Australas Phys Eng Sci Med (2014) 37:691–703 DOI 10.1007/s13246-014-0306-5
SCIENTIFIC PAPER
Enhancing the properties of beam forming bolus in hyperthermia: numerical simulation and empirical verification Seyed Ali Aghayan • Dariush Sardari • Seied Rabi Mehdi Mahdavi • Maryam Mohammadi
Received: 25 May 2014 / Accepted: 8 October 2014 / Published online: 16 October 2014 Australasian College of Physical Scientists and Engineers in Medicine 2014
Abstract In this paper we present a simulation study of the induced specific absorption rate (SAR) within the phantom produced by radiofrequency radiation from a 8 MHz capacitive applicator. The main focus of the current study is on demonstrating the beam shaping properties of the bolus system as well as its effect on controlling the therapeutic area. Different electrical conductivities and geometries of the bolus were considered in the simulation of induced SAR distributions in a muscle-equivalent model with uniform dielectric properties. To validate the presented model, we carried out a comparison between the SAR simulation results and the temperature measurements in an agar split-phantom and an excellent agreement was observed. Keywords Absorbing bolus Beam forming Hyperthermia SAR Simulation Temperature Therapeutic area
S. A. Aghayan (&) D. Sardari Department of Engineering, Science and Research Branch, Islamic Azad University, 14155-775, Tehran 14778 93855, Islamic Republic of Iran e-mail: [email protected] S. R. M. Mahdavi Department of Medical Physics, Tehran University of Medical Science, Tehran, Islamic Republic of Iran M. Mohammadi Department of Physics, Shahrood Branch, Islamic Azad University, Shahrood, Iran
Introduction Hyperthermia is a procedure for treating cancer in which the tissue temperatures in the tumor are raised to the range of 42–45 C [1]. A major challenge for hyperthermia technology is to produce uniformly elevated temperatures throughout a tumor volume [2]. Temperature homogeneity in tissue during treatment is determined by several factors that include the radiation pattern of the heating applicator, variability in the composition of tissue which determines absorption rate of energy and transfer of thermal energy due to conduction and blood flow [3]. Although the bolus is an essential part of the complete hyperthermia set-up, its influence on the resulting SAR distribution and the final performance of the electromagnetic (EM) applicator generally receives limited attention [4]. The use of absorbing structures with the view of reducing the treatment limiting effects of hot spots was first introduced by De Leeuw et al. [5]. Detailed analysis of the effects of an absorbing structure, using simulation tools, might lead to improvement of its efficiency by modification of shape and/or composition [6]. In this paper a simulation method for determining the induced SAR patterns of RF capacitive applicator inside the tissue is discussed. To demonstrate the role of the bolus design in the SAR focusing, two types of bolus configurations were examined: the conventional bolus and the absorbing one. In the latter one an additional L-form patch is inserted inside the conventional bolus. Much work has been reported on determining the SAR distributions with the aim of improving the bolus performance in the microwave hyperthermia. In these studies either the numerical or the experimental methods have been utilized. Van Rhoon et al. [7] noted the potential effect of water bolus size and shape on the resulting SAR distribution. Their measurements indicated the possibility that advantages of the Lucite
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Fig. 1 a Model used for simulation; b Model of the ‘absorbing bolus’ in HFSS
Table 1 Dielectric properties used for simulation at 8 MHz [21, 22] Material type
Electrical conductivity (S m-1) Relative permittivity
Muscle phantom 0.62
81
Saline (salinity 5.5 g l-1)
Saline (salinity 16 g l-1)
Saline (salinity 26.5 g l-1)
1
3
5
75
71.5
68
Fig. 2 Set up of verification measurement in phantom
cone applicator may be counteracted by the application of an unfavorable water bolus configuration. De Bruinje et al. [4] presented the effects of water bolus dimensions and configuration on the effective field size of the Lucite cone applicator for superficial hyperthermia. Shearer et al. [8] have attempted to increase the heating area by modifying the coupling water bolus used with microwave waveguide applicators. This was accomplished by introducing a centrally located saline-filled ellipsoidal absorbing patch into
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the bolus. This design was later extended empirically to include an array of smaller microwave absorbing pads filled with saline solution [9]. Sherar et al. [10] also described an optimization procedure based on the finite element method with edge-elements to perform three-dimensional simulations of this bolus design in order to improve its performance. Kumaradas and Sherar [3] developed a prototype dynamically modifiable square array of saline filled patches
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Fig. 3 The effect of electrical conductivity and thickness of the saline pad on the SAR pattern in the transversal plane located at z = 0.03 m
which attenuate microwave energy for superficial treatments that use external microwave applicators. Although there is much work on the bolus performance in microwave hyperthermia, there is only few work
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Fig. 4 The effect of electrical conductivity and thickness of the saline pad on the SAR pattern in the transversal plane located at z = 0.04 m
addressing this issue in RF capacitive hyperthermia. Michiyama and Nikawa [11] considered a phantom and a human model in the SAR simulations to optimize the
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Table 2 Summary of the results obtained for Area80 and the corresponding SAR values in both bolus configurations
r = 5 S m-1 Dd (m)
0.005
2
0.005
19.4
19.8
21.3
Saline Pad
2
80 % SARmax(W kg-1)
18.7
-1
Area80 (%)
40
80
70
90
Absorbing bolus
2
0.002
2
0.002
80 % SARmax(W kg-1)
34.1
38.3
36.6
39.6
Area80 (%)
5
70
5
50
12
6 σ effect z=0.04m
Thicness effect z=0.04m σ effect z=0.03m Thicness effect z=0.03m
5
10
4
8
3
6
2
4
1
0.01
0.015
0.02
0.025
0.03
Thickness ( × 10-3m)
Fig. 5 Normalized Area80 versus electrical conductivity and thickness of the saline pad in the transversal plane located at z = 0.03 m and z = 0.04 m
Thickness = 0.005m Dr (S m-1)
r=5Sm Dda (m)
-1
Variation in thickness
Z = 0.04 m
Thickness = 0.005m Dr (S m-1)
Electrical Conductivity (S.m )
a
Z = 0.03 m
2
Normalized Area 80
radius and location of the electrodes during RF heating. Kroeze et al. [6] investigated the use of solid absorber blocks to reduce peripheral hot spots with the aid of numerical simulation tools. The influence of the size and the salinity of the rectangular solid absorbers on the SAR distribution were discussed. Computational models of power deposition patterns and temperature distributions in human bodies have helped researchers to understand and improve hyperthermia treatments [12–18]. The simulation method developed in this paper addresses some parameters concerning in both target bolus designs including electrical conductivity of bolus and its thickness. These parameters affect the transmitted component of electrical field which plays an important role in deposition of power inside the phantom. Several series of induced SAR simulations were performed based on the variation of each parameter in both bolus designs. The homogenous model was considered in the current study. This model allows us to compare numerical simulations directly with the simple phantom models that can be constructed and tested in the laboratory [19]. Although, in this study there is not any intent to simulate the temperature, we can take an advantage of
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knowing the SAR profiles as an indication of where the temperature may increase throughout the phantom. The main focus of the previous studies on the performance of the bolus in the RF capacitive hyperthermia was mainly based on theories and simulations. However, this paper collated the simulation result with experimental measurement in a muscle-equivalent phantom. It is indicated that the bolus system configuration can be one of the key elements determining the quality of the RF capacitive hyperthermia of an arbitrary target tumors. The promising results show the bolus ability in controlling the SAR profile in tissue. It can also give the possibility to predict whether there is a successful treatment for the tumor shape. In addition, a capability for comparing the effectiveness of different bolus designs on any tumor shape before the treatment will be provided. The present paper is organized as follows: an explanation of the model and the proposed method for improving the beam shaping feature of bolus, a discussion on the output of the performed SAR simulations, empirical verifications of the simulations and finally an overall conclusion and some suggestions for further research.
