Radiosurgical Treatment Planning of AVM following Embolization with Onyx: A Cautionary Note

Abstract

Treatment of arterio-venous malformations (AVM) of the brain represents a challenge due to the size and location of the nidus proper and its proximity to the cerebrovascular circulation. Recent advances in catheter techniques and new embolization materials, in particular Onyx (a novel liquid agent that is less adhesive and slowly polymerizing), have increased the probability of achieving total and near-total obliteration. In planning radiosurgical cases following such embolizations, one must be cognizant of the distortions introduced by Onyx on imaging studies (CT/MR). Between 12/06 and 12/08 we treated 13 AVM patients with SRS after Onyx embolization. One ml of Onyx stored in a circular plastic membrane (15mm diameter and 2cm high) was irradiated to define the attenuation of the onyx per mm thickness. We calculated the difference in the attenuation compared to water. Then, the Onyx and the projected volume of the brain were contoured in the planning system and a density correction was applied for the heterogeneity of the virtual organ that was drawn (i.e., onyx and brain). A pinpoint chamber was used for relative measurements (0.015cm3) due to the small amounts of onyx collected. The brain tissue around the Onyx had to be contoured due to the high intensity projection of the Onyx on CT images, which caused the CT to apply an incorrect brain density adjacent to the Onyx, thereby directly impacting the radiation calculation. We compared the monitor unit per arc of treatment between the corrected density value and the standard heterogeneity correction of the planning system. The absorption of the onyx with thickness of about 5mm was up to 2.3% higher compared to 5mm of water. The difference in the dose calculation due to the absorption differences, from a 30 degrees radiation arc treatment through the Onyx was higher by up to 30%. In contrast, the dose calculation for the same arc that does not pass through the Onyx caused a reduction of up to 26% due to the distortion of the images and assigned density values similar to air around the Onyx. The use of Onyx brought about significant image artefacts on MR and especially on CT. Larger Onyx-embolized AVMs showed significant imaging artefacts especially on CT, which rendered this imaging modality useless for radiation treatment planning purposes. These findings show the importance of the correction that needs to be applied manually in the planning system. For patient with large areas and volumes of onyx, this result underscores the importance of the correction that must be applied. A dose calculation may be completely wrong, particularly for AVM- radiosurgery patients receiving high doses in single fractions with no margins.