**4. Microwave applicators for "BPH" thermotherapy**

In all above mentioned key activities we need to do numerical simulations of SAR and temperature 3D distribution. Temperature distribution inside area of biological tissue heated by microwave energy can be calculated from well known formula:

$$
\rho\_t c\_t \frac{\partial T}{\partial t} = \mathbf{y}\_t \Delta T - \chi (\mathbf{T} - \mathbf{T\_b}) + \mathbf{q} \tag{1}
$$

where t is the time.

*q = q(x,y,z,t)* is energy delivered by EM field, *T = T(x,y,z,t)* is designates the temperature, *Tb = Tb(x,y,z,to)* is the temperature of blood,

Physical meaning and values of the here used constants for the case of high water contents tissue are:

rt = 0.965 [g/cm3] is density of biological tissue (BT). ct = 3 586 [mJ/g/C] is specific heat of BT. k = 5.45 [mW/cm3/C] is blood flow and temperature capacity of BT. gt = 5.84 [mW/cm/C] is spec. temp. conductivity of BT.

Possibilities of analytical solution of this equation are very limited to only a few cases - like e.g. "one dimensional" case of plane wave penetrating in homogeneous phantom. Therefore computers are to be used to solve this equation to obtain the temperature *T*(*x,y,z,t*) time dependence and space distribution. For the treatment planning of microwave thermotherapy we use software product SEMCAD. Fig. 13 gives basic idea about SAR calculated for the case of the monopole intracavitary applicator.

#### **4.1 Applicators for "BPH" treatment**

We have investigated basic types of microwave intracavitary applicators suitable for BPH treatment, i.e. monopole, dipole and a helical coil structures. These applicators are designed to work either at 915 or at 434 MHz. We would like to discuss its effective heating depth, based on the comparison of the theoretical and experimental results. Basic mechanisms and parameters influencing (limiting) heating effective depth are described and explained in ref. (Vrba et al., 1996, 1999).

Fig. 13. Calculated SAR of BPH applicator

The basic type of intracavitary applicator is a monopole applicator. The construction of this applicator is very simple, but calculated and measured SAR distribution along the applicator is more complicated, than has been the first idea. SAR can be measured either in water phantom or by infrared camera.

#### **4.2 Evaluation of BPH applicators**

516 Advances in Cancer Therapy

In all above mentioned key activities we need to do numerical simulations of SAR and temperature 3D distribution. Temperature distribution inside area of biological tissue

Physical meaning and values of the here used constants for the case of high water contents

Possibilities of analytical solution of this equation are very limited to only a few cases - like e.g. "one dimensional" case of plane wave penetrating in homogeneous phantom. Therefore computers are to be used to solve this equation to obtain the temperature *T*(*x,y,z,t*) time dependence and space distribution. For the treatment planning of microwave thermotherapy we use software product SEMCAD. Fig. 13 gives basic idea about SAR

We have investigated basic types of microwave intracavitary applicators suitable for BPH treatment, i.e. monopole, dipole and a helical coil structures. These applicators are designed to work either at 915 or at 434 MHz. We would like to discuss its effective heating depth, based on the comparison of the theoretical and experimental results. Basic mechanisms and parameters influencing (limiting) heating effective depth are described and explained in ref.

The basic type of intracavitary applicator is a monopole applicator. The construction of this applicator is very simple, but calculated and measured SAR distribution along the

డ௧ ൌ ߛ௧οܶ െ ɖሺെୠሻ (1)

**4. Microwave applicators for "BPH" thermotherapy** 

heated by microwave energy can be calculated from well known formula:

ߩ௧ܿ௧ డ்

*q = q(x,y,z,t)* is energy delivered by EM field, *T = T(x,y,z,t)* is designates the temperature, *Tb = Tb(x,y,z,to)* is the temperature of blood,

rt = 0.965 [g/cm3] is density of biological tissue (BT).

gt = 5.84 [mW/cm/C] is spec. temp. conductivity of BT.

calculated for the case of the monopole intracavitary applicator.

k = 5.45 [mW/cm3/C] is blood flow and temperature capacity of BT.

ct = 3 586 [mJ/g/C] is specific heat of BT.