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SAR ¼
r E2 ; q
ð1Þ
where r is the electrical conductivity of the medium and E is the local electric field. Following the calculation of the SAR, the temperature distribution inside the tissue heated by EM energy could be obtained by using the bioheat equation: qC
oT ¼ rðkrT Þ þ xbl qbl Cbl ðTa T Þ þ Qm þ q SAR, ot ð2Þ
Fig. 6 Comparison between the contours representing Area80 in the transversal plane located at a z = 0.03 m, b z = 0.04 m. Solid line thickness of pad = 0.005 m, r = 3 S m-1, dashed line thickness of pad = 0.005 m, r = 5 S m-1, dotted line thickness of pad = 0.01 m, r = 5 S m-1
Materials and methods In this study the characteristics of RF heating is considered in terms of induced SAR. The power dissipated by the RF beam into the tissue is known as SAR. The SAR (units of W kg-1) is the more useful concept in our work and is given by:
where T(x, y, z, t) is the temperature of the tissue (C); Ta is the arterial temperature (C); k is the thermal conductivity of tissue (W m-1 C-1); Cbl is the Heat capacity of blood (J kg-1 C-1); C is the heat capacity of tissue (J kg-1 C-1); qbl is the density of blood (kg m-3); q is the density of tissue (kg m-3); xbl is the blood perfusion (ml s-1 ml); Qm is the tissue metabolism (W m-3). The simulation package from Ansoft Corporation includes High Frequency Structure Simulator (HFSS) for EM Simulation. HFSS is a powerful finite electromagnetic (FEM) solver for EM problems. The SAR patterns were determined from the HFSS simulations. In this work, the homogeneous muscle-equivalent model with uniform dielectric properties is considered. The electrodes are set outside of the phantom model. The bolus system which cools the phantom’s surface and produces the impedance matching, consists of saline which is embedded in a thin casing silicone layer. Figure 1a shows the model with square aluminum electrodes and the bolus system. The dielectric properties of the model are summarized in Table 1. During the simulation procedure, two different bolus configurations were analyzed: The first design which is referred to ‘saline pad’ bolus, has the uniform salinity and the second one is referred to ‘absorbing bolus’. The latter design, compared to the first one, has an additional L-form saline patch and could be filled individually with appropriate salinity, Fig. 1b. The objective of the ‘absorbing bolus’ is analyzing the ability of a specific bolus configuration to shape the SAR pattern to the arbitrary target region. Saline absorbs RF energy and causes the L-form patch to attenuate the RF energy passing through it. The electrical properties of the target ‘L’-shaped region were assumed as same as the native tissue. Moreover, the effect of electrical conductivity and thickness is also investigated. Therefore, three different parameters were varied during this study: the bolus system configuration, the electrical conductivity and the thickness of saline. It has been shown previously [20] that the maximum of temperature rise is most likely to occur at 0.02–0.03 m below the surface of the phantom. Therefore, the different transversal planes located at this depth are considered in our study.
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Fig. 7 The effect of electrical conductivity and thickness of the L-form patch on the SAR pattern in the transversal plane located at z = 0.03 m
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Fig. 8 The effect of electrical conductivity and thickness of the L-form patch on the SAR pattern in the transversal plane located at z = 0.04 m
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9 σ
5
7
3
5
1
0.01
0.015
0.02
0.025
0.03
Thickness ( × 10-3m)
effect z=0.04m Thickness effect z=0.04m σ effect z=0.03m Thickness effect z=0.03m
-1
Electrical Conductivity (S.m )
Fig. 9 Normalized Area80 versus electrical conductivity and thickness of the saline patch in the transversal plane located at z = 0.03 m and z = 0.04 m
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3
Normalized Area80 Fig. 10 Comparison between the contours representing Area80 in the c transversal plane located at a z = 0.03 m, b z = 0.04 m. Solid line thickness of patch = 0.005 m, r = 3 S m-1, dashed line thickness of patch = 0.005 m, r = 5 S m-1, dotted line thickness of patch = 0.007 m, r = 5 S m-1
Experimental verification To confirm the validity of the SAR simulation results, the temperature measurements were made through the phantom after radiation absorption. An experimental setup as shown in Fig. 2, was prepared. A rectangular muscleequivalent agar phantom of 0.12 9 0.12 9 0.12 m3, composed of water (95.66 %), NaN3 (0.1 %), NaCl (0.24 %) and agar (4 %) was prepared [21]. To evaluate the validity of the SAR simulations, the phantom was then split into the ‘planes of interest’ which represent the desired planes in which the thermometry is performed. To do this, two different planes at z = 0.03 m and z = 0.04 m below the surface of the phantom were chosen. The phantom has approximately the same electrical properties at room temperature as the corresponding tissue at body temperature. The applicator which has been made of an ultrafast MOSFET driver (DEIC420) operating at 8 MHz and 50 W power is supplied by half size clock oscillator (XO-52). The square electrodes made of aluminum with 0.06 m in side length were prepared to connect to the RF generator through coaxial wires. The interaction between the electric field (generated between the parallel-opposed electrodes) and the material results in a heat distribution throughout the phantom. Experiments were separated by sufficient time intervals in order to return back to baseline temperature (20 C ± 1 C). The experimental bolus systems were constructed as previously described. The ‘saline pad’ bolus consists of two 0.12 9 0.12 m2 thin silicone pads. The pads which
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698 Fig. 11 Comparison between the SAR pattern at z = 0.03 m and the c measured temperatures in the corresponding transversal plane: a thickness of pad = 0.005 m, r = 3 S m-1, b thickness of pad = 0.005 m, r = 5 S m-1, c thickness of pad = 0.01 m, r = 5 S m-1
could measure up to 0.01 m in thickness when filled with saline, were connected to the circulating system via the inflow and the out-flow connectors at the edges. Temperature of the cooling saline was kept constant at 10 C. On the other hand, the general structure of the ‘absorbing bolus’ was similar to the ‘saline pad’ one, except that it has an additional L-form saline patch as shown schematically in Fig. 1b. To prevent mixture of NaCl to the surrounding saline, the L-form patch was wrapped in a thin silicone cover and was secured into the ‘saline pad’ bolus using its in-flow and out-flow connectors. The ‘absorbing bolus’ was constructed to change the RF beam shape, thus the shape, the thickness and the saline concentration of the additional patch determine the SAR pattern inside the phantom. The phantom was surrounded by bolus system and electrodes in such a way that they were parallel to the ‘planes of interest’. After exposure of 8 MHz radiation, the segments of the phantom were immediately removed and then the temperature in the ‘planes of interest’ was measured using a portable infrared thermometer K5500 with an accuracy of 0.1 C. Moreover, a thermometry system (Extech 421509) consists of Teflon-coated thermocouples was used to control the total time of the process.
