Kaatee R S, Crezee J, Kanis A P, Lagendijk J J, Levendag P C, Visser A G
Dr Daniel den Hoed Cancer Center/University Hospital Rotterdam Dijkzigyt, Department of Radiation Oncology, The Netherlands.
Phys Med Biol. 1997 Jun;42(6):1087-108. doi: 10.1088/0031-9155/42/6/007.
In interstitial heating one of the main requirements for achieving a certain elevated temperature in a tumour is that the effective power per applicator (Peff), i.e. the power which is actually deposited in the tissue, is sufficiently high. In this paper this requirement is discussed for the applicators of the 27 MHz multielectrode current source (MECS) interstitial hyperthermia (IHT) system. To minimize power reflection, the applicator impedance was matched with the generator impedance by adjusting the length of the coaxial cable in between. Transmission line losses, applicator efficiency and subsequently Peff were computed for several applicator types. The actual Peff per electrode was obtained from calorimetric measurements. Experiments with RC loads, which can be seen as perfect applicators, were performed to investigate the effect of mismatching on Peff. Applicator losses were measured for clinically used applicators, both single- and dual-electrode, utilizing saline phantoms. A simple spherical tumour model, using the effective heat conductivity (keff) to account for heat transport, was used to estimate Peff for a given tumour size, implant size and applicator density. Computations of Peff of various MECS-IHT electrodes were in close agreement with the phantom measurements. Most of the initial generator power was absorbed in the transmission line (60-65%). The efficiency of the applicators was about 65%. For both single- and dual-electrode applicators the effective electrode power was found to be about 1 W. Model calculations show that Peff of 1 W is sufficient to reach a minimum tumour temperature of 43 degrees C in well perfused tumours (keff = 3 W m-1 degree C-1), using a typical implant with 2 cm electrodes and 1.5 cm spacing. Mismatching can considerably affect Peff. Both a reduction to almost zero and a two-fold increase are possible. However, because the matching theory is well understood, mismatching is not a serious problem in clinical practice and can even be used to increase Peff if necessary. We conclude that the applicator design and the impedance matching method chosen in the MECS system allow heating to temperatures in the therapeutic range with implants used in clinical practice.
在间质热疗中，要使肿瘤达到一定的高温，一个主要要求是每个施源器的有效功率（Peff），即实际沉积在组织中的功率，要足够高。本文针对27 MHz多电极电流源（MECS）间质热疗（IHT）系统的施源器讨论了这一要求。为使功率反射最小化，通过调整其间同轴电缆的长度，使施源器阻抗与发生器阻抗匹配。计算了几种施源器类型的传输线损耗、施源器效率以及随后的Peff。每个电极的实际Peff通过量热测量获得。进行了使用RC负载（可视为理想施源器）的实验，以研究失配对Peff的影响。利用盐水仿体测量了临床使用的单电极和双电极施源器的施源器损耗。使用有效热导率（keff）来考虑热传递的简单球形肿瘤模型，用于估计给定肿瘤大小、植入物大小和施源器密度下的Peff。各种MECS - IHT电极的Peff计算结果与仿体测量结果密切吻合。大部分初始发生器功率被传输线吸收（60 - 65%）。施源器的效率约为65%。对于单电极和双电极施源器，有效电极功率均约为1 W。模型计算表明，对于灌注良好的肿瘤（keff = 3 W m-1℃-1），使用电极长度为2 cm、间距为1.5 cm的典型植入物，1 W的Peff足以使肿瘤达到最低温度43℃。失配会对Peff产生显著影响。Peff既可能降低到几乎为零，也可能增加两倍。然而，由于匹配理论已被充分理解，失配在临床实践中并非严重问题，必要时甚至可用于增加Peff。我们得出结论，MECS系统中选择的施源器设计和阻抗匹配方法能够使用临床实践中使用的植入物将温度加热到治疗范围。