**4.1 Applicators for "BPH" treatment** 

Fig. 13. Calculated SAR of BPH applicator

(Vrba et al., 1996, 1999).

where t is the time.

tissue are:

One of our tools for experimental evaluation of microwave applicators is the apparatus, which enable us to do 3D measurements of SAR distribution. It can be used for both local external and intracavitary applicators.

The basic part of this apparatus is big salt water phantom (water with 0.3% to 0.6% NaCl) and the measurement probe with the possibility of 3D scan around the applicators. As probes we use a Light Emitting Diodes (LED) with a fibber optic link connection to interface of the computer. The purpose of this link is to reduce influence of the metallic (i.e. conductive) components from the measured electromagnetic field. The schematics of the apparatus for 3D measurements of SAR distribution is shown in Fig. 14.

Fig. 14. Apparatus for 3D SAR measurements

During measurements of SAR along the applicator we have found, that typically there is not only a one main SAR maximum (first from the right side), but also a second and/or higher order maxima can be created, being produced by outside back wave propagating along the coaxial cable, see Fig. 15a. In Fig. 15b. SAR distribution improvement (i.e. reduction of second maximum) can be noticed for the case of dipole like applicator. To eliminate this second maximum and optimise the focusing of SAR in predetermined area of biological tissue needs to use the helical coil antenna structure.

After coil radius and length optimisation we have obtained very good results of SAR distribution, see Fig. 16a,b. Some problems can be with the antenna self-heating, but it can be reduced by cooler at the end of applicator tip.

Prospective Applications of Microwaves in Medicine 519

approximation, i.e. the results with accuracy better than 5 % for higher frequencies (f >100 MHz) and/or bigger radii (R > 3mm). For lower frequencies (up to 100 MHz) combined with small radius of the cavity (R < 2 mm) the accuracy is cca 10 %. It is not possible to achieve a heating penetration depth (i.e. 50 % decrease) higher than *R* at any frequency and in any propagation medium. The small cavity radius plays a dominant role in the

Fig. 17. Effective depth of heating d with respect to frequency f [MHz] and radius R [mm]

1000

2450

f [MHz]

27 434

20 100 200

<sup>R</sup> R=100

50

d [mm]

25

20 15 10

> 5 3 1

Infrared camera is according to our experiences a very efficient tool for "SAR" measurements of microwave intracavitary applicators. In Fig. 18. there is the typical measured heating pattern of monopole (a), dipole (b), and helical coil antennas (c). This pattern can be obtained by heating suitable phantoms of biological tissue – mimicring the dielectric and thermal properties of the prostate tissue. Such phantoms can be made on the basis of agar or so called superstuff material. The pattern is colour-coded according to a linearly decreasing scale of white-yellow-red, where white is the maximum temperature. A diagrammatic catheter is inserted in each figure; the orientation of the bladder neck in a patient is indicated by a dashed line. Note the long back heating tails with a monopole antenna (Fig. 18a.) which is caused by microwave currents that flow backwards along the cable and cause the feeding cable to radiate. The radiation pattern from a dipole antenna (Fig. 18b.) is generally well confined without any excessive back heating. The dipole antenna is suitable for prostates with axial length > 40 mm. The helical coil antenna (Fig. 18c.) has the shortest and most focussed heating and would be the choice for small prostates, 25 – 40 mm

**4.4 SAR measurements by infrared camera** 

in length.

penetration depth.

30

25

20

15

10

5

10 0

Fig. 15. a) Monopole applicator, b) Dipole applicator

Fig. 16. a) Helical coil applicator, b) Temperature field around the helix

#### **4.3 Effective treatment depth**

We have studied the theoretical limits of intracavitary applicator heating depth. We have found the basic relation for determination of the limit of maximum heating depth for the case of "very long" intracavitary applicator. We suppose excitation of an ideal radial wave around radiating applicator.

