Radiosurgical treatment planning of AVM following embolization with Onyx: possible dosage error in treatment planning can be averted

Abstract

Treatment of arteriovenous malformations (AVM) of the brain is challenging 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 such as Onyx (a liquid agent that is less adhesive and slowly polymerizing) have increased the probability of achieving obliteration. When planning radiosurgical cases following such embolization, however, one must be cognizant of the distortions introduced by this novel substance on imaging studies. A sample of Onyx was irradiated to define the attenuation per mm thickness. The difference in attenuation compared to water was determined. Dose calculations were performed using 3 methods of inhomogeneity corrections. Homogeneous calculations were compared to "standard" heterogeneity corrections and to "modified" heterogeneity corrections by assigning individual electron densities to the normal brain and the Onyx. The difference between the attenuation of water in comparison to the Onyx was approximately 3% for beam energy of 6 MV. Best calculation results were achieved when using the modified inhomogeneity corrections which were based on the actual attenuation of the Onyx. The use of Onyx caused significant image artifact on MR and especially CT. As such, a correction must be manually introduced into the planning system to account for this potential error. Otherwise, dose calculation may be unreliable and could have dire consequences for patients receiving high doses of radiotherapy.

Investigation of dosimetric differences between the TMR 10 and convolution algorithm for Gamma Knife stereotactic radiosurgery

“…Potential dosimetric inaccuracies have been suggested with onyx embolization material and the effect of other embolization agents has not been fully understood (16) . In our study four AV M patients had undergone partial glue embolization, and the dose shift with the convolution algorithm in these cases was comparable to subjects who had not had endovascular treatment before GKR.…”

Stereotactic radiosurgery for arteriovenous malformations after Onyx embolization: a case-control study

“…9 Shtraus et al found that the measured attenuation of Onyx placed in a bottle was around 3% higher than the attenuation of water. 58 The biomaterial properties of Onyx appear to be reflected in the current clinical study. The difference in obliteration rate was not statistically significant between patients who did not undergo prior embolization and those that did, when they had comparable AVM volume, grading, and radiation dose.…”

“…9,35 The possible dosage attenuation caused by previous embolic materials can be averted with Onyx embolization. 58 The present study aims to evaluate the outcome of SRS treatment of AVMs after Onyx embolization. These cases were matched to a cohort of patients who underwent SRS as an upfront treatment.…”

Liquid Embolization Material Reduces the Delivered Radiation Dose: Clinical Myth or Reality?

“…Wilczek et al 10 tested the dose perturbation induced by different stents in brachytherapy and noticed that the shielding effect exists and depends on the strut thickness and mesh attenuation rather than on the atomic number of the metal. Shtraus et al 11 found that the measured attenuation of Onyx placed in a bottle was 3% higher than the attenuation of water. Light ion beams can also be used in radiosurgery, with clinically and radiobiologically interesting results, especially for large BAVMs.…”