Department of Radiation Oncology, Fox Chase Cancer Center, Philadelphia, PA 19111, USA.
Phys Med Biol. 2013 Aug 21;58(16):5653-72. doi: 10.1088/0031-9155/58/16/5653. Epub 2013 Jul 29.
A motorized electron multileaf collimator (eMLC) was developed as an add-on device to the Varian linac for delivery of advanced electron beam therapy. It has previously been shown that electron beams collimated by an eMLC have very similar penumbra to those collimated by applicators and cutouts. Thus, manufacturing patient specific cutouts would no longer be necessary, resulting in the reduction of time taken in the cutout fabrication process. Moreover, cutout construction involves handling of toxic materials and exposure to toxic fumes that are usually generated during the process, while the eMLC will be a pollution-free device. However, undulation of the isodose lines is expected due to the finite size of the eMLC. Hence, the provided planned target volume (PTV) shape will not exactly follow the beam's-eye-view of the PTV, but instead will make a stepped approximation to the PTV shape. This may be a problem when the field edge is close to a critical structure. Therefore, in this study the capability of the eMLC to achieve the same clinical outcome as an applicator/cutout combination was investigated based on real patient computed tomographies (CTs). An in-house Monte Carlo based treatment planning system was used for dose calculation using ten patient CTs. For each patient, two plans were generated; one with electron beams collimated using the applicator/cutout combination; and the other plan with beams collimated by the eMLC. Treatment plan quality was compared for each patient based on dose distribution and dose-volume histogram. In order to determine the optimal position of the leaves, the impact of the different leaf positioning strategies was investigated. All plans with both eMLC and cutouts were generated such that 100% of the target volume receives at least 90% of the prescribed dose. Then the percentage difference in dose between both delivery techniques was calculated for all the cases. The difference in the dose received by 10% of the volume of the target was showing a mean percentage difference of 1.57%± 1.65, while the difference in the dose received by 99% of the volume was showing a mean percentage difference of 1.08%± 0.78. The mean percentage volume of Lung receiving a percentage dose equal to or greater than 20% of the prescribed dose was found to be 8.55%± 7.3 and 8.67%± 7 for the eMLC and applicator/cutout combination delivery methods respectively. Results have shown that target coverage and critical structure sparing can be effectively achieved by electron beams collimated with the eMLC. Positioning the eMLC leaves in such a way to avoids shielding any part of the projected treatment volume is most conservative and would be the recommended method to define the actual leaf position for the eMLC defined field. More optimal leaf positions can be achieved in shaping the same treatment field through the interplay of different leaf positioning strategies. We concluded that the eMLC represents an effective time saving and pollution-free device that can completely replace patient specific cutouts.
一种电动电子多叶准直器（eMLC）作为瓦里安直线加速器的附加设备开发，用于提供先进的电子束治疗。以前已经表明，由 eMLC 准直的电子束具有与应用器和切孔准直的电子束非常相似的半影。因此，不再需要制造特定于患者的切孔，从而减少了切孔制造过程中的时间。此外，切孔的构造涉及处理有毒材料和接触在处理过程中通常产生的有毒烟雾，而 eMLC 将是一种无污染的设备。然而，由于 eMLC 的有限尺寸，预计等剂量线会波动。因此，所提供的计划靶区（PTV）形状不会完全遵循 PTV 的光束视线，而是会对 PTV 形状进行阶梯近似。当射野边缘接近关键结构时，这可能会成为一个问题。因此，在这项研究中，基于真实患者的计算机断层扫描（CT），研究了 eMLC 实现与应用器/切孔组合相同临床结果的能力。使用基于内部蒙特卡罗的治疗计划系统使用十个患者的 CT 进行剂量计算。对于每个患者，生成了两个计划；一个使用应用器/切孔组合准直的电子束；另一个计划使用 eMLC 准直的电子束。基于剂量分布和剂量-体积直方图，比较了每个患者的治疗计划质量。为了确定叶片的最佳位置，研究了不同叶片定位策略的影响。为了用 eMLC 和切孔生成所有计划，使得目标体积的 100%接收至少 90%的规定剂量。然后，计算了两种输送技术之间的剂量差异百分比。所有病例均显示，10%目标体积接收的剂量差异平均百分比为 1.57%±1.65，而 99%目标体积接收的剂量差异平均百分比为 1.08%±0.78。结果表明，使用 eMLC 准直的电子束可以有效地实现靶区覆盖和关键结构保护。以避免屏蔽任何部分投影治疗体积的方式定位 eMLC 叶片是最保守的，并且将是定义 eMLC 定义场实际叶片位置的推荐方法。通过不同叶片定位策略的相互作用，可以实现相同治疗场的更优化叶片位置。我们得出结论，eMLC 是一种有效的节省时间和无污染的设备，可以完全替代特定于患者的切孔。