Mentioned results can be simply interpreted by following figure, where on the horizontal axis is the working frequency of thermotherapy apparatus and on vertical axis there is effective depth of electromagnetic wave penetration. As a third parameter playing an important role there is a diameter of a discussed intracavitary applicator.

Very important is that in this case we are dealing with a radial wave, not the plane wave, and that's why the penetration depth is smaller than penetration depth of plane wave. Some works published in this field give too optimistic results. Measurements discussed without theoretical analysis can give results influenced by thermal conductivity of mostly used agar phantom of muscle tissue. As the real heating depth is typically a few millimetres (in the best case up to approx. 1 cm under the surface of the cavity), thermal conductivity of the phantom material can easily cause errors of several tenth of percents.

Ideal intracavitary applicator irradiates an ideal cylindrical wave into the biological tissue surrounding the cavity. According to our experience Fig. 17. gives very good

100

50

No mr

S

A

R

%( )

a)

a) b)


0 60


d (mm)

b)

a)

d (mm)

b)

a)

a) b)

b) 20 mm

We have studied the theoretical limits of intracavitary applicator heating depth. We have found the basic relation for determination of the limit of maximum heating depth for the case of "very long" intracavitary applicator. We suppose excitation of an ideal radial wave

Mentioned results can be simply interpreted by following figure, where on the horizontal axis is the working frequency of thermotherapy apparatus and on vertical axis there is effective depth of electromagnetic wave penetration. As a third parameter playing an

Very important is that in this case we are dealing with a radial wave, not the plane wave, and that's why the penetration depth is smaller than penetration depth of plane wave. Some works published in this field give too optimistic results. Measurements discussed without theoretical analysis can give results influenced by thermal conductivity of mostly used agar phantom of muscle tissue. As the real heating depth is typically a few millimetres (in the best case up to approx. 1 cm under the surface of the cavity), thermal conductivity of the

Ideal intracavitary applicator irradiates an ideal cylindrical wave into the biological tissue surrounding the cavity. According to our experience Fig. 17. gives very good

Fig. 16. a) Helical coil applicator, b) Temperature field around the helix

d (mm)

important role there is a diameter of a discussed intracavitary applicator.

phantom material can easily cause errors of several tenth of percents.

Fig. 15. a) Monopole applicator, b) Dipole applicator



0 60

100

50

No mr

S

100

50

No mr

S

A

R

%( )

A

R

%( )

**4.3 Effective treatment depth** 

around radiating applicator.

approximation, i.e. the results with accuracy better than 5 % for higher frequencies (f >100 MHz) and/or bigger radii (R > 3mm). For lower frequencies (up to 100 MHz) combined with small radius of the cavity (R < 2 mm) the accuracy is cca 10 %. It is not possible to achieve a heating penetration depth (i.e. 50 % decrease) higher than *R* at any frequency and in any propagation medium. The small cavity radius plays a dominant role in the penetration depth.

Fig. 17. Effective depth of heating d with respect to frequency f [MHz] and radius R [mm]

#### **4.4 SAR measurements by infrared camera**

Infrared camera is according to our experiences a very efficient tool for "SAR" measurements of microwave intracavitary applicators. In Fig. 18. there is the typical measured heating pattern of monopole (a), dipole (b), and helical coil antennas (c). This pattern can be obtained by heating suitable phantoms of biological tissue – mimicring the dielectric and thermal properties of the prostate tissue. Such phantoms can be made on the basis of agar or so called superstuff material. The pattern is colour-coded according to a linearly decreasing scale of white-yellow-red, where white is the maximum temperature. A diagrammatic catheter is inserted in each figure; the orientation of the bladder neck in a patient is indicated by a dashed line. Note the long back heating tails with a monopole antenna (Fig. 18a.) which is caused by microwave currents that flow backwards along the cable and cause the feeding cable to radiate. The radiation pattern from a dipole antenna (Fig. 18b.) is generally well confined without any excessive back heating. The dipole antenna is suitable for prostates with axial length > 40 mm. The helical coil antenna (Fig. 18c.) has the shortest and most focussed heating and would be the choice for small prostates, 25 – 40 mm in length.

