Aim: 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.
Material and methods: 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.
Results and discussion: 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.
Conclusions: 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.