Yohannes Indra, Prasetio Heru, Bert Christoph
University Hospital Erlangen.
J Appl Clin Med Phys. 2015 Nov 8;16(6):84–90. doi: 10.1120/jacmp.v16i6.5492.
Noncoplanar fields are normally used to improve the dose conformity of the target while sparing organs at risk. One of the methods to verify the dose distribution from the noncoplanar fields is by comparing their planar dose distributions from the treatment planning system (TPS) and the measured ones; for example, using film or electronic portal imaging devices (EPID). The planar dose distributions of the measurement tools, that are normally perpendicular to the central axis of the beam, can be calculated by creating special structures to mimic them in the TPS. With TPS commercially available today, however, it is not easy to create these special structures, especially in the noncoplanar configuration. For this work, we have written in-house scripts in the Pinnacle(3) TPS that can create the structures and define them as virtual planes. These virtual planes can be generated for any arbitrary gantry and couch angles, as well as source to planar distance, so that the planar dose maps at these planes can be computed. Two independent quality assurance (QA) tools were used to validate the planar dose distributions computed using the scripts for three open fields and one IMRT field at several different couch angles. The absolute planar dose patterns measured by the QA tools for all fields at all couch angles were found to be in good agreement, more than 95% (gamma criteria 3% delta dose and 3 mm distance to agreement), with the calculated ones by TPS. The results of this feasibility study can be valuable either for pretreatment dose verification or for in vivo dosimetry that directly implements the planar dose distributions from the TPS, particularly for the noncoplanar fields.
非共面射野通常用于提高靶区的剂量适形度，同时保护危及器官。验证非共面射野剂量分布的方法之一是比较治疗计划系统（TPS）中的平面剂量分布与测量得到的平面剂量分布；例如，使用胶片或电子射野影像装置（EPID）。测量工具的平面剂量分布通常垂直于射束中心轴，可通过在TPS中创建特殊结构来模拟它们进行计算。然而，就目前市面上的TPS而言，创建这些特殊结构并不容易，尤其是在非共面配置中。在本研究中，我们在Pinnacle(3) TPS中编写了内部脚本，可创建这些结构并将其定义为虚拟平面。这些虚拟平面可针对任意机架角和治疗床角以及源到平面的距离生成，从而能够计算这些平面处的平面剂量图。使用两种独立的质量保证（QA）工具，在几个不同的治疗床角下，对使用脚本计算得到的三个开放射野和一个调强放疗（IMRT）射野的平面剂量分布进行验证。发现QA工具在所有治疗床角下对所有射野测量得到的绝对平面剂量模式与TPS计算得到的结果高度一致，一致性超过95%（γ标准为3%剂量差异和3 mm距离一致性）。这项可行性研究的结果对于预处理剂量验证或直接实施TPS平面剂量分布的体内剂量测定都可能具有重要价值，特别是对于非共面射野。