Prospective Applications of Microwaves in Medicine 521

In present time four research institutions here in the Czech Republic run research projects focused on studies of interactions between EM field and biological systems. These institutions are technically supported by Dept. of EM Field of the Czech Technical University in Prague. In this contribution we would like to give more details about that projects and obtained technical results (i.e. description of developed exposition systems). Three of discussed projects (1 in Germany and 2 here in Czech Republic) are basic research for simulation of the microwave hyperthermia treatment. Other two projects (both in Czech

In the modern view, cancer is intended as a complex illness, involving the cells that undergo to transformation, their environment, and the general responses at biochemical and biological levels induced in the host. Consequently, the anti-cancer treatment protocols need to be multi-modal to reach curative effects. Especially after the technical improvements achieved in the last 15 years by bio-medical engineering, microscopy devices, and molecular biology methods, the combinations of therapeutic procedures are growing in interest in

The combination of applied biological research together to the physical sciences can offer important perspectives in anticancer therapy (e.g. different methodologies and technical

The modern bioengineering knowledge applied to traditional tools, as the microscopy, has largely renewed and expanded the fields of their applications (e.g.: in vivo imaging), pushing the interest for direct morpho-functional investigations of the biomedical problems.

Very good results of EM field expositions in biological experiments can be obtained by simple but efficient waveguide applicators see example in Fig. 19. Waveguide offer a very big advantage – in approximately of fifty percents of its aperture the irradiated electromagnetic field is very near to a plane wave, which is basic assumption for good

Here described system is being used (shared) for research projects by two institutions (Institute of Radiation Oncology in Prague and Institute of Microbiology of the Czech

**5. Technical equipment for research of biological effects of EM field** 

Republic are focused on simulation of the case of exposition by mobile phone.

devices for application of energies to pathological tissues).

homogeneity of the heating and optimal treatment penetration.

Fig. 19. Waveguide applicator for biological experiments

basic and clinical research.

**5.1 Waveguide applicator** 

Academy of Sciences).

The pattern is colour-coded according to a linearly decreasing scale of white-yellow-red , where white is the maximum temperature. A diagrammatic catheter is inserted in each figure; the orientation of the bladder neck in a patient is indicated by a dashed line.

The pattern is colour-coded according to a linearly decreasing scale of white-yellow-red , where white is the maximum temperature. A diagrammatic catheter is inserted in each figure; the orientation of the bladder neck in a patient is indicated by a dashed line.

Moreover, by using infra-red camera we can follow a lot of other properties resp. behaviour of intracavitary applicators itself, like e.g. standing waves along feeding coaxial cable (i.e. the risk of existence of unwanted SAR and/or temperature maxima), possible overheating of some specific parts of tested applicator itself etc.

Fig. 18. The heating pattern for different antennas: a) the monopole, b) the dipole, c) the

figure; the orientation of the bladder neck in a patient is indicated by a dashed line.

figure; the orientation of the bladder neck in a patient is indicated by a dashed line.

of some specific parts of tested applicator itself etc.

The pattern is colour-coded according to a linearly decreasing scale of white-yellow-red , where white is the maximum temperature. A diagrammatic catheter is inserted in each

4b) Dipole

4a) Monopole

4c) Helical coil

The pattern is colour-coded according to a linearly decreasing scale of white-yellow-red , where white is the maximum temperature. A diagrammatic catheter is inserted in each

Moreover, by using infra-red camera we can follow a lot of other properties resp. behaviour of intracavitary applicators itself, like e.g. standing waves along feeding coaxial cable (i.e. the risk of existence of unwanted SAR and/or temperature maxima), possible overheating

helical coil.