Results Simulation results of SAR distributions In order to present a quantitative analysis, we define a percentage Area80 as the relative increase of the contour area where the SAR is 80 % of its maximum amount in each pair of the following figures. In this section, simulated SAR profiles inside the model at frequency of 8 MHz with two different bolus configurations are presented with the aim of evaluating the bolus system performance. The bolus system in both cases was placed between the electrodes and the phantom. Firstly, the ‘saline pad’ bolus was employed in the SAR simulation. Figures 3 and 4 show the SAR patterns produced from heating the phantom with ‘saline pad’ bolus in transversal planes located at z = 0.03 m and z = 0.04 m of the phantom, respectively. As was found, utilizing the ‘saline pad’ bolus results in a Gaussian SAR pattern. Figure 3a and b shows the results of SAR patterns obtained from introducing a 0.005 m thick ‘saline pad’ with two different electrical conductivities of 3 and 5 S m-1 in transversal plane located at z = 0.03 m of the
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Fig. 12 Comparison between the SAR pattern at z = 0.04 m and the c measured temperatures in the corresponding transversal plane: a thickness of pad = 0.005 m, r = 3 S m-1, b thickness of pad = 0.005 m, r = 5 S m-1, c thickness of pad = 0.01 m, r = 5 S m-1
phantom. The calculated Area80 among these figures is 40 % which reveals the increase of therapeutic area with increasing the electrical conductivity. The same procedure was repeated in order to see the effect of electrical conductivity in transversal plane located at z = 0.04 m. Figure 4a and b indicates the similar results revealed from Fig. 3a and b except the amount of Area80 which becomes 70 % this time. Therefore, increasing the electrical conductivity has the desired effect of flattening out on the therapeutic area. Shown in Figs. 3b, c and 4b, c are the effect of introducing a saline pad with two different thicknesses of 0.005 m and 0.01 m on the SAR profiles in the transversal planes located at z = 0.03 m and z = 0.04 m, respectively. The electrical conductivity of saline was 5 S m-1 in these series of simulations. Results of Area80 value obtained from SAR patterns at z = 0.03 m and z = 0.04 m are 80 % and 90 %, respectively. Calculation of Area80 indicates that decreasing the thickness of saline pad makes the therapeutic area larger. The computed results of Area80 (which were obtained from the variation in either the electrical conductivity or the thickness of saline.) as well as the corresponding SAR values at z = 0.03 m and z = 0.04 m are summarized in Table 2. It is noticed that decreasing the pad thickness as well as increasing the electrical conductivity of saline has a similar effect on the effective therapeutic area. However, a comparison among Area80 values indicates that the effect of saline thickness is more pronounced. In Fig. 5 the normalized values of Area80 in each case study are plotted against the electrical conductivity and the thickness of saline. In addition, it could be extended the concept of to Area80 to more than two contours. To provide a comprehensive representation of Area80, the contours corresponding to 80 % SARmax in all case studies at z = 0.03 m and z = 0.04 m are compared in Fig. 6a and b, respectively. Obviously from the figures, we can conclude that increasing the electrical conductivity and decreasing the thickness of bolus make the therapeutic area larger. In the second series of simulations, we wish to expose an L-form target tumor, thus the ‘absorbing bolus’ was replaced with the ‘saline pad’ bolus. The objective of the ‘absorbing bolus’ is to change the Gaussian SAR pattern into an L-form one and to shape the therapeutic area with respect to the tumor shape. The simulated SAR patterns produced from heating the phantom with ‘absorbing bolus’ in the transversal planes located at z = 0.03 m and
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700 Fig. 13 Comparison between the SAR pattern at z = 0.03 m and the c measured temperatures in the corresponding transversal plane: a thickness of patch = 0.005 m, r = 3 S m-1, b thickness of patch = 0.005 m, r = 5 S m-1, c thickness of patch = 0.007 m, r = 5 S m-1
z = 0.04 m of the phantom are depicted in Figs. 7 and 8, respectively. Clearly, utilizing the ‘absorbing bolus’ has the desired effect of shaping the therapeutic area. Figures 7a, b and 8a, b investigate the effect of patch electrical conductivity on the produced SAR patterns at depths z = 0.03 m and z = 0.04 m of the phantom, respectively. In this series of simulations, a 0.005 m thick L-form patch with two different electrical conductivities of 3 and 5 S m-1 was introduced within a 0.01 m thick saline pad with the electrical conductivity of 1 S m-1. The computed Area80 value becomes 5 % in both cases. Therefore, it is found that as the electrical conductivity of the additional patch increases, the therapeutic area increases as well. Moreover, for different patch thicknesses, the simulated SAR profiles at z = 0.03 m and z = 0.04 m of the phantom are shown in Figs. 7b, c and 8b, c, respectively. In this series of simulations, an L-form patch with two different thicknesses of 0.005 m and 0.007 m was employed. The electrical conductivity of patch was 5 S m-1. The calculated Area80 values at z = 0.03 m and z = 0.04 m become 70 and 50 %, respectively. Therefore, reduction of the patch thickness has the desired effect of flattening out on the therapeutic area. Table 2 summarizes the computed results of Area80 (which were obtained from the variation in either the electrical conductivity or the thickness of saline.) and the corresponding SAR values at z = 0.03 m and z = 0.04 m. It is indicated that raising the electrical conductivity of saline and reducing the pad thickness has a similar influence on the effective therapeutic area. However, comparing Area80 values shows that the influence of saline thickness is more significant. The normalized values of Area80 in each case study in terms of the electrical conductivity and the thickness of saline are shown in Fig. 9. Moreover, the comparison between the contours corresponding to 80 % SARmax in each case study at z = 0.03 m and z = 0.04 m are indicated in Fig. 10a and b, respectively. It is clear from the figures that the increase in electrical conductivity and the decrease in thickness of the bolus will flat out the therapeutic area. Experimental results of heating the phantom In the following, the accuracy of the presented simulations was assessed through several experiments. As we know, the temperature variation inside the phantom could be estimated from the SAR values according to [23]:
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Fig. 14 Comparison between the SAR pattern at z = 0.04 m and the c measured temperatures in the corresponding transversal plane: a thickness of patch = 0.005 m, r = 3 S m-1, b thickness of patch = 0.005 m, r = 5 S m-1, c thickness of patch = 0.007 m, r = 5 S m-1
