates, found that radiosurgery did not change the annual bleeding rates during the latency interval [17, 18]. Karlsson et al. [19] reported 1,604 patients with a total of 2,340 patient-years at risk of a hemorrhage. They detected a decrease in hemorrhage rates within 6 months after radiosurgery and patients who received higher radiation doses were conferred the greatest protection from bleeding. When considered together, these three reports with 2,000 patients and more than 3,000

patient-years at risk of hemorrhage document quite convincingly that radiosurgery does not increase the annual bleeding rate for AVM patients prior to obliteration. In fact, the data suggest that even patients with incomplete obliteration may have some protection against the risk of future bleeding. Nevertheless, the continued risk of hemorrhage during the delay in obliteration is a drawback of radiosurgery.

Some papers [20] highlighted the late radiological changes seen on MRI, noted many years after the malformations had been shown by angiography to be obliterated. In most cases this is merely an imaging finding, but in a selected small proportion of cases clinical symptoms may arise. Surgery for these cysts (including one case operated upon in our department) found no neoplastic change. The long-term significance of these fluid-filled cavities is uncertain but it is likely that most will be innocuous and similar to the porencephalic cyst seen after surgical removal of any large mass lesion.

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