Department of Medical Physics and Biomedical Engineering, School of Medicine, Tehran University of Medical Sciences, International Campus (TUMS-IC), Tehran, Iran.
Department of Physics, Faculty of Natural and Computational Sciences, Debre Tabor University, Debre Tabor, Ethiopia.
Radiat Environ Biophys. 2020 May;59(2):295-306. doi: 10.1007/s00411-020-00835-0. Epub 2020 Mar 31.
The main aim of this study was to investigate the dosimetric characteristics of the INTRABEAM ® system in the presence of air gaps between the surface of applicators (APs) and tumor bed. Additionally, the effect of tissue heterogeneities was another focus. Investigating the dosimetric characteristics of the INTRABEAM® system is essential to deliver the required dose to the tumor bed correctly and reduce the delivered dose to the ribs and lung. Choosing the correct AP size and fitting it to the lumpectomy cavity is essential to remove the effect of air gaps and avoid inaccurate dose delivery. Consequently, the Geant4 toolkit was used to simulate the INTRABEAM ® system with spherical APs of various sizes. The wall effect of the ion chamber (IC) PTW 34013 used in the present study was checked. The simulations were validated in comparison with measurements, and then used to calculate any inaccuracies in dose delivery in the presence of 4- and 10-mm air gaps between the surface of the APs and the tumor bed. Also, the doses received due to tissue heterogeneities were characterized. It turned out that measurements and simulations were approximately in agreement (± 2%) for all sizes of APs. The perturbation factor introduced by the IC due to differences in graphite-coated polyethylene and air as compared to the phantom material was approximately equal to one for all AP. The greatest relative dose delivery difference was observed for an AP with a diameter of 1.5 cm, i.e., 44% and 70% in the presence of 4- and 10-mm air gaps, respectively. In contrast, the lowest relative dose delivery difference was observed for an AP with a diameter of 5 cm, i.e., 24% and 42% in the presence of 4- and 10-mm air gaps, respectively. Increasing APs size showed a decrease in relative dose delivery difference due to the presence of air gaps. In addition, the undesired dose received by the ribs turned out to be higher when a treatment site closer to the ribs was assumed. The undesired dose received by the ribs increased as the AP size increased. The lung dose turned out to be decreased due to the shielding effect of the ribs, small lung density, and long separation distance from the AP surface.
本研究的主要目的是研究 INTRABEAM ® 系统在施源器（APs）表面与肿瘤床之间存在气隙时的剂量学特性。此外，组织异质性的影响也是另一个关注点。研究 INTRABEAM ® 系统的剂量学特性对于正确向肿瘤床提供所需剂量以及降低肋骨和肺部的剂量非常重要。选择正确的 AP 大小并将其适配于乳房切除术腔对于消除气隙的影响并避免剂量不准确输送至关重要。因此，使用 Geant4 工具包对各种大小的球形 AP 的 INTRABEAM ® 系统进行了模拟。检查了本研究中使用的 PTW 34013 离子室（IC）的壁效应。将模拟结果与测量结果进行了比较验证，然后用于计算在 APs 表面与肿瘤床之间存在 4 和 10mm 气隙时剂量输送的任何不准确之处。此外，还对由于组织异质性而产生的剂量进行了特征描述。结果表明，对于所有 AP 尺寸，测量值和模拟值大致一致（±2%）。与模拟材料相比，由于石墨涂覆的聚乙烯和空气与 IC 之间的差异，引入的扰动因子对于所有 AP 大约等于一。对于直径为 1.5cm 的 AP，观察到最大的相对剂量输送差异，即在存在 4 和 10mm 气隙的情况下，分别为 44%和 70%。相比之下，对于直径为 5cm 的 AP，观察到最低的相对剂量输送差异，即在存在 4 和 10mm 气隙的情况下，分别为 24%和 42%。随着 AP 尺寸的增加，由于气隙的存在，相对剂量输送差异减小。此外，当假设治疗部位更接近肋骨时，肋骨接收到的不期望剂量更高。随着 AP 尺寸的增加，肋骨接收到的不期望剂量增加。由于肋骨的屏蔽效应、肺密度较小以及与 AP 表面的长距离分离，肺剂量减小。