SAR ¼ C
DT ; Dt
ð3Þ
where C is the heat capacity of the phantom (which is 4.2 9 103 J kg-1 C-1 in our model [21] ) and Dt is the period of heating. Thus, the simulated SAR values could be regarded as a determining factor for temperature increase inside the phantom exposed to an external source. In the following figures, the measured temperatures and the corresponding SAR patterns are comprised. In first series of experiments performed with ‘saline pad’, the phantom was placed between the pads. After 1,200 s of exposure to 8 MHz radiation, the temperature along the x and y axis of the ‘planes of interest’ were measured in every point with equal 0.01 m distance. These directions were chosen due to the symmetry consideration related to Gaussian SAR patterns. The effect of electrical conductivity and thickness of saline on the measured temperature were examined in the following way. First, the effect of the saline electrical conductivity was investigated. Two pairs of saline pads with 0.005 m thickness were filled with NaCl solution. The salinity of one pair was 16 g l-1 and the other one was 26.5 g l-1 corresponding to the electrical conductivities of 3 and 5 S m-1, respectively [22]. After exposure, temperature were measured in the planes located at z = 0.03 m and z = 0.04 m of the phantom. Results together with the relevant SAR patterns are compared in Figs. 11a, b and 12a, b, respectively. It is observed that the temperature variations are in a good agreement with the SAR variations. Second, to see the effect of saline thickness, two pairs of saline pads with salinity of 26.5 g l-1 were constructed. The thickness of one pair was set to 0.005 m and the other one to 0.01 m. A comparison between the results of temperature readings in the planes located at z = 0.03 m and z = 0.04 m of the phantom and the corresponding SAR patterns is visualized in Figs. 11b, c and 12b, c. In another series of experiments, the accuracy of the SAR simulations obtained from ‘absorbing bolus’ were examined. The phantom was placed between the pads and after 600 s of exposure to 8 MHz radiation, the temperature measurements were carried out with equal spacing in the directions shown in the following figures. The L-form patch is also indicated in the following figures. First, similar to the strategy of the ‘saline pad’ experiments, the effect of the electrical conductivity of the L-form patch was considered. A 0.01 m thick bolus pad containing NaCl
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solution with the salinity of 5.5 g l-1 corresponding to the electrical conductivity of 1 S m-1 [22] was prepared. Then, one pair of L-form patch was filled with 16 g l-1 NaCl solution and the salinity of the other one was set to 26.5 g l-1. The thickness of both pairs of patches was 0.005 m. Shown in Figs. 13a, b and 14a, b is a comparison between the SAR profiles and the corresponding temperature measurements in the planes located at z = 0.03 m and z = 0.04 m of the phantom, respectively. Second, in order to examine the effect of the patch thickness on the measured temperatures, two pairs of patches were filled with NaCl solution with salinity of 26.5 g l-1 until one pair reached to the thickness of 0.005 m and the other one to 0.007 m. These patches were then set inside a 10 mm thick bolus pad containing NaCl solution with the salinity of 5.5 g l-1. A comparison between temperature measurements in the planes located at z = 0.03 m and z = 0.04 m and the corresponding SAR patterns are shown in Figs. 13b, c and 14b, c, respectively. It is clear from the presented results, that all measurements and the simulations have the same reasonable variations. Here, the temperature inside the phantom was not simulated. However, the SAR patterns can indicate the temperature variation throughout the phantom. Our empirical results confirm that computer-aided simulation of induced SAR distribution is a reliable and accurate way to provide the clinical operation. The geometry of the tumor should be first scanned using an MRI or CT or other noninvasive imaging techniques. Then, the best configuration of the additional patches could be examined to predict whether there is a successful treatment. In addition, a capability for comparing the effectiveness of different bolus designs on any tumor shape before the treatment will be provided.
Conclusion This work describes the design of a modified bolus that uses an absorbing patch to improve the SAR patterns of 8 MHz capacitive applicator. The absorber developed here is a saline-filled patch situated inside the bolus system. The conventional boluses, regardless to the tumor shape, produce approximately the large and uniform therapeutic areas. However, saline absorbs a larger fraction of the RF energy flowing through the patch. Thus, the ‘absorbing bolus’ provides a spatial control of the RF beam profile inside the patch and any arbitrary shaped tumor. We show the possibility of controlling the therapeutic area according to the tumor shape through modification of the absorber geometry, i.e. the shape and the thickness of the additional patch as well as its electrical conductivity. Moreover, because the simulated SAR results can determine the temperature increase inside the phantom, the accuracy of
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the model was assessed through several series of empirical temperature measurements in a split-muscle-equivalent phantom. It is confirmed that the temperature changes inside the phantom closely reflect the reasonable agreement with the results of the simulated SAR as predicted by the model. In the perspective of the proposed model, we believe that the ‘‘absorbing bolus’’ could be made of the dynamically additional solid or liquid patches which reasonably must resemble the tumor shape and placed inside the bolus system or between the applicator and the bolus. This model could also be coupled into a proper optimization algorithm to achieve the best choice of both electrical and thermal properties of the additional patches. Regarding to the optimization procedure, one could also quantitatively determine the efficiency of the beam forming performance of the proposed ‘‘absorbing bolus’’ configuration for a given tumor shape. In this respect, we suggest that it would be necessary to consider two main factors in the SAR pattern simultaneously: the percentage of the tumor region covered by Area80 and the uniformity of SAR contour inside the tumor region. These factors could be calculated and then compared to those obtained in the presence of ‘‘saline pad’’. As a result, the desired SAR patterns which have the greatest compatibility with a given tumor shape could be obtained. Therefore, the proposed model shows great promise for further research in both model and clinical performance.
