Technical Nuances Of Stereotactic Radiosurgery Planning After Partial Onyx Embolization Of Arteriovenous Malformations

John YK Lee1, Melandee Brown2, Linda Bagley2, Michael Stiefel2

1University of Pennsylvania 2University of Pennsylvania, Philadelphia, USA

Keywords: arteriovenous malformation, Onyx, embolization, radiosurgery, gamma knife

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     Onyx (ethylene vinyl copolymer) is a recently approved embolic agent that has been increasingly used to treat AVMs. In some large AVMs, we embolize part of the AVM with Onyx and then treat the residual volume with stereotactic radiosurgery but exclude the previously embolized volume. 
     SRS planning is difficult in these situations because of the unique imaging characteristics of Onyx. Onyx contains radiopaque tantalum, and therefore CT scans demonstrate significant beam-hardening artifacts. In addition, Onyx appears hypointense on T2-weighted MR sequences, and thus makes it difficult to differentiate flow voids versus Onyx. We describe techniques to overcome these difficulties.
     We conducted a literature review using the Pubmed database with the following search criteria: “Onyx”, “stereotactic radiosurgery”, “gamma knife” and identified two papers that described the unique imaging characteristics of onyx avm embolization and radiosurgery. At the Penn Gamma Knife center, we have treated two patients with extremely large AVMs who have required both embolization and radiosurgery. We reviewed the techniques and nuances necessary to define their nidus.
     Based on literature review, CT scans and CT-angiograms are not useful imaging modalities after onyx embolization.  Standard T1, contrast-enhanced SPGR studies are useful, but T2-weighted studies may be limited.  Time of flight magnetic resonance imaging angiography (TOF-MRA) may be a useful adjunct to standard imaging modalities.  Conventional angiography remains a mainstay for defining vessels. We recommend defining the onyx embolization mass first before attempting to define the AVM nidus, especially in large AVMs.  
     This is a retrospective study
     We describe technical nuances of stereotactic radiosurgery planning in patients who have been partially embolized with Onyx. 
     We currently do not include the embolized volume within the treated volume, and therefore we review the literature and describe techniques to help differentiate nidus versus Onyx.


Project Roles:

J. Lee (), M. Brown (), L. Bagley (), M. Stiefel ()