Evaluation and survey of microwave and radiofrequency applicators

The Performance of a New 915-MHz Direct Contact Applicator with Reduced Leakage

The technical feasibility of commercially developing a safe and effective direct contact diathermy applicator operating at the industrial, scientific, medical (ISM) frequency of 915 MHz is demonstrated. The basic design consists of a circular waveguide which is internally loaded with two orthogonal pairs of forward ridges to obtain circular polarization and two rear ridges with a probe to excite the guide. Two prototype designs are considered: the small applicator (15 cm diameter) has one annular choke covered with a 2.5-cm thick microwave absorber, and the large applicator (25 cm diameter) has two additional concentric chokes to limit leakage radiation. The performance of the applicators was evaluated in terms of the requirements of a ORH microwave diathermy test protocol to control stray radiation and deliver a thermally effective absorbed dose rate to simulated muscle tissue of a phantom with a 1-cm or 2-cm fat layer. The net power required to deliver a thermally effective 235-W/kg specific absorption rate (SAR) to such a planar phantom was determined. For this net power, leakage levels considerably less than 5 mW/cm2 (at 5 cm from applicator-phantom boundary) were obtained for the applicators in direct contact with the phantom. If a small spacing (1 cm) between these applicators and planar phantoms is introduced, the net power required to deliver an effective SAR to a phantom and the associated leakage can become excessive. For the small applicator, the required net power for inducing an SAR of 235 W/kg in muscle tissue of a 1-cm fat layer phantom is about 330 W and the leakage is about 120 mW/cm2. For a 2-cm fat layer phantom, these values are somewhat higher. For the large applicator, using a 1-cm fat layer phantom, the values are about 200 W and about 17 mW/cm2. Again, for a 2-cm fat layer phantom, these values are somewhat higher.

A 433 MHz Lucite cone waveguide applicator for superficial hyperthermia

The effective field size (EFS, SAR > or = 50% of the maximum SAR at 1 cm depth) of a conventional 433 MHz water filled waveguide applicator (32 cm2, aperture area 100 cm2) has been increased by: (1) replacement of the two diverging brass side walls which are parallel to the direction of the electric field by Lucite walls; and (2) Placement of a heterogeneous permittivity in the centre of the aperture. SAR distributions were measured at several depths in layered fat-muscle phantoms. With Lucite side walls the SAR distribution becomes wider in the H-plane of the aperture, resulting in a circular SAR distribution. In this situation the EFS is 67 cm2. Additional insertion of a PVC cone with a top angle of 15 degrees at the centre of the aperture increases the EFS to 91 +/- 6 cm2 for a waterbolus of 18 x 18 x 1 cm3. The experiments also demonstrated that the resulting EFS is affected by the waterbolus size and shape. Calorimetric measurements showed that the efficiency of the improved applicator is comparable to the efficiency of the conventional water filled waveguide applicator, 50 and 56% respectively. The modifications reported provide a simple and inexpensive means to increase the EFS and can be easily implemented in water filled waveguide applicators.

Large stationary microstrip arrays for superficial microwave hyperthermia at 433 MHz: SAR analysis and clinical data

Superficial hyperthermia with present day applicators provides a challenge when tumours exceed several cm in diameter. Unless microstrip applicators are scanned, the usable heat region often falls short of treating the entire region with 50% power or specific absorption rate (SAR). New microstrip applicator designs were evaluated through SAR analysis and compared to the traditional microstrip applicators used in the clinic at Dartmouth over the past six years. The new designs included fabricating thin archimedean spirals (1.0 mm strip width) incorporating dielectric substrate (epsilon = 5.3-10.8). The designs were optimized at 433 MHz for an arm length of 59 cm. Measurements in a plane 1.0 cm from the surface showed that thin spirals outperformed traditional designs by increasing the 50% SAR area by a factor of 2.5, while maintaining the same physical size. Arrays of four elements were fabricated from thin spirals, although SAR evaluation showed only 10-20% SAR between elements. Since this was deemed unacceptable and the design goal was to fabricate a stationary applicator that had at least 50% SAR between elements, dual element designs were created with gradually overlapped elements. It was found that overlapping three coils of the spiral created a large region that equalled or exceeded 50% SAR that could not be matched by single applicators. Coherent operation of the dual spiral array resulted in more central power deposition and incoherent operation resulted in more peripheral power deposition. SAR measurements at the fat/muscle interface showed an elongated heating pattern in hydroxyethylcellulose muscle equivalent phantom. Power deposition 1.0 cm deep in muscle retained the same basic size and shape with or without the fat layer. Patient treatments for chestwall tumours confirmed that the dual overlapping applicator heated a larger region without the sharp temperature peak associated with single applicators.