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SCIENTIFIC PAPER
Enhancing the properties of beam forming bolus in hyperthermia: numerical simulation and empirical verification Seyed Ali Aghayan • Dariush Sardari • Seied Rabi Mehdi Mahdavi • Maryam Mohammadi
Received: 25 May 2014 / Accepted: 8 October 2014 / Published online: 16 October 2014 Australasian College of Physical Scientists and Engineers in Medicine 2014
Abstract In this paper we present a simulation study of the induced specific absorption rate (SAR) within the phantom produced by radiofrequency radiation from a 8 MHz capacitive applicator. The main focus of the current study is on demonstrating the beam shaping properties of the bolus system as well as its effect on controlling the therapeutic area. Different electrical conductivities and geometries of the bolus were considered in the simulation of induced SAR distributions in a muscle-equivalent model with uniform dielectric properties. To validate the presented model, we carried out a comparison between the SAR simulation results and the temperature measurements in an agar split-phantom and an excellent agreement was observed. Keywords Absorbing bolus Beam forming Hyperthermia SAR Simulation Temperature Therapeutic area
S. A. Aghayan (&) D. Sardari Department of Engineering, Science and Research Branch, Islamic Azad University, 14155-775, Tehran 14778 93855, Islamic Republic of Iran e-mail: [email protected] S. R. M. Mahdavi Department of Medical Physics, Tehran University of Medical Science, Tehran, Islamic Republic of Iran M. Mohammadi Department of Physics, Shahrood Branch, Islamic Azad University, Shahrood, Iran
Introduction Hyperthermia is a procedure for treating cancer in which the tissue temperatures in the tumor are raised to the range of 42–45 C [1]. A major challenge for hyperthermia technology is to produce uniformly elevated temperatures throughout a tumor volume [2]. Temperature homogeneity in tissue during treatment is determined by several factors that include the radiation pattern of the heating applicator, variability in the composition of tissue which determines absorption rate of energy and transfer of thermal energy due to conduction and blood flow [3]. Although the bolus is an essential part of the complete hyperthermia set-up, its influence on the resulting SAR distribution and the final performance of the electromagnetic (EM) applicator generally receives limited attention [4]. The use of absorbing structures with the view of reducing the treatment limiting effects of hot spots was first introduced by De Leeuw et al. [5]. Detailed analysis of the effects of an absorbing structure, using simulation tools, might lead to improvement of its efficiency by modification of shape and/or composition [6]. In this paper a simulation method for determining the induced SAR patterns of RF capacitive applicator inside the tissue is discussed. To demonstrate the role of the bolus design in the SAR focusing, two types of bolus configurations were examined: the conventional bolus and the absorbing one. In the latter one an additional L-form patch is inserted inside the conventional bolus. Much work has been reported on determining the SAR distributions with the aim of improving the bolus performance in the microwave hyperthermia. In these studies either the numerical or the experimental methods have been utilized. Van Rhoon et al. [7] noted the potential effect of water bolus size and shape on the resulting SAR distribution. Their measurements indicated the possibility that advantages of the Lucite
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Fig. 1 a Model used for simulation; b Model of the ‘absorbing bolus’ in HFSS
Table 1 Dielectric properties used for simulation at 8 MHz [21, 22] Material type
Electrical conductivity (S m-1) Relative permittivity
Muscle phantom 0.62
81
Saline (salinity 5.5 g l-1)
Saline (salinity 16 g l-1)
Saline (salinity 26.5 g l-1)
1
3
5
75
71.5
68
Fig. 2 Set up of verification measurement in phantom
cone applicator may be counteracted by the application of an unfavorable water bolus configuration. De Bruinje et al. [4] presented the effects of water bolus dimensions and configuration on the effective field size of the Lucite cone applicator for superficial hyperthermia. Shearer et al. [8] have attempted to increase the heating area by modifying the coupling water bolus used with microwave waveguide applicators. This was accomplished by introducing a centrally located saline-filled ellipsoidal absorbing patch into
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the bolus. This design was later extended empirically to include an array of smaller microwave absorbing pads filled with saline solution [9]. Sherar et al. [10] also described an optimization procedure based on the finite element method with edge-elements to perform three-dimensional simulations of this bolus design in order to improve its performance. Kumaradas and Sherar [3] developed a prototype dynamically modifiable square array of saline filled patches
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Fig. 3 The effect of electrical conductivity and thickness of the saline pad on the SAR pattern in the transversal plane located at z = 0.03 m
which attenuate microwave energy for superficial treatments that use external microwave applicators. Although there is much work on the bolus performance in microwave hyperthermia, there is only few work
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Fig. 4 The effect of electrical conductivity and thickness of the saline pad on the SAR pattern in the transversal plane located at z = 0.04 m
addressing this issue in RF capacitive hyperthermia. Michiyama and Nikawa [11] considered a phantom and a human model in the SAR simulations to optimize the
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Table 2 Summary of the results obtained for Area80 and the corresponding SAR values in both bolus configurations
r = 5 S m-1 Dd (m)
0.005
2
0.005
19.4
19.8
21.3
Saline Pad
2
80 % SARmax(W kg-1)
18.7
-1
Area80 (%)
40
80
70
90
Absorbing bolus
2
0.002
2
0.002
80 % SARmax(W kg-1)
34.1
38.3
36.6
39.6
Area80 (%)
5
70
5
50
12
6 σ effect z=0.04m
Thicness effect z=0.04m σ effect z=0.03m Thicness effect z=0.03m
5
10
4
8
3
6
2
4
1
0.01
0.015
0.02
0.025
0.03
Thickness ( × 10-3m)
Fig. 5 Normalized Area80 versus electrical conductivity and thickness of the saline pad in the transversal plane located at z = 0.03 m and z = 0.04 m
Thickness = 0.005m Dr (S m-1)
r=5Sm Dda (m)
-1
Variation in thickness
Z = 0.04 m
Thickness = 0.005m Dr (S m-1)
Electrical Conductivity (S.m )
a
Z = 0.03 m
2
Normalized Area 80
radius and location of the electrodes during RF heating. Kroeze et al. [6] investigated the use of solid absorber blocks to reduce peripheral hot spots with the aid of numerical simulation tools. The influence of the size and the salinity of the rectangular solid absorbers on the SAR distribution were discussed. Computational models of power deposition patterns and temperature distributions in human bodies have helped researchers to understand and improve hyperthermia treatments [12–18]. The simulation method developed in this paper addresses some parameters concerning in both target bolus designs including electrical conductivity of bolus and its thickness. These parameters affect the transmitted component of electrical field which plays an important role in deposition of power inside the phantom. Several series of induced SAR simulations were performed based on the variation of each parameter in both bolus designs. The homogenous model was considered in the current study. This model allows us to compare numerical simulations directly with the simple phantom models that can be constructed and tested in the laboratory [19]. Although, in this study there is not any intent to simulate the temperature, we can take an advantage of
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knowing the SAR profiles as an indication of where the temperature may increase throughout the phantom. The main focus of the previous studies on the performance of the bolus in the RF capacitive hyperthermia was mainly based on theories and simulations. However, this paper collated the simulation result with experimental measurement in a muscle-equivalent phantom. It is indicated that the bolus system configuration can be one of the key elements determining the quality of the RF capacitive hyperthermia of an arbitrary target tumors. The promising results show the bolus ability in controlling the SAR profile in tissue. It can also give the possibility to predict whether there is a successful treatment for the tumor shape. In addition, a capability for comparing the effectiveness of different bolus designs on any tumor shape before the treatment will be provided. The present paper is organized as follows: an explanation of the model and the proposed method for improving the beam shaping feature of bolus, a discussion on the output of the performed SAR simulations, empirical verifications of the simulations and finally an overall conclusion and some suggestions for further research.
