Large Arteriovenous Malformations
Gamma Knife Surgery for Large AVMs
At the University of Virginia, we have recently begun to treat patients with large, surgically unresectable arteriovenous malformations (AVM's). A patient's clinical information, neurological examination, and radiological images are evaluated by a multidisciplinary team of neurosurgeons and neuroradiologists. For those patients deemed to be candidates, Gamma Knife surgery and embolization may be employed to treat the patient's AVM. If embolization is utilized, it may be performed before or after radiosurgery.
Gamma Knife surgery has been very successful at treating small to medium sized AVM's. Obliteration rates for AVM's less than 3 cm in maximal diameter are approximately 80 to 85%. Although the obliteration rate for large AVM's is likely to be less than for small and medium sized AVM's, initial experiences at our center and others have been promising. Moreover, for many of these large AVM's, microsurgical resection may be contraindicated because of the location, size, and proximity to eloquent structures. As such, Gamma Knife surgery with or without embolization may represent the best chance for successful obliteration of the AVM.
In preparation for the Gamma Knife surgery, patients undergo placement of a stereotactic frame in the operating room. The operating room affords a sterile environment for this procedure. In addition, an anesthesiologist provides monitored anesthesia so that the patient is completely comfortable during this procedure. A stereotactic MRI and angiogram are then obtained for each patient. This information is sent electronically to the Gamma Knife suite for dose planning.
During Gamma Knife dose planning, we prefer to use multiple isocenters for irradiation to obtain a highly conformal, homogeneous radiation dose to the AVM center or nidus. The AVM margin is typically covered by the 55 to 60% isodose level, and the 70% isodose level covers the main part of the nidus.
For AVM's with a volume of 10 to 20 cc, the marginal dose is usually 18 Gy. If the AVM volume is between 20 to 30 cc, the marginal dose is reduced slightly to 16 to 17 Gy. For AVM's with a volume of 30 to 40 cc, the marginal dose is about 16 to 16.5Gy. Our maximal dose is usually around 30 Gy (range, 32 to 28 Gy) for large AVM's treated in a single session.
For extremely large AVM's with volumes of 40 to 50 cc, we utilize a staged radiosurgical approach. Our strategy is to divide one large AVM into two parts. Each part is treated separately by the aforementioned methodology. Half of the AVM is treated in the first session, and the other part is treated 3 to 6 months later.
As part of the treatment, every attempt is made to minimize the risks and maximize the benefits of radiosurgery. Patients are followed up with MRI's every 6 months after Gamma Knife surgery. At the 3 year time point, patients undergo a diagnostic cerebral angiogram to determine if the AVM has been successfully obliterated. If the AVM has decreased in volume but still remains patent, repeat radiosurgery may be indicated.