Stereotactic Radiosurgery of Cranial Arteriovenous Malformations and Dural Arteriovenous Fistulas

Stereotactic radiosurgery

Stereotactic radiosurgery (SRS) is a technique in radiosurgery that delivers a conformal dose of intense radiation on a target, such as an AVM. A number of different technologies have been developed to achieve this result: Gamma Knife; linear accelerators (LINACs; e.g., CyberKnife, Synergy S, and Trilogy); and proton-beam. Gamma Knife uses a radioactive source (cobalt) and aims radiation through a circular array of ports to focus the beams at a single point. LINACs, which are made by a number

Mechanism and techniques of radiosurgery for AVMs and AVFs

Radiosurgery works to obliterate the AVM through two stages. Initially, radiosurgery damages the endothelial cells.²⁰ The intimal layer thickens because of smooth muscle proliferation and deposits of hyaline, calcium, and collagen in the extracellular matrix.20, 21, 22 Ultimately, radiosurgery effects thrombosis and necrosis of AVM vessels resulting in a durable response that lasts at least a decade after radiosurgery.21, 23 It is important to note that this process can take years to work. The

Choosing the appropriate AVM and AVF for radiosurgery

In determining the role of SRS for AVM patients, various scales are used to guide treatment. In unhemorrhaged AVMs, case selection can be guided by a number of AVM grading scales, such as the Spetzler-Martin scale (SM), modified Spetzler-Martin scale (mSM), and Pittsburgh radiosurgery-based AVM grading scale (Table 2).37, 38 Key components shared by these scales are AVM size and location. Although the SM scale was developed for predicting surgical morbidity and mortality, its role in guiding

Factors impacting outcome of radiosurgery in AVMs and AVFs

To determine the factors that affect the outcome of SRS, one must acknowledge that there are varying opinions on the definition of successful treatment of an AVM or AVF. Success of SRS has been defined in terms of hemorrhage prevention and symptom management, or nidus obliteration. Typically, the most severe consequence of AVMs and AVFs is morbidity and mortality from hemorrhage, so seizure control is less well studied. However, only a subset of AVMs and AVFs hemorrhage each year, which limits

Radiosurgery outcomes in treatment of AVMs and AVFs

SRS may aim to control AVM symptoms or prevent morbidity and mortality from AVM hemorrhage, but many studies focus on the prevention of hemorrhage. In addition to hemorrhage events, studies also measure AVM nidus or AVF obliteration, which is typically very strongly associated and is thought to be causatively related to hemorrhage.

Limitations and complications of radiosurgery

Although SRS is suitable for treating many AVMs, there are specific challenges that limit its application. In addition to adverse effects and complications of SRS, the most distinct limitation is the delayed effect of SRS on nidus obliteration.

The time from SRS treatment to maximum nidus obliteration is typically cited as 24 to 36 months. An exponential decay model estimated an average of 44% decrease in volume annually.⁷⁹ The obliteration rate of SRS has been found to depend on the dose

Nuances and pearls in treating patients with AVMs and AVFs using radiosurgery

There are situations where radiosurgery may be useful, but where traditional single-dose radiosurgery does not offer a viable solution, such as geometrically complex AVMs or extremely large AVMs. A number of variations have been explored but are less well studied. Because the radiobiology of radiosurgery is not completely understood, it has been difficult to predict which of these modifications will improve obliteration rate or decrease hemorrhage. In addition, the applicable patient population

Summary

SRS is a complementary treatment option to microsurgery and embolization for AVMs and AVFs. AVM and AVF patient selection needs to be carefully considered given the immediate benefit and high success rate of microsurgery. Cases that are not amenable to surgery may also be poorly suited for monotherapy SRS and combination therapy should be considered. History of hemorrhage or embolization, size of the AVM or AVF, proximity to radiosensitive structures, and patient age are key factors in