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SAR ¼
r E2 ; q
ð1Þ
where r is the electrical conductivity of the medium and E is the local electric field. Following the calculation of the SAR, the temperature distribution inside the tissue heated by EM energy could be obtained by using the bioheat equation: qC
oT ¼ rðkrT Þ þ xbl qbl Cbl ðTa T Þ þ Qm þ q SAR, ot ð2Þ
Fig. 6 Comparison between the contours representing Area80 in the transversal plane located at a z = 0.03 m, b z = 0.04 m. Solid line thickness of pad = 0.005 m, r = 3 S m-1, dashed line thickness of pad = 0.005 m, r = 5 S m-1, dotted line thickness of pad = 0.01 m, r = 5 S m-1
Materials and methods In this study the characteristics of RF heating is considered in terms of induced SAR. The power dissipated by the RF beam into the tissue is known as SAR. The SAR (units of W kg-1) is the more useful concept in our work and is given by:
where T(x, y, z, t) is the temperature of the tissue (C); Ta is the arterial temperature (C); k is the thermal conductivity of tissue (W m-1 C-1); Cbl is the Heat capacity of blood (J kg-1 C-1); C is the heat capacity of tissue (J kg-1 C-1); qbl is the density of blood (kg m-3); q is the density of tissue (kg m-3); xbl is the blood perfusion (ml s-1 ml); Qm is the tissue metabolism (W m-3). The simulation package from Ansoft Corporation includes High Frequency Structure Simulator (HFSS) for EM Simulation. HFSS is a powerful finite electromagnetic (FEM) solver for EM problems. The SAR patterns were determined from the HFSS simulations. In this work, the homogeneous muscle-equivalent model with uniform dielectric properties is considered. The electrodes are set outside of the phantom model. The bolus system which cools the phantom’s surface and produces the impedance matching, consists of saline which is embedded in a thin casing silicone layer. Figure 1a shows the model with square aluminum electrodes and the bolus system. The dielectric properties of the model are summarized in Table 1. During the simulation procedure, two different bolus configurations were analyzed: The first design which is referred to ‘saline pad’ bolus, has the uniform salinity and the second one is referred to ‘absorbing bolus’. The latter design, compared to the first one, has an additional L-form saline patch and could be filled individually with appropriate salinity, Fig. 1b. The objective of the ‘absorbing bolus’ is analyzing the ability of a specific bolus configuration to shape the SAR pattern to the arbitrary target region. Saline absorbs RF energy and causes the L-form patch to attenuate the RF energy passing through it. The electrical properties of the target ‘L’-shaped region were assumed as same as the native tissue. Moreover, the effect of electrical conductivity and thickness is also investigated. Therefore, three different parameters were varied during this study: the bolus system configuration, the electrical conductivity and the thickness of saline. It has been shown previously [20] that the maximum of temperature rise is most likely to occur at 0.02–0.03 m below the surface of the phantom. Therefore, the different transversal planes located at this depth are considered in our study.
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Fig. 7 The effect of electrical conductivity and thickness of the L-form patch on the SAR pattern in the transversal plane located at z = 0.03 m
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Fig. 8 The effect of electrical conductivity and thickness of the L-form patch on the SAR pattern in the transversal plane located at z = 0.04 m
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9 σ
5
7
3
5
1
0.01
0.015
0.02
0.025
0.03
Thickness ( × 10-3m)
effect z=0.04m Thickness effect z=0.04m σ effect z=0.03m Thickness effect z=0.03m
-1
Electrical Conductivity (S.m )
Fig. 9 Normalized Area80 versus electrical conductivity and thickness of the saline patch in the transversal plane located at z = 0.03 m and z = 0.04 m
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3
Normalized Area80 Fig. 10 Comparison between the contours representing Area80 in the c transversal plane located at a z = 0.03 m, b z = 0.04 m. Solid line thickness of patch = 0.005 m, r = 3 S m-1, dashed line thickness of patch = 0.005 m, r = 5 S m-1, dotted line thickness of patch = 0.007 m, r = 5 S m-1
Experimental verification To confirm the validity of the SAR simulation results, the temperature measurements were made through the phantom after radiation absorption. An experimental setup as shown in Fig. 2, was prepared. A rectangular muscleequivalent agar phantom of 0.12 9 0.12 9 0.12 m3, composed of water (95.66 %), NaN3 (0.1 %), NaCl (0.24 %) and agar (4 %) was prepared [21]. To evaluate the validity of the SAR simulations, the phantom was then split into the ‘planes of interest’ which represent the desired planes in which the thermometry is performed. To do this, two different planes at z = 0.03 m and z = 0.04 m below the surface of the phantom were chosen. The phantom has approximately the same electrical properties at room temperature as the corresponding tissue at body temperature. The applicator which has been made of an ultrafast MOSFET driver (DEIC420) operating at 8 MHz and 50 W power is supplied by half size clock oscillator (XO-52). The square electrodes made of aluminum with 0.06 m in side length were prepared to connect to the RF generator through coaxial wires. The interaction between the electric field (generated between the parallel-opposed electrodes) and the material results in a heat distribution throughout the phantom. Experiments were separated by sufficient time intervals in order to return back to baseline temperature (20 C ± 1 C). The experimental bolus systems were constructed as previously described. The ‘saline pad’ bolus consists of two 0.12 9 0.12 m2 thin silicone pads. The pads which
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698 Fig. 11 Comparison between the SAR pattern at z = 0.03 m and the c measured temperatures in the corresponding transversal plane: a thickness of pad = 0.005 m, r = 3 S m-1, b thickness of pad = 0.005 m, r = 5 S m-1, c thickness of pad = 0.01 m, r = 5 S m-1
could measure up to 0.01 m in thickness when filled with saline, were connected to the circulating system via the inflow and the out-flow connectors at the edges. Temperature of the cooling saline was kept constant at 10 C. On the other hand, the general structure of the ‘absorbing bolus’ was similar to the ‘saline pad’ one, except that it has an additional L-form saline patch as shown schematically in Fig. 1b. To prevent mixture of NaCl to the surrounding saline, the L-form patch was wrapped in a thin silicone cover and was secured into the ‘saline pad’ bolus using its in-flow and out-flow connectors. The ‘absorbing bolus’ was constructed to change the RF beam shape, thus the shape, the thickness and the saline concentration of the additional patch determine the SAR pattern inside the phantom. The phantom was surrounded by bolus system and electrodes in such a way that they were parallel to the ‘planes of interest’. After exposure of 8 MHz radiation, the segments of the phantom were immediately removed and then the temperature in the ‘planes of interest’ was measured using a portable infrared thermometer K5500 with an accuracy of 0.1 C. Moreover, a thermometry system (Extech 421509) consists of Teflon-coated thermocouples was used to control the total time of the process.