Clinical hyperthermia with a new device: the current sheet applicator

PURPOSE

The current sheet applicator (CSA) is a newly developed microwave hyperthermia device. Advantages over commercial microwave applicators include its small size and high ratio of heating area to physical aperture area. These physical characteristics make the CSA excellent for heating constricted areas and allow the use of arrays of CSAs over large surfaces. This study examines the clinical efficacy of the CSA for heating superficial malignant tumors.

METHODS AND MATERIALS

From December 1989 through October 1991, 19 patients with recurrent or metastatic superficial malignant tumors were treated once or twice weekly to 30 hyperthermia fields using one to four CSAs. Each field received from one to four hyperthermia treatments for a total of 74 treatments. The treatment objective was to elevate the tumor temperature to a minimum of 42.5 degrees C for 30 min (2 patients) or 60 min (17 patients). Intratumor temperatures were measured with percutaneous fiberoptic thermometry probes. All patients received concurrent fractionated radiation therapy with total dose ranging from 20 to 65 Gy (median 46 Gy). Seventeen of the 30 fields had been previously irradiated to a median dose of 50 Gy.

RESULTS

Mean values for the maximum temperature, average temperature, and minimum temperature were 43.6 degrees C +/- 1.0, 42.2 degrees C +/- 1.4, and 41.0 degrees C +/- 1.5, respectively. Mean values for T50 and T90 were 42.2 degrees C +/- 1.1 and 41.0 degrees C +/- 1.3, respectively. The overall response rate for all assessable fields was 96%. Only Only three responding tumors have progressed with a median follow-up period of 6 months. Treatment related morbidity was generally mild and self-limited.

CONCLUSION

The CSA is a promising new microwave hyperthermia device capable of heating superficial tumors to therapeutic temperatures. When used in combination with radiotherapy, response rates are excellent without excessive toxicity.

Feasibility of estimating the temperature distribution in a tumor heated by a waveguide applicator

The feasibility of using a 2-dimensional (2D) modeling approach for retrospectively describing complete temperature distributions in the midplane of a tumor during a clinical hyperthermia treatment was tested. An experimental treatment, using a 915-MHz waveguide applicator to heat a large melanoma in a dog, was modeled. Detailed measurements of temperatures were made during the treatment. The steady-state blood flow distribution at the midplane was imaged by positron emission tomography (PET), and these data were used to prescribe the modeled perfusion pattern. A 2D finite element method (FEM) was used to approximate the solution to Maxwell's Equations to obtain the specific absorption rate (SAR) distribution. The blood-flow estimates, assumed material properties, SAR distribution, and temperature boundary conditions were then used with the same mesh in a second FEM program to obtain a solution to the bioheat transfer equation. This latter routine was embedded in a state-and-parameter-estimation program that systematically varied selected parameters until the differences between computed and measured temperatures were minimized. Optimizations were performed independently for three subsets of the measured temperature data to assess the sensitivity of the predicted temperature field to the number of measurements. The calculated temperature distributions that resulted were similar to each other, and the predicted temperatures at the sensor points excluded from these optimizations were in reasonable agreement with the measurements. However, lack of unique blood flow values following optimization indicates that the methods of estimating blood flow will need to be improved or that there are problems with model mismatch. This work is a clinical case study of an evolving 2D system of thermal dosimetry which relies on both empirical and theoretical concepts. The methodology is being evaluated for its ability to generate prognostically significant descriptors of the treatment temperature field.

Theoretical and experimental comparison of three types of electromagnetic hyperthermia applicator

There is evidence that heating of malignant tissue in the treatment of cancer may be beneficial and so the performance of different applicator designs needs to be established. A theoretical model of the dielectric loaded waveguide applicator is compared with models of two other applicators which depend on energy radiated from conductors carrying high frequency current. The latter are exemplified by the compact resonant patch applicator and the lightweight inductive current sheet applicator. It is shown that heating profiles and field penetration of each applicator are similar for equal radiating areas, and these results have been substantiated experimentally. Impedance match as a function of frequency and load is also compared for the three types of applicator.

