Marini P, Guiot C, Baiotto B, Gabriele P
Dipartimento di Neuroscienze, Università degli Studi, Turin, Italy.
Radiol Med. 2001 Sep;102(3):159-67.
The bolus (silicon + deionized water) is used in order to achieve a good electromagnetic match between the biological tissue and the applicator surface and to locally modify the skin temperature, to regulate it based on the patient's sensitivity. Therefore, it is very important to know the electromagnetic power deposition and how the bolus can induce a variation in the thermal map, both superficially and deeply. The aim of this study is to evaluate the applicator characteristics in the same conditions as those indicated by the ESHO guidelines for clinical practice and to investigate the penetration depth in the absence and in the presence of a very effective circulating bolus. The first condition is used by the manufacturer to characterize MW applicator while the second condition is used by medical equipes delivering HT treatments.
The specific absorption rate (SAR) generated by a microwave planar applicator (H2, CFMA, Istok,Russia) operating at 433 MHz connected with the hyperthermic equipment ALBA (Restek, Italy) was studied in a muscle-equivalent polyacrilamide phantom. IsoSAR lines were detected on a liquid crystal sheet with a video camera. Images analysis was performed using the graphic software Paint Shop Pro 6. This method allows to evaluate PD in the phantom under the plane of MW applicator. The hyperthermic equipment ALBA is used in the Radiotherapy Unit of the Mauriziano Umberto I Hospital in Turin for both oncologic and psychiatric treatments.
The actual pattern of measured SAR mainly depends on the applicator and the phantom characteristics, but also the thickness and the efficiency of the bolus inserted between the applicator and the patient's skin have been shown to affect the results. In this study the penetration depth was measured on 5 applicator sections both with non circulating bolus (static) and with circulating bolus (dynamic). The penetration depth is reduced with the dynamic bolus with respect to the static bolus in all sections, and specifically: in section I it is reduced from 19.46 +/- 1.49 mm to 17.22 +/- 0.71 mm; in section II from 38.17 +/- 4.77 mm to 26.91 +/- 1.48 mm; in section III from 39.81 +/- 3.24 mm to 30.38+/- 4.56 mm; in section IV from 42.12 +/- 1.67 mm to 33.11 +/- 1.89 mm; and finally in section V from 39.83 +/- 4.14 mm to 31.064 +/- 1.57 mm. The above reduction ranges between 11.5 and 29 % with an average value of 21.64 % Our results suggest that different conditions at the interface between bolus and phantom resulting from the use of a dynamic or static bolus produce considerable changes on the heated volume dimension and on the SAR local pattern. The effect of the convective heat losses due to the circulating bolus' assessed by our measurements is not negligible and should be taken into account when evaluating the SAR distribution before clinical treatments.
To disregard the heat losses due to a circulating bolus in MW applicator characterization can severely affect the SAR estimation. This may have serious consequences on clinical applications, since the temperature in the heated lesion can be lower than expected, and the clinical effectiveness of the therapeutical session can be therefore severely reduced. Since at the moment inexpensive and accurate non-invasive temperature monitoring systems are not available, a sound knowledge of the PD of the MW applicator is of primary importance for clinical applications and for a treatment plan allowing to deliver customized treatment to each patient.
使用团注物（硅 + 去离子水）以实现生物组织与施照器表面之间良好的电磁匹配，并局部改变皮肤温度，根据患者的敏感度对其进行调节。因此，了解电磁功率沉积以及团注物如何在浅表和深部引起热图变化非常重要。本研究的目的是在与欧洲热疗肿瘤学会（ESHO）临床实践指南所规定的相同条件下评估施照器的特性，并研究在不存在和存在非常有效的循环团注物的情况下的穿透深度。第一种情况被制造商用于表征微波施照器，而第二种情况被提供热疗治疗的医疗设备所采用。
在肌肉等效聚丙烯酰胺体模中研究了与热疗设备ALBA（意大利Restek公司）相连的、工作频率为433 MHz的微波平面施照器（俄罗斯Istok公司的H2，CFMA）产生的比吸收率（SAR）。用摄像机在液晶片上检测等SAR线。使用图形软件Paint Shop Pro 6进行图像分析。该方法可评估微波施照器平面下方体模中的功率沉积。热疗设备ALBA在都灵的毛里齐亚诺·翁贝托一世医院放疗科用于肿瘤和精神疾病治疗。
实测SAR的实际模式主要取决于施照器和体模的特性，但施照器与患者皮肤之间插入的团注物的厚度和效率也已显示会影响结果。在本研究中，在5个施照器截面处测量了无循环团注物（静态）和有循环团注物（动态）时的穿透深度。在所有截面中，动态团注物相对于静态团注物的穿透深度均减小，具体如下：在第I截面中，从19.46±1.49 mm减小到17.22±0.71 mm；在第II截面中，从38.17±4.77 mm减小到26.91±1.48 mm；在第III截面中，从39.81±3.24 mm减小到30.38±4.56 mm；在第IV截面中，从42.12±1.67 mm减小到33.11±1.89 mm；最后在第V截面中，从39.83±4.14 mm减小到31.064±1.57 mm。上述减小范围在11.5%至29%之间，平均值为21.64%。我们的结果表明，使用动态或静态团注物导致团注物与体模界面处的不同条件会对加热体积尺寸和SAR局部模式产生相当大的变化。我们的测量评估了循环团注物引起的对流热损失的影响不可忽略，在临床治疗前评估SAR分布时应予以考虑。
在微波施照器特性表征中忽略循环团注物引起的热损失会严重影响SAR估计。这可能对临床应用产生严重后果，因为加热病变部位的温度可能低于预期，从而治疗疗程的临床有效性可能会严重降低。由于目前尚无廉价且准确的非侵入性温度监测系统，对微波施照器的功率沉积有充分了解对于临床应用以及制定允许为每个患者提供定制治疗的治疗计划至关重要。