Results Simulation results of SAR distributions In order to present a quantitative analysis, we define a percentage Area80 as the relative increase of the contour area where the SAR is 80 % of its maximum amount in each pair of the following figures. In this section, simulated SAR profiles inside the model at frequency of 8 MHz with two different bolus configurations are presented with the aim of evaluating the bolus system performance. The bolus system in both cases was placed between the electrodes and the phantom. Firstly, the ‘saline pad’ bolus was employed in the SAR simulation. Figures 3 and 4 show the SAR patterns produced from heating the phantom with ‘saline pad’ bolus in transversal planes located at z = 0.03 m and z = 0.04 m of the phantom, respectively. As was found, utilizing the ‘saline pad’ bolus results in a Gaussian SAR pattern. Figure 3a and b shows the results of SAR patterns obtained from introducing a 0.005 m thick ‘saline pad’ with two different electrical conductivities of 3 and 5 S m-1 in transversal plane located at z = 0.03 m of the
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Fig. 12 Comparison between the SAR pattern at z = 0.04 m and the c measured temperatures in the corresponding transversal plane: a thickness of pad = 0.005 m, r = 3 S m-1, b thickness of pad = 0.005 m, r = 5 S m-1, c thickness of pad = 0.01 m, r = 5 S m-1
phantom. The calculated Area80 among these figures is 40 % which reveals the increase of therapeutic area with increasing the electrical conductivity. The same procedure was repeated in order to see the effect of electrical conductivity in transversal plane located at z = 0.04 m. Figure 4a and b indicates the similar results revealed from Fig. 3a and b except the amount of Area80 which becomes 70 % this time. Therefore, increasing the electrical conductivity has the desired effect of flattening out on the therapeutic area. Shown in Figs. 3b, c and 4b, c are the effect of introducing a saline pad with two different thicknesses of 0.005 m and 0.01 m on the SAR profiles in the transversal planes located at z = 0.03 m and z = 0.04 m, respectively. The electrical conductivity of saline was 5 S m-1 in these series of simulations. Results of Area80 value obtained from SAR patterns at z = 0.03 m and z = 0.04 m are 80 % and 90 %, respectively. Calculation of Area80 indicates that decreasing the thickness of saline pad makes the therapeutic area larger. The computed results of Area80 (which were obtained from the variation in either the electrical conductivity or the thickness of saline.) as well as the corresponding SAR values at z = 0.03 m and z = 0.04 m are summarized in Table 2. It is noticed that decreasing the pad thickness as well as increasing the electrical conductivity of saline has a similar effect on the effective therapeutic area. However, a comparison among Area80 values indicates that the effect of saline thickness is more pronounced. In Fig. 5 the normalized values of Area80 in each case study are plotted against the electrical conductivity and the thickness of saline. In addition, it could be extended the concept of to Area80 to more than two contours. To provide a comprehensive representation of Area80, the contours corresponding to 80 % SARmax in all case studies at z = 0.03 m and z = 0.04 m are compared in Fig. 6a and b, respectively. Obviously from the figures, we can conclude that increasing the electrical conductivity and decreasing the thickness of bolus make the therapeutic area larger. In the second series of simulations, we wish to expose an L-form target tumor, thus the ‘absorbing bolus’ was replaced with the ‘saline pad’ bolus. The objective of the ‘absorbing bolus’ is to change the Gaussian SAR pattern into an L-form one and to shape the therapeutic area with respect to the tumor shape. The simulated SAR patterns produced from heating the phantom with ‘absorbing bolus’ in the transversal planes located at z = 0.03 m and
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700 Fig. 13 Comparison between the SAR pattern at z = 0.03 m and the c measured temperatures in the corresponding transversal plane: a thickness of patch = 0.005 m, r = 3 S m-1, b thickness of patch = 0.005 m, r = 5 S m-1, c thickness of patch = 0.007 m, r = 5 S m-1
z = 0.04 m of the phantom are depicted in Figs. 7 and 8, respectively. Clearly, utilizing the ‘absorbing bolus’ has the desired effect of shaping the therapeutic area. Figures 7a, b and 8a, b investigate the effect of patch electrical conductivity on the produced SAR patterns at depths z = 0.03 m and z = 0.04 m of the phantom, respectively. In this series of simulations, a 0.005 m thick L-form patch with two different electrical conductivities of 3 and 5 S m-1 was introduced within a 0.01 m thick saline pad with the electrical conductivity of 1 S m-1. The computed Area80 value becomes 5 % in both cases. Therefore, it is found that as the electrical conductivity of the additional patch increases, the therapeutic area increases as well. Moreover, for different patch thicknesses, the simulated SAR profiles at z = 0.03 m and z = 0.04 m of the phantom are shown in Figs. 7b, c and 8b, c, respectively. In this series of simulations, an L-form patch with two different thicknesses of 0.005 m and 0.007 m was employed. The electrical conductivity of patch was 5 S m-1. The calculated Area80 values at z = 0.03 m and z = 0.04 m become 70 and 50 %, respectively. Therefore, reduction of the patch thickness has the desired effect of flattening out on the therapeutic area. Table 2 summarizes the computed results of Area80 (which were obtained from the variation in either the electrical conductivity or the thickness of saline.) and the corresponding SAR values at z = 0.03 m and z = 0.04 m. It is indicated that raising the electrical conductivity of saline and reducing the pad thickness has a similar influence on the effective therapeutic area. However, comparing Area80 values shows that the influence of saline thickness is more significant. The normalized values of Area80 in each case study in terms of the electrical conductivity and the thickness of saline are shown in Fig. 9. Moreover, the comparison between the contours corresponding to 80 % SARmax in each case study at z = 0.03 m and z = 0.04 m are indicated in Fig. 10a and b, respectively. It is clear from the figures that the increase in electrical conductivity and the decrease in thickness of the bolus will flat out the therapeutic area. Experimental results of heating the phantom In the following, the accuracy of the presented simulations was assessed through several experiments. As we know, the temperature variation inside the phantom could be estimated from the SAR values according to [23]:
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Fig. 14 Comparison between the SAR pattern at z = 0.04 m and the c measured temperatures in the corresponding transversal plane: a thickness of patch = 0.005 m, r = 3 S m-1, b thickness of patch = 0.005 m, r = 5 S m-1, c thickness of patch = 0.007 m, r = 5 S m-1
