Department of Radiation Oncology, Lahey Hospital and Medical Center, Beth Israel Lahey Health, Burlington, MA, USA.
J Appl Clin Med Phys. 2021 Mar;22(3):261-272. doi: 10.1002/acm2.13174. Epub 2021 Feb 18.
We have determined the optimal method for modeling kyphoplasty cement to enable accurate dose calculations in the Eclipse treatment planning system (TPS). The cement studied (Medtronic Kyphon HV-R®) consists of 30% Barium, 68% polymethylmethacrylate (PMMA), and 2% benzoyl peroxide, formulated to be radiopaque with kV imaging systems. Neither Barium nor PMMA have a high physical density, resulting in different interaction characteristics for megavoltage treatment beams compared to kV imaging systems. This can lead to significant calculation errors if density mapping is performed using a standard CT number to density curve. To properly characterize the cement for dose calculation, we 3D printed a hemi-cylindrical container to fit adjacent to a micro-chamber insert for an anthropomorphic phantom, and filled the container with Kyphon cement. We CT scanned the combination, modeled the cement with multiple material assignments in the TPS, designed plans with different field sizes and beam geometry for five photon modes, and measured the doses for all plans. All photon energies show significant error in calculated dose when the cement is modeled based on the CT number. Of the material assignments we evaluated, polytetrafluoroethylene (PTFE) showed the best overall agreement with measurement. Calculated and measured doses agree within 3.5% for a 340-degree arc technique (which averages transmission and scatter effects) with the Acuros XB algorithm and PTFE as the assigned material. To confirm that PTFE is a reasonable substitute for kyphoplasty cement, we performed measurements in a slab phantom using rectangular inserts of cement and PTFE, showing average agreement of all photon modes within 2%. Based on these findings, we conclude that the PTFE material assignment provides acceptable dose calculation accuracy for the AAA and Acuros XB photon algorithms in the Eclipse TPS. We recommend that the cement be delineated as a structure and assigned the PTFE material for accurate dose calculation.
我们已经确定了为经皮椎体后凸成形术骨水泥建模的最佳方法，以在 Eclipse 治疗计划系统（TPS）中实现准确的剂量计算。所研究的骨水泥（美敦力 Kyphon HV-R®）由 30%的钡、68%的聚甲基丙烯酸甲酯（PMMA）和 2%的过氧化苯甲酰组成，配方为在千伏成像系统中具有放射性。钡和 PMMA 的实际密度都不高，与千伏成像系统相比，对兆伏治疗束有不同的相互作用特征。如果使用标准 CT 数到密度曲线进行密度映射，这可能会导致显著的计算误差。为了正确描述骨水泥进行剂量计算，我们 3D 打印了一个半圆柱形容器，以适合毗邻人体模型中的微室插件，并在容器中填充 Kyphon 骨水泥。我们对组合进行 CT 扫描，在 TPS 中使用多个材料分配对骨水泥进行建模，为五种光子模式设计了不同射野大小和射束几何形状的计划，并测量了所有计划的剂量。所有光子能量在根据 CT 数对骨水泥进行建模时，计算剂量都存在显著误差。在我们评估的材料分配中，聚四氟乙烯（PTFE）与测量结果的总体一致性最好。使用 Acuros XB 算法和 PTFE 作为分配材料，对于 340 度弧形技术（平均传输和散射效应），计算剂量和测量剂量的一致性在 3.5%以内。为了确认 PTFE 是经皮椎体后凸成形术骨水泥的合理替代品，我们在平板模型中使用水泥和 PTFE 的矩形插件进行了测量，所有光子模式的平均一致性在 2%以内。基于这些发现，我们得出结论，在 Eclipse TPS 中，PTFE 材料分配为 AAA 和 Acuros XB 光子算法提供了可接受的剂量计算准确性。我们建议将骨水泥描绘为一种结构，并将 PTFE 材料分配给它，以进行准确的剂量计算。