Heating system with a lens applicator for 430 MHz microwave hyperthermia

To attain deep and localized heating for hyperthermic treatment of cancers, metal plate lens applicators that can converge microwave electromagnetic (EM) energy in the lossy medium such as human muscle with a computer-controlled heating system have been developed. This paper describes a system operating at 430 MHz. Results of an electric-field distribution calculation and a heating experiment made on a saline solution phantom show that the maximum heating depth is over 60 mm using the lens applicator, which is twice as deep as that obtained with a conventional waveguide applicator. The experimental results agree well with the theoretical ones. A computer-controlled heating system has been developed using the applicator. Experimental results show that fluctuations in temperature at locations in the heating region of the saline phantom were maintained within +/- 0.3 degrees C of the present temperatures. These results indicate that the system can be used for the clinical hyperthermia treatment of cancer.

Heating characteristics of a 430 MHz microwave heating system with a lens applicator in phantoms and miniature pigs

Heating experiments on phantoms and miniature pigs were performed using two types of lens applicator and a conventional waveguide applicator fed by a 430 MHz microwave heating system (HTS-100, manufactured by Tokyo Keiki Co., Ltd). Temperature distributions in agar phantoms and miniature pigs were measured at regular intervals during heating. The following results were thus obtained: (1) The effective heating depth varied with the focal length, showing the lens effect. (2) Using a four-aperture lens applicator in miniature pigs, a heating depth of 6 cm was obtained. This almost doubles the depth of conventional waveguide applicator. The heating area was 12 X 6 cm at the depth of 4 cm. (3) Dielectric properties of the buttocks of the miniature pig were virtually identical to those of the agar phantom containing 0.35% NaCl and 0.05% NaN3.

Experimental assessment of phased-array heating of neck tumours

An investigation of phased-array microwave systems (PAMS) for non-invasively inducing hyperthermia, primarily in neck lesions, has been done with implications for applications at other sites such as lung and pelvis. Our general approach was to combine numerical and analytical approaches with parallel experimental studies. In this paper we will concentrate only on the experimental aspects. The object, such as a homogeneous cylindrical phantom or a neck phantom, was encircled with several standard applicators driven by a single source, but with relative phase and amplitude control over each applicator. The relative phases of the applicators were adjusted by using an implanted monopole antenna connected to an HP network analyser. Power was applied and the specific absorption rate (SAR) was determined by using split phantoms and thermography or by measuring temperature transients dT/dt, recorded by implanted thermometer probes. We found that at 915 MHz for our applicators (SMA Co.) the centre of an 11 cm diameter muscle-like phantom heated to about 33% of the value at the surface in front of the applicator. Similarly, we were able to show significant SAR at the centre of realistically sized neck phantoms using four phased apertures of 915 MHz. Furthermore, substantial improvement was observed if the frequency was lowered to about 400 MHz.

TRIPAS: a triapplicator system with relocatable 'hot spot' at tissue depth

Solving the problem of heat focusing and standardization of the clinical application of hyperthermia requires a mathematical prediction model. The model should include the medium constitutive parameter, and be able to predict positioning of the microwave applicators to optimize treatment planning and provide for reproducible treatment set-up. We present a configuration of 3 applicators subtended by an equilateral triangle in order to target and relocate a 'hot spot' for improved treatment of deep tumors. A simple geometric analysis is illustrated. The microwave beam absorption profile, from the three power sources, was obtained from phantom studies depicting the radiative heat pattern for the triapplicator system (TRIPAS). A complex mathematical model was developed to demonstrate interaction of the beams in the medium. It was observed empirically that under coherent propagation in the near field electromagnetic (EM) waves tend to add at the center, while varying the propagation axial focal length caused a relocation of the summing focal points. Mathematical prediction correlated very well with the phantom studies. SAR values above 100 W/kg were achieved at 12.5 cm phantom depth, creating a relocatable 'hot spot' at the concentric foci of the 3 air cooled horn microwave applicators operating at 300 MHz.

Spiral microstrip hyperthermia applicators: technical design and clinical performance

Spiral microstrip microwave (MW) antennas have been developed and adapted for use as clinical hyperthermia applicators. The design has been configured in a variety of forms including single fixed antenna applicators, multi-element arrays, and mechanically scanned single or paired antennas. The latter three configurations have been used to allow an expansion of the effective heating area. Specific absorption rate (SAR) distributions measured in phantom have been used to estimate the depth and volume of effective heating. The estimates are made using the bioheat equation assuming uniformly perfused tissue. In excess of 500 treatments of patients with advanced or recurrent localized superficial tumors have been performed using this applicator technology. Data from clinical treatments have been analyzed to quantify the heating performance and verify the suitability of these applicators for clinical use. Good microwave coupling efficiency together with the compact applicator size have proved to be valuable clinical assets.