SAR ¼ C
DT ; Dt
ð3Þ
where C is the heat capacity of the phantom (which is 4.2 9 103 J kg-1 C-1 in our model [21] ) and Dt is the period of heating. Thus, the simulated SAR values could be regarded as a determining factor for temperature increase inside the phantom exposed to an external source. In the following figures, the measured temperatures and the corresponding SAR patterns are comprised. In first series of experiments performed with ‘saline pad’, the phantom was placed between the pads. After 1,200 s of exposure to 8 MHz radiation, the temperature along the x and y axis of the ‘planes of interest’ were measured in every point with equal 0.01 m distance. These directions were chosen due to the symmetry consideration related to Gaussian SAR patterns. The effect of electrical conductivity and thickness of saline on the measured temperature were examined in the following way. First, the effect of the saline electrical conductivity was investigated. Two pairs of saline pads with 0.005 m thickness were filled with NaCl solution. The salinity of one pair was 16 g l-1 and the other one was 26.5 g l-1 corresponding to the electrical conductivities of 3 and 5 S m-1, respectively [22]. After exposure, temperature were measured in the planes located at z = 0.03 m and z = 0.04 m of the phantom. Results together with the relevant SAR patterns are compared in Figs. 11a, b and 12a, b, respectively. It is observed that the temperature variations are in a good agreement with the SAR variations. Second, to see the effect of saline thickness, two pairs of saline pads with salinity of 26.5 g l-1 were constructed. The thickness of one pair was set to 0.005 m and the other one to 0.01 m. A comparison between the results of temperature readings in the planes located at z = 0.03 m and z = 0.04 m of the phantom and the corresponding SAR patterns is visualized in Figs. 11b, c and 12b, c. In another series of experiments, the accuracy of the SAR simulations obtained from ‘absorbing bolus’ were examined. The phantom was placed between the pads and after 600 s of exposure to 8 MHz radiation, the temperature measurements were carried out with equal spacing in the directions shown in the following figures. The L-form patch is also indicated in the following figures. First, similar to the strategy of the ‘saline pad’ experiments, the effect of the electrical conductivity of the L-form patch was considered. A 0.01 m thick bolus pad containing NaCl
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solution with the salinity of 5.5 g l-1 corresponding to the electrical conductivity of 1 S m-1 [22] was prepared. Then, one pair of L-form patch was filled with 16 g l-1 NaCl solution and the salinity of the other one was set to 26.5 g l-1. The thickness of both pairs of patches was 0.005 m. Shown in Figs. 13a, b and 14a, b is a comparison between the SAR profiles and the corresponding temperature measurements in the planes located at z = 0.03 m and z = 0.04 m of the phantom, respectively. Second, in order to examine the effect of the patch thickness on the measured temperatures, two pairs of patches were filled with NaCl solution with salinity of 26.5 g l-1 until one pair reached to the thickness of 0.005 m and the other one to 0.007 m. These patches were then set inside a 10 mm thick bolus pad containing NaCl solution with the salinity of 5.5 g l-1. A comparison between temperature measurements in the planes located at z = 0.03 m and z = 0.04 m and the corresponding SAR patterns are shown in Figs. 13b, c and 14b, c, respectively. It is clear from the presented results, that all measurements and the simulations have the same reasonable variations. Here, the temperature inside the phantom was not simulated. However, the SAR patterns can indicate the temperature variation throughout the phantom. Our empirical results confirm that computer-aided simulation of induced SAR distribution is a reliable and accurate way to provide the clinical operation. The geometry of the tumor should be first scanned using an MRI or CT or other noninvasive imaging techniques. Then, the best configuration of the additional patches could be examined to predict whether there is a successful treatment. In addition, a capability for comparing the effectiveness of different bolus designs on any tumor shape before the treatment will be provided.
Conclusion This work describes the design of a modified bolus that uses an absorbing patch to improve the SAR patterns of 8 MHz capacitive applicator. The absorber developed here is a saline-filled patch situated inside the bolus system. The conventional boluses, regardless to the tumor shape, produce approximately the large and uniform therapeutic areas. However, saline absorbs a larger fraction of the RF energy flowing through the patch. Thus, the ‘absorbing bolus’ provides a spatial control of the RF beam profile inside the patch and any arbitrary shaped tumor. We show the possibility of controlling the therapeutic area according to the tumor shape through modification of the absorber geometry, i.e. the shape and the thickness of the additional patch as well as its electrical conductivity. Moreover, because the simulated SAR results can determine the temperature increase inside the phantom, the accuracy of
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the model was assessed through several series of empirical temperature measurements in a split-muscle-equivalent phantom. It is confirmed that the temperature changes inside the phantom closely reflect the reasonable agreement with the results of the simulated SAR as predicted by the model. In the perspective of the proposed model, we believe that the ‘‘absorbing bolus’’ could be made of the dynamically additional solid or liquid patches which reasonably must resemble the tumor shape and placed inside the bolus system or between the applicator and the bolus. This model could also be coupled into a proper optimization algorithm to achieve the best choice of both electrical and thermal properties of the additional patches. Regarding to the optimization procedure, one could also quantitatively determine the efficiency of the beam forming performance of the proposed ‘‘absorbing bolus’’ configuration for a given tumor shape. In this respect, we suggest that it would be necessary to consider two main factors in the SAR pattern simultaneously: the percentage of the tumor region covered by Area80 and the uniformity of SAR contour inside the tumor region. These factors could be calculated and then compared to those obtained in the presence of ‘‘saline pad’’. As a result, the desired SAR patterns which have the greatest compatibility with a given tumor shape could be obtained. Therefore, the proposed model shows great promise for further research in both model and clinical performance.
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