Department of Radiation Oncology, Washington University School of Medicine, St. Louis, Missouri, USA.
J Neurosurg. 2011 Aug;115(2):364-70. doi: 10.3171/2011.3.JNS10717. Epub 2011 Apr 15.
The effectiveness of Gamma Knife stereotactic surgery to obliterate brain arteriovenous malformations (AVMs) may be diminished by the preoperative adjunctive use of endovascular liquid embolic agents. The purpose of the present investigation was to determine if commercially available liquid embolic agents reduce the radiation dose to the target because of attenuation of the (60)Co beam.
The apparent linear attenuation coefficients for 120- to 140-keV radiographs in embolized regions were retrieved from CT scans for several patients with AVMs who had undergone embolization procedures with liquid embolic agents to reduce nidal volumes. Based on these coefficients and a virtual model of Gamma Knife surgery (GKS) with basic ray tracing, the authors obtained the path lengths and densities of the embolized regions. The attenuation of (60)Co beams was then calculated for various sizes and positions of embolized AVM regions and for the number of beams used for treatment. Published experiments for several high-atomic-number materials were used to estimate the effective (60)Co beam attenuation coefficients for the N-butyl cyanoacrylate (NBCA, suspended in ethiodized oil) and ethylene vinyl alcohol copolymer (EVOH, with suspended micronized tantalum powder, Onyx) used in the AVM embolizations. Dose reductions during GKS were calculated for a theoretical model based on the CT-documented apparent linear attenuation coefficients and for the (60)Co energy attenuation coefficient. Dose measurements were obtained in a phantom study with EVOH for comparison with the estimates generated from the two attenuation coefficients.
Based on CT (keV) apparent attenuation coefficients, the authors' theoretical model predicted that the cumulative effect of either of the embolic agents decreased the number of kilovoltage photons in an embolized nidus by -8% to -15% because of the increased atomic number and density of NBCA and Onyx. However, in using the effective attenuation coefficient for the (60)Co energies as is used in GKS, the authors' theoretical model yielded only a 0.2% dose reduction per beam and a < 0.01%-0.2% dose reduction in total. These theoretical results were validated by measurements in a head phantom containing Onyx.
Dose reduction due to attenuation of the (60)Co beam by the AVM embolization material was negligible for both NBCA and EVOH because of the high-energy (60)Co beam.
术前使用血管内液体栓塞剂辅助伽玛刀立体定向手术治疗脑动静脉畸形（AVM）可能会降低疗效。本研究旨在确定市售液体栓塞剂是否会因 60Co 射线的衰减而降低靶区剂量。
对行液体栓塞术以减少瘤巢体积的 AVM 患者的 CT 扫描，获取栓塞区域的 120-140keV 射线的表观线性衰减系数。基于这些系数和伽玛刀手术（GKS）的虚拟模型（基本射线追踪），作者获得了栓塞区域的路径长度和密度。然后，计算了不同大小和位置的栓塞 AVM 区域以及用于治疗的光束数量的 60Co 射线的衰减。使用几种高原子序数材料的已发表实验来估算 N-丁基氰基丙烯酸酯（NBCA，悬浮于乙碘油中）和乙烯-乙烯醇共聚物（EVOH，悬浮有微米级钽粉，Onyx）在 AVM 栓塞中使用的有效 60Co 射线衰减系数。根据 CT 记录的表观线性衰减系数和 60Co 能量衰减系数，基于理论模型计算 GKS 期间的剂量减少。通过 EVOH 的体模研究进行剂量测量，以与从两个衰减系数生成的估计值进行比较。
根据 CT（keV）表观衰减系数，作者的理论模型预测，由于 NBCA 和 Onyx 的原子序数和密度增加，任一栓塞剂的累积效应都会使栓塞瘤巢中的千伏光子数量减少 8%至 15%。然而，在使用 GKS 中使用的 60Co 能量的有效衰减系数时，作者的理论模型仅导致每束射线减少 0.2%的剂量，总剂量减少<0.01%-0.2%。这些理论结果通过含有 Onyx 的头部体模的测量得到了验证。
由于 AVM 栓塞材料对 60Co 射线的衰减，NBCA 和 EVOH 的剂量减少可以忽略不计，因为 60Co 射线能量较高。