Comparison of two-dimensional numerical approximation and measurement of SAR in a muscle equivalent phantom exposed to a 915 MHz slab-loaded waveguide

Computer predictions of the specific absorption rate (SAR) distribution in a uniform muscle-equivalent phantom with an overlying bolus have been compared to those measured experimentally. The microwave source was a 10 cm x 10 cm slab-loaded waveguide applicator operating at 915 MHz. The modelling technique (theory) combines the equivalence principle and a two-dimensional finite element technique to determine the incident and the scattered electric fields separately. The E-field was measured using a small dipole device oriented parallel to the polarized field of the waveguide source. Comparisons of the predicted and measured SAR were made for various bolus properties, and reasonable agreement with theory was found in each case. The results demonstrate the usefulness of numerical modelling in characterizing the fields from microwave applicators used in clinical hyperthermia.

Noninvasive microwave phased arrays for local hyperthermia: a review

Microwave energy has proven useful for treating superficial tumours in the head, neck and chest regions. Currently, multi-element phased arrays are being proposed to upgrade clinical capabilities for localized microwave hyperthermia. When compared with a single radiating element, phased array applicators are expected to provide deeper tissue penetration, reduce undesired heating of normal tissues between the applicator and tumour, and improve local control of the tumour temperature distribution. This paper surveys recent developments in the design and characterization of phased arrays, identifies anatomical and physiological factors that complicate successful clinical treatment and discusses the current state of phased array hardware development for hyperthermia.

Comparative study of shortwave heating patterns in phantoms with polyethylene and silk partitions

Specific absorption rate (SAR) and effective depths of heating patterns induced by a shortwave, pancake diathermy applicator in fat-muscle phantom are measured. Midplane partitions of polyethylene and silk screen with and without contact chemicals are used. Thermographically obtained SAR data show nearly the same value for silk-screen partitions with and without contact chemicals and slightly lower values with polyethylene partitions, provided that the partition midplanes are tightly pressed against each other. Thermometry data indicate that for low-power exposures the major error in thermographic measurements obtained after termination of heating is due to thermal diffusion and not evaporative cooling in the opened midplane of the phantom.

Introduction to hyperthermia device evaluation

From 10/81 to 1/87, the National Cancer Institute (NCI) of the U.S. Department of Public Health Services (PHS) contracted with four institutions to evaluate hyperthermia technology to be applied in the treatment of human malignancy. During the five year period 1981-1986, data were collected which now reside in a consensus database representing treatments of 792 separate sites or fields in a total of 573 patients. These patients were treated with one or more of 49 devices by the four institutions. Sixteen ultrasound (US) devices were evaluated in 195 sites. Nine magnetic induction (MI) devices were evaluated in 208 sites. Twenty radiative electromagnetic (EM) devices were evaluated in 488 sites. Four interstitial (IRF) devices were evaluated in 37 sites. Many sites were treated with multiple hyperthermia modalities. This first in a series of 13 reports describes the general objectives of the Contractors' Group, basic methods of device evaluation and brief details of the large variety of devices tested.

Determination of power deposition patterns for localized hyperthermia: a steady-state analysis

Hyperthermia applicator design has concentrated on developing systems that allow control of power deposition patterns. In this paper, a method is detailed which uses the steady-state bioheat transfer equation and the target temperature distributions in normal and tumour tissue to calculate the desired steady-state power deposition patterns. This prospective thermal dosimetry approach is demonstrated analytically for three tumour models: an infinite half-space model; an infinite cylinder model; and a spherical model. A three-dimensional numerical method is demonstrated for two different tumour geometries and further applications of this method are discussed.

Absorbed power distributions from two tilted waveguide applicators

One major problem in microwave-induced clinical hyperthermia treatment of superficial tumours is to obtain therapeutic temperatures at the tumour periphery and adequate deep heating when using a single applicator. The use of multiple applicators has therefore been investigated in order to improve the power distribution. Anatomical surface topography often permits the application of two tilted applicators, e.g. in the head and neck area, on extremities and on large protruding tumours. Theoretical calculations of the absorbed power distribution from such an applicator configuration were performed in a homogeneous muscle equivalent medium. The power distribution from two conventional radiative apertures (TE10) was studied at different frequencies, aperture sizes, tilting angles and non-coherent or coherent fields in phase, both theoretically and with phantom experiments using a thermographic camera. With controlled phase relations between two tilted applicators excited at the lower microwave or upper radiofrequency band, the absorbed power distribution can be varied in a wide range. The theoretical calculations and thermographic phantom experiments in simple geometries give valuable information on absorbed power distributions and guidance for the location of temperature probes in clinical hyperthermia